Plant Biology

Plant cells vs. Animal cells

Plant Cells

Most cells are not visible with the naked eye. However, with microscopes of various types, plant cells can be readily viewed and studied. In young parts of plant and fruits, cell shapes are generally round, while in older sections, the cells are somewhat boxlike with up to 14 sides as they become packed together. 

A plant cell is bounded by a cell wall and the living portion of the cell is within the walls and is divided into two portions: the nucleus, or central control center; and the cytoplasm, a fluid in which membrane bound organelles are found. Between the primary cell walls of adjacent plant cells, lies a pectic middle lamella. There can be a secondary cell wall which would be located just to the inside of the primary wall. Both walls consist mainly of cellulose, but the secondary cell wall may contain lignin and other substances. The outer boundary of the protoplasm (cytoplasm and nucleus) is a sandwich-like, flexible plasma membrane. This membrane regulates what enters and leaves the plant cell. Plant cell organelles include: endoplasmic reticulum, with and without ribosomes attached; Golgi bodies, mitochondria, and plastids. Plastids are chloroplasts, chromoplasts or leucoplasts—depending on the color and likewise the function. Chloroplasts are of specific interest to those studying plants. A plant cell also, obviously, contains a nucleus which is bounded by a nuclear envelope with pores. The pores in the nuclear envelope allow for movement of substances in and out of the nucleus. Within the nucleus is a number of chromosomes. The number present is specific to the organism and it will be later noted how sex cells contain one-half the number of chromosomes, and restore chromosome number upon fertilization. All of these organelles and the nucleus are suspended in the cytoplasm. The cytoplasm has movements that are referred to as cytoplasmic streaming or cyclosis.  The particular function of the other organelles contained in plant cells can be reviewed below:

1. The nucleus is in the center of most cells. Some cells contain multiple nuclei, such as skeletal muscle, while some do not have any, such as red blood cells. The nucleus is the largest membrane-bound organelle. Specifically, it is responsible for storing and transmitting genetic information. The nucleus is surrounded by a selective nuclear envelope. The nuclear envelope is composed of two membranes joined at regular intervals to form circular openings called nuclear pores. The pores allow RNA molecules and proteins modulating DNA expression to move through the pores and into the cytosol. The selection process is controlled by an energy-dependent process that alters the diameter of the pores in response to signals. Inside the nucleus, DNA and proteins associate to form a network of threads called chromatin. The chromatin becomes vital at the time of cell division as it becomes tightly condensed thus forming the rodlike chromosomes with the enmeshed DNA. Inside the nucleus is a filamentous region called the nucleolus. This serves as a site where the RNA and protein components of ribosomes are assembled. The nucleolus is not membrane bound, but rather just a region.
2. Ribosomes are the sites where protein molecules are synthesized from amino acids. They are composed of proteins and RNA. Some ribosomes are found bound to granular endoplasmic reticulum, while others are free in the cytoplasm. The proteins synthesized on ribosomes bound to granular endoplasmic reticulum are transferred from the lumen (open space inside endoplasmic reticulum) to the golgi apparatus for secretion outside the cell or distribution to other organelles. The proteins that are synthesized of free ribosomes are released into the cytosol.
3. The endoplasmic reticulum (ER) is collectively a network of membranes enclosing a singular continuous space. As mentioned earlier, granular endoplasmic reticulum is associated with ribosomes (giving the exterior surface a rough, or granular appearance). Sometimes granular endoplasmic reticulum is referred to as rough ER. The granular ER is involved in packaging proteins for the golgi apparatus. The agranular, or smooth, ER lacks ribosomes and is the site of lipid synthesis. In addition, the agranular ER stores and releases calcium ions   (Ca ²⁺ ).
4. The golgi apparatus is a membranous sac that serves to modify and sort proteins into secretory/transport vesicles. The vesicles are then delivered to other cell organelles and the plasma membrane. Most cells have at least one golgi apparatus, although some may have multiple. The apparatus is usually located near the nucleus.
5. Endosomes are membrane-bound tubular and vesicular structures located between the plasma membrane and the golgi apparatus. They serve to sort and direct vesicular traffic by pinching off vesicles or fusing with them.
6. Mitochondria are some of the most important structures in the cell. They are they site of various chemical processes involved in the synthesis of energy packets called ATP (adenosine triphosphate). Each mitochondrion is surrounded by two membranes. The outer membrane is smooth, while the inner one is folded into tubule structures called cristae. Mitochondria are unique in that they contain small amounts of DNA containing the genes for the synthesis of some mitochondrial proteins. The DNA is inherited solely from the mother. Cells with greater activity have more mitochondria, while those that are less active have less need for energy producing mitochondria.
7. Lysosomes are bound by a single membrane and contain highly acidic fluid. The fluid acts as digesting enzymes for breaking down bacteria and cell debris. They play an important from in the cells of the immune system.
8. Peroxisomes are also bound by a single membrane. They consume oxygen and work to drive reactions that remove hydrogen from various molecules in the form of hydrogen peroxide. They are important in maintaining the chemical balances within the cell.
9. The cytoskeleton is a filamentous network of proteins that are associated with the processes that maintain and change cell shape and produce cell movements in animal and bacteria cells. In plants, it is responsible for maintaining structures within the plant cell, rather then whole cell movement. The cytoskeleton also forms tracks along which cell organelles move propelled by contractile proteins attached to their various surfaces. Like a little highway infrastructure inside the cell. Three types of filaments make up the cytoskeleton.
   1. Microfilaments are the thinnest and most abundant of the cytoskeleton proteins. They are composed of actin, a contractile protein, and can be assembled and disassembled quickly according to the needs of the cell or organelle structure.
   2. Intermediate filaments are slightly larger in diameter and are found most extensively in regions of cells that are going to be subjected to stress. Once these filaments are assembled they are not capable of rapid disassembly.
   3. Microtubules are hollow tubes composed of a protein called tubulin. They are the thickest and most rigid of the filaments. Microtubules are present in the axons and long dendrite projections of nerve cells. They are capable of rapid assembly and disassembly according to need. Microtubules are structured around a cell region called the centrosome, which surrounds two centrioles composed of 9 sets of fused microtubules. These are important in cell division when the centrosome generates the microtubluar spindle fibers necessary for chromosome separation.
10. Chloroplasts
    It is necessary to note a bit about the form of chloroplasts, as you will encounter them throughout this tutorial. Inside a chloroplast is a matrix called the stroma. Enzymes are found in the stroma as well as grana—stacks of coin-shaped discs, called thylakoids. It is within the thylakoids that photosynthesis takes place. Note that chloroplasts, like mitochondria contain their own DNA. They do rely on proteins from the nucleus, and are considered semi-autonomous organelles. Photosynthesis will be discussed in greater detail in the Plant Metabolism tutorial.
11. Vacuoles
    Plant cells are also notorious for having huge vacuoles. Up to 90% of the volume of a mature cell may be taken up by a single large vacuole or several vacuoles. The vacuole is bound by a special membrane, called the tonoplast, and contains cell sap—which is composed of dissolved substances and may include pigments.

Cell Cycle

The cell cycle contains the process in which cells are either dividing or in between divisions. Cells that are not actively dividing are said to be in interphase, which has three distinct periods of intense activity that precedes the division of the nucleus, or mitosis. The division of the rest of the cell occurs as an end result of mitosis and this process occurs in regions of active cell division, called meristems. Meristems will be looked at in the plant tissue tutorial.

Mitosis is a process within the cell cycle that is divided into four phases which we will sum up here:

1. Prophase—the chromosomes and their usual two-stranded nature becomes apparent, the nuclear envelope breaks down.
2. Metaphase—the chromosomes become aligned at the equator of the cell. A spindle composed of spindle fibers is developed and some attach to the chromosomes at their centromere.
3. Anaphase—the sister chromatids of each chromosome, that is now called the daughter chromosomes, separate lengthwise and each group of daughter chromosomes migrates to the opposite ends of the cell.
4. Telophase—the groups of daughter chromosomes are grouped within a developing nuclear envelope which makes them separate nuclei. A wall forms between the two sets of daughter chromosomes thus creating two daughter cells.

In plants, as the cell wall is developing, droplets or vesicles of pectin merge forming a cell plate that eventually will become the middle lamella of the new cell wall.

Plant Cells Compared with Animal Cells

Animal cells do not have a cell wall. Instead of a cell wall, the plasma membrane (usually called cell membrane when discussing animal cells) is the outer boundary of animal cells. Animal tissues therefore require either external or internal support from some kind of skeleton.  Frameworks of rigid cellulose fibrils thicken and strengthen the cell walls of higher plants.  Plasmodesmata that connect the protoplasts of higher plant cells do not have a counterpart in the animal cell model.  During telophase of mitosis, a cell plate is formed as the plant cell begins its division.  In animal cells, the cell pinches in the center to form two cells; no cell plate is laid down.  Centrioles are generally not found in higher plant cells, while they are found in animal cells. Animal cells do not have plastids, which are common in plant cells (chloroplasts). Both cell types have vacuoles, however, in animal cells vacuoles are very tiny or absent, while in plant cells vacuoles are generally quite large.

Plant Tissues

Plants are composed of three major organ groups: roots, stems and leaves. As we know from other areas of biology, these organs are comprised of tissues working together for a common goal (function). In turn, tissues are made of a number of cells which are made of elements and atoms on the most fundamental level. In this section, we will look at the various types of plant tissue and their place and purpose within a plant. It is important to realize that there may be slight variations and modifications to the basic tissue types in special plants.

Plant tissues are characterized and classified according to their structure and function. The organs that they form will be organized into patterns within a plant which will aid in further classifying the plant. A good example of this is the three basic tissue patterns found in roots and stems which serve to delineate between woody dicot, herbaceous dicot and monocot plants. We will look at these classifications later on in the tutorial.

Meristematic Tissues

Tissues where cells are constantly dividing are called meristems or meristematic tissues. These regions produce new cells. These new cells are generally small, six-sided boxlike structures with a number of tiny vacuoles and a large nucleus, by comparison. Sometimes there are no vacuoles at all. As the cells mature the vacuoles will grow to many different shapes and sizes, depending on the needs of the cell. It is possible that the vacuole may fill 95% or more of the cell’s total volume.

There are three types of meristems:

1. Apical Meristems
2. Lateral Meristems
3. Intercalary Meristems

Apical meristems are located at or near the tips of roots and shoots. As new cells form in the meristems, the roots and shoots will increase in length. This vertical growth is also known as primary growth. A good example would be the growth of a tree in height. Each apical meristem will produce embryo leaves and buds as well as three types of primary meristems: protoderm, ground meristems, and procambium.  These primary meristems will produce the cells that will form the primary tissues.

Lateral meristems account for secondary growth in plants. Secondary growth is generally horizontal growth. A good example would be the growth of a tree trunk in girth. There are two types of lateral meristems to be aware of in the study of plants.

The vascular cambium, the first type of lateral meristem, is sometimes just called the cambium. The cambium is a thin, branching cylinder that, except for the tips where the apical meristems are located, runs the length of the roots and stems of most perennial plants and many herbaceous annuals. The cambium is responsible for the production of cells and tissues that increase the thickness, or girth, of the plant.

The cork cambium, the second type of lateral meristem, is much like the vascular cambium in that it is also a thin cylinder that runs the length of roots and stems. The difference is that it is only found in woody plants, as it will produce the outer bark.

Both the vascular cambium and the cork cambium, if present, will begin to produce cells and tissues only after the primary tissues produced by the apical meristems have begun to mature.

Intercalary meristems are found in grasses and related plants that do not have a vascular cambium or a cork cambium, as they do not increase in girth. These plants do have apical meristems and in areas of leaf attachment, called nodes, they have the third type of meristematic tissue. This meristem will also actively produce new cells and is responsibly for increases in length. The intercalary meristem is responsible for the regrowth of cut grass.

There are other tissues in plants that do not actively produce new cells. These tissues are called nonmeristematic tissues. Nonmeristematic tissues are made of cells that are produced by the meristems and are formed to various shapes and sizes depending on their intended function in the plant. Sometimes the tissues are composed of the same type of cells throughout, or sometimes they are mixed.  There are simple tissues and complex tissues to consider, but we will start with the simple tissues for the sake of discussion.

Simple Tissues

There are three basic types, named for the type of cell that makes up their composition.

1. Parenchyma cells form parenchyma tissue. Parenchyma cells are the most abundant of cell types and are found in almost all major parts of higher plants (we will discuss higher plants later in the tutorial). These cells are basically sphere shaped when they are first made. However, these cells have thin walls, which flatten at the points of contact when many cells are packed together. Generally, they have many sides with the majority having 14 sides. These cells have large vacuoles and may contain various secretions including starch, oils, tannins, and crystals. Some parenchyma cells have many chloroplasts and form the tissues found in leaves. This type of tissue is called chlorenchyma. The chief function of this type of tissue is photosynthesis, while parenchyma tissues without chloroplasts are generally used for food or water storage. Additionally, some groups of cells are loosely packed together with connected air spaces, such as in water lilies, this tissue is called aerenchyma tissue. These type of cells can also develop irregular extensions of the inner wall which increases overall surface area of the plasma membrane and facilitates transferring of dissolved substances between adjacent cells.  Parenchyma cells can divide if they are mature, and this is vital in repairing damage to plant tissues. Parenchyma cells and tissues comprise most of the edible portions of fruit.
2. Collenchyma cells form collenchyma tissue. These cells have a living protoplasm, like parenchyma cells, and may also stay alive for a long period of time. Their main distinguishing difference from parenchyma cells is the increased thickness of their walls. In cross section, the walls looks uneven. Collenchyma cells are found just beneath the epidermis and generally they are elongated and their walls are pliable in addition to being strong. As a plant grows these cells and the tissues they form, provide flexible support for organs such as leaves and flower parts. Good examples of collenchyma plant cells are the ‘strings’ from celery that get stuck in our teeth.
3. Sclerenchyma cells form sclerenchyma tissue. These cells have thick, tough secondary walls that are imbedded with lignin. At maturity, most sclerenchyma cells are dead and function in structure and support. Sclerenchyma cells can occur in two forms:
   1. Sclereids are sclerenchyma cells that are randomly distributed throughout other tissues. Sometimes they are grouped within other tissues in specific zones or regions. They are generally as long as they are wide.  An example, would be the gritty texture in some types of pears. The grittiness is due to groups of sclereid cells. Sclereids are sometimes called stone cells.
   2. Fibers are sometimes found in association with a wide variety of tissues in roots, stems, leaves and fruits. Usually fiber cells are much longer than they are wide and have a very tiny cavity in the center of the cell. Currently, fibers from over 40 different plant families are used in the manufacture of textiles, ropes, string and canvas goods to name a few.

Secretory Cells and Tissues

As a result of cellular processes, substances that are left to accumulate within the cell can sometimes damage the protoplasm. Thus it is essential that these materials are either isolated from the protoplasm in which they originate, or be moved outside the plant body. Although most of these substances are waste products, some substances are vital to normal plant functions. Examples: oils in citrus, pine resin, latex, opium, nectar, perfumes and plant hormones. Generally, secretory cells are derived from parenchyma cells and may function on their own or as a tissue. They sometimes have great commercial value.

Complex Tissues

Tissues composed of more than one cell type are generically referred to as complex tissues. Xylem and phloem are the two most important complex tissues in a plant, as their primary functions include the transport of water, ions and soluble food substances throughout the plant. While some complex tissues are produced by apical meristems, most in woody plants are produced by the vascular cambium and is often referenced as vascular tissue. Other complex tissues include the epidermis and the periderm. The epidermis consists primarily of parenchyma-like cells and forms a protective covering for all plant organs. The epidermis includes specialized cells that allow for the movement of water and gases in and out of the plant, secretory glands, various hairs, cells in which crystals are accumulated and isolated, and other cells that increase absorption in the roots.  The periderm is mostly cork cells and therefore forms the outer bark of woody plants. It is considered to be a complex tissue because of the pockets of parenchyma cells scattered throughout.

Xylem

Xylem is an important plant tissue as it is part of the ‘plumbing’ of a plant.  Think of bundles of  pipes running along the main axis of stems and roots. It carries water and dissolved substances throughout and consists of a combination of parenchyma cells, fibers, vessels, tracheids and ray cells.  Long tubes made up of individual cells are the vessels, while vessel members are open at each end. Internally, there may be bars of wall material extending across the open space. These cells are joined end to end to form long tubes. Vessel members and tracheids are dead at maturity. Tracheids have thick secondary cell walls and are tapered at the ends. They do not have end openings such as the vessels. The tracheids ends overlap with each other, with pairs of pits present. The pit pairs allow water to pass from cell to cell. While most conduction in the xylem is up and down, there is some side-to-side or lateral conduction via rays. Rays are horizontal rows of long-living parenchyma cells that arise out of the vascular cambium. In trees, and other woody plants, ray will radiate out from the center of stems and roots and in cross-section will look like the spokes of a wheel.

Phloem

Phloem is an equally important plant tissue as it also is part of the ‘plumbing’ of a plant. Primarily, phloem carries dissolved food substances throughout the plant. This conduction system is composed of sieve-tube member and companion cells, that are without secondary walls.  The parent cells of the vascular cambium produce both xylem and phloem. This usually also includes fibers, parenchyma and ray cells. Sieve tubes are formed from sieve-tube members laid end to end. The end walls, unlike vessel members in xylem, do not have openings. The end walls, however, are full of small pores where cytoplasm extends from cell to cell. These porous connections are called sieve plates. In spite of the fact that their cytoplasm is actively involved in the conduction of food materials, sieve-tube members do not have nuclei at maturity. It is the companion cells that are nestled between sieve-tube members that function in some manner bringing about the conduction of food. Sieve-tube members that are alive contain a polymer called callose. Callose stays in solution as long at the cell contents are under pressure. As a repair mechanism, if an insect injures a cell and the pressure drops, the callose will precipitate. However, the callose and a phloem protein will be moved through the nearest sieve plate where they will for a plug. This prevents further leakage of sieve tube contents and the injury is not necessarily fatal to overall plant turgor pressure.

Epidermis

The epidermis is also a complex plant tissue, and an interesting one at that. Officially, the epidermis is the outermost layer of cells on all plant organs (roots, stems, leaves). The epidermis is in direct contact with the environment and therefore is subject to environmental conditions and constraints. Generally, the epidermis is one cell layer thick, however there are exceptions such as tropical plants where the layer may be several cells thick and thus acts as a sponge. Cutin, a fatty substance secreted by most epidermal cells, forms a waxy protective layer called the cuticle.  The thickness of the cuticle is one of the main determiners of how much water is lost by evaporation. Additionally, at no extra charge, the cuticle provides some resistance to bacteria and other disease organisms. Some plants, such as the wax palm, produce enough cuticle to have commercial value: carnauba wax. Other wax products are used as polishes, candles and even phonographic records. Epidermal cells are important for increasing absorptive surface area in root hairs. Root hairs are essentially tubular extensions of the main root body composed entirely of epidermal cells. Leaves are not left out. They have many small pores called stomata that are surrounded by pairs of specialized epidermal cells called guard cells. Guard cells are unique epidermal cells because they are of a different shape and contain chloroplasts. They will be discussed in detail later on in the tutorial. There are other modified epidermal cells that may be glands or hairs that repel insects or reduce water loss.

Periderm

In woody plants, when the cork cambium begins to produce new tissues to increase the girth of the stem or root the epidermis is sloughed off and replaced by a periderm. The periderm is made of semi-rectangular and boxlike cork cells. This will be the outermost layer of bark. These cells are dead at maturity.  However, before the cells die, the protoplasm secretes a fatty substance called suberin into the cell walls. Suberin makes the cork cells waterproof and aids in protecting tissues beneath the bark.  There are parts of the cork cambium that produce pockets of loosely packed cork cells. These cork cells do not have suberin imbedded in their cell walls. These loose areas are extended through the surface of the periderm and are called lenticels. Lenticels function in gas exchange between the air and the stem interior. At the bottom of the deep fissures in tree bark are the lenticels. 

Stems

External Form of a woody twig

A woody twig, or stem, is an axis with leaves attached. The leaves are arranged in various ways around and on the axis. You may hear them described as alternate, or alternately arranged, opposite or oppositely arranged, or if they are found in groups of three or more they may be referred to as whorled. The region, just a general area in this case, where the leaves attach to the stem are called nodes. The region of stem between two nodes is called the internode. The leaf blade is attached to the stem via a stalk called the petiole.  In the angle, or axil, formed between the petiole and the stem you will find the axillary bud. Axillary refers to a structure that forms an armpit, just for trivia’s sake. These buds can become new branches or they may have tissues that will form into flowers for the next season.  Most buds are protected by bud scales which fall off as bud tissue begins to grow. In general, at the tip of a twig a terminal (or ending) bud is present. It is larger than the axillary buds and produces tissues to extend twig length during the growing season.  When the bud scales of a terminal bud fall off they leave scars on the twig. You can calculate the age of a twig by counting up the terminal bud scale scars. There are other scars on twigs that may look like terminal bud scars that are left by paired appendages called stipules which are found at the base of a petiole in the axil.

Trees and shrubs that lose their leaves every year, deciduous plants, have characteristic leaf scars with dormant, or not active, axillary buds directly above them. Sometimes tiny bundle scars can be seen. These scars are found in the leaf scar and mark the location of food and water conducting tissues.  The shape and arrangement of the bundle scars can help distinguish deciduous trees in the winter months when the leaf structures are absent.

The origin and development of stems 

Recall that the apical meristem is responsible for vertical growth, or increase in length of a stem. Prior to the start of the growing season, the cells in the apical meristem are dormant. The apical meristem is protected at the tip of the twig, by the covering bud scales and by the leaf primordial. The leaf primordia are tiny embryonic leaves that will develop into mature leaves after bud scales drop off and growth commences. When a seed germinates or a bud begins to grow, the cells in the apical meristem undergo mitosis. From these cells three primary meristems will develop:

 

1. The outermost meristem is the protoderm, which gives rise to the epidermis. This layer is usually one cell thick and becomes coated with a waxy cuticle.
2. The second layer is the procambium, which is a cylinder of strands. This layer gives rise to the primary xylem and primary phloem cells.
3. The innermost meristem is the ground meristem from which arises two tissues composed of parenchyma cells. The tissue in the center of the stem is the pith. These cells are large and may break down shortly after being formed which leaves a cylindrical hollow area.  If they do not break down, they will be compressed by new additions to the plant girth by other meristems. The second parenchyma tissue that arises is called cortex. Cortex may be quite extensive and also crushed or replaced in woody stems. The function of both tissues is food storage. If chloroplasts are present the tissues may function in producing food.

 

It is important to note that all five of the above mentioned tissues—epidermis, primary xylem, primary phloem, pith and cortex—are produced by the apical meristem and are thus primary tissues as the plant is increasing in length.  Xylem and phloem tissue branch off from the main vascular cylinder and enter into the leaf or bud. Each branching of vascular tissue is called a trace. Each trace branch leaves a small thumbnail shaped gap in the cylinder of tissue and are called leaf gaps and bud gaps.

In between the primary xylem and primary phloem a thin band of cells retains its meristematic nature. This band becomes the vascular cambium of one of the two lateral meristems.

In woody plants, and some others, a second cambium arises from the cortex or sometimes the epidermis or phloem. The second cambium is called the cork cambium or phellogen and is responsible for producing cork cells. Recall that the cork cells become filled with suberin which waterproofs the cells. The resulting cork tissue constitutes the out bark of woody plants and functions to reduce water loss and to protect the stem against mechanical injury. We will revisit the role of cork later on in discussing biotechnology and propagation. For now, though, understand that cork tissue cuts off food and water supplies to the epidermis which results in a sloughing off. Also understand that cork tissues do not form a solid cylinder around the exterior of a woody stem. This is to allow vital gas exchange with the environment. 

Before we go on, it is important to remember the difference between monocots and dicots, the two main divisions of flowering plants. Most of the distinguishing revolves around the seed leaves, which are called cotyledons. Cotyledons function in storing food needed by the young seedling until true leaves grow and are able to take over the food supplying function.

 

1. Monocotyledon (monocot) plants—These plants form from seeds that have one embryonic seed leaf (hence the ‘mono’ in monocot).
2. Dicotyledon (dicot) plants—These plants form from seeds that have two (hence the ‘di’ in dicot) embryonic seed leaf.

 

Cone bearing trees, conifers such as pines, have multiple cotyledons, usually eight, in their seed structure.

There are four tissue patterns to be aware of in the study of plants.

1. Steles—steles are a central cylinder in most younger stems and roots, composed of primary xylem, phloem and the pith, if present. Sometimes referred to as eusteles, which are vascular bundles in higher vascular plants.
2. Herbaceous Dicotyledonous Stems—Herbaceous refers to non-woody plants. Plants that die after going from seed to maturity are called annuals. In general, most monocots are annuals, but there are annual dicot plants as well. Annual dicots are mostly composed of primary tissues, although there may be some minimal secondary growth. Remember, the plant only lives a year, so extensive secondary growth, or increases in width, really doesn’t make sense as far as using the plant resources. A cross-section of a herbaceous dicot stem will show discrete patches of xylem and phloem, vascular bundles, that are arranged in a proper ring separating the cortex and the pith. If secondary xylem and phloem are to emerge, they will arise from between the two primary tissues.  Monocots will be discussed shortly.
3. Woody Dicotyledonous Stems—Wood is essentially secondary xylem growth. These stems look similar to herbaceous dicot stems up until the vascular cambium and the cork cambium start functioning.  The differences are then quite obvious. While some tropical trees demonstrate year round secondary growth, most trees in temperate climates grow in the spring and summer and cease through the winter. In the springtime, when water and resources are plentiful, the vascular cambium produces large xylem cells. During the summer months when resources and water may be lacking or reduced, the xylem cells are small. Pressed up against the large, light colored xylem cells, the small xylem cells look like a thin dark ring. One year of xylem growth, called an annual ring, can be measured as the distance between the dark rings—or the distance between summer xylem growth. Summer growth is called summer wood while the large spring cells are called spring wood. Much can be learned about the local environmental conditions through the years by looking at tree rings. If water is plentiful the rings will be wider than usual. Years with fires and blights will be evident, as well as insect infestations and fungal infections. All this by looking at a cross section of a tree. In conifers, vessels and fibers are absent and thus the wood consists mainly of tracheids. It is important therefore to remember that environmental conditions affect xylem production and the dark rings may not be completely visible, one year’s growth is what constitute an annual ring, not just dark circles.

The vascular cambium produces more xylem than phloem. In fact, the phloem will be difficult to locate as the cells are thinner than xylem and more likely to collapse under the pressure of the cambiums. Phloem grows to the outside of the vascular cambium and xylem grows to the inside. The oldest xylem is in the very center of the stem/trunk. The wood in the center is called heartwood. It is usually darker as the vessels and tracheids are filled with old resins, gums and tannins. The younger wood where the xylem is still functioning is toward the outside of the stem nearest the cambium and is lighter in color. This younger wood is called sapwood.  The main role of heartwood is structure and support, since it is unable to conduct water and nutrients. The heartwood sometimes rots out of a otherwise living tree. Sapwood develops at roughly the same rate that heartwood is ‘retiring’ and thus vital conducting functions are not compromised. Recall that conifers do not have vessels or fibers and are primarily tracheids. Conifers have resin canals scattered throughout the xylem tissue. Conifers are primarily considered to be softwoods while the wood of woody dicot trees are considered to be hardwood.

Bark is all of the tissues outside of the cambium, including the phloem. Some have gone so far as to distinguish between inner bark—primary and secondary phlolem and outer bark—the periderm, which consists of cork tissue and cork cambium. The cells in these layers only function briefly as they usually become crushed and then slough off. New layers are annually produced by the cambiums. The youngest phloem cells are the ones nearest the vascular cambium and are most active in transporting nutrients, sugars and water. Mature bark may be composed of alternating layers of crushed phloem and cork.

1. Monocotyledonous Stems—These plants are usually grass or grass-like and do not grow to great size. Monocot stems do not have vascular cambiums or cork cambiums, as growth will not be laterally. The vascular bundles produced by the procambium are scattered throughout the stem, rather than organized in rings as in woody dicot stems.  Every bundle is oriented with the xylem toward the center of the stem and the phloem toward the stem surface. The xylem in the vascular bundle generally consists of two large vessels with some small vessels in between them, while the phloem consists of sieve tubes and companion cells. The entire vascular bundle is wrapped in a sheath of sclerenchyma cells. The background tissue between vascular bundles is not divided into cortex and pith in monocots, but they do have similar function and appearance as the parenchyma cells in cortex and pith. The concentration of bundles and bands of sclerenchyma cells, give the stem the flexibility and strength to withstand the elements—such as a summer rainstorm. In grasses, there is an intercalary meristem at the base o each internode which contributes to growth in length, like apical meristems. During the growing season, the stems of the grasses elongate rapidly. Because there is no vascular cambium that would produce tissues to increase the girth of the plant, the growth is columnar with very little variation in diameter between the top and the bottom of the plant. 

Palm trees are special, because they grow to considerable size, however this is primarily due to the subsequent division and growth of their parenchyma cells. All this growth occurs without a true cambium developing. Other monocot stems have adaptations that allow for specialized growth. Monocot fibers, such as manila hemp and sisal, come from stems and leaves and are used for commercial products however, their fibers are not as strong as dicot fibers.

Rhizomes—horizontal stems that grow beneath the ground, but near the surface

of the soil. They resemble roots, but are actually modified stems with

scale-like leaves and buds at each axillary node. In addition, adventitious roots are produced along the rhizome on the lower surface in order to increase absorption surface area.

Runners and Stolons—Runners are horizontal stems that grow above ground,

usually along the surface (compare with rhizomes). Strawberry plants produce runners after the first flowering of the season, they may extend out up to 3 feet or more beyond the parent plant. Along the runner, adventitious buds will develop in order to propagate new plants. Stolons are similar to runners, except that they grow roughly vertically beneath the surface of the soil. Irish potato plants have tubers at the tips of stolons.

Tubers—Tubers develop at the tips of stolons. The plant accumulates food at the

stolon and the area swells at the internodes. When the tuber is mature the stolon will die and the ‘eyes’ of the potato are actually nodes arranged in a spiral around the modified stem. Each eye has an axillary bud in the axil of a tiny leaf, which is not always visible in maturing tubers.

Bulbs—These are actually large buds with a small stem at the lower end that is

surrounded with fleshy leaves. Onions, irises and tulips are good examples of bulbs and their main function is food storage.

Corms—On first glance you might think these guys are bulbs, however, the

differences lie beneath the thin layer of leaves covering the outside of the corm. Adventitious roots form beneath the fullness of the base. Corms function in storing food. Crocuses and gladioli are good examples of plants with corms.

Cladophylls—These are usually called the prickly part of a cactus. Cladophylls

are flattened and somewhat leaf-like in appearance. They center of each cladophyll usually has a node with small scalelike leaves complete with axillary buds. The scaly look to asparagus are cladophylls. These specialized stems are not only restricted to cacti, but are found in some orchids and greenbriars.

Other Specialized Stems—Cacti usually have modifications in their stem or

‘trunk’ structure in order to hold extra water. Other stems may be modified into thorns or briars. It is important to remember that not all thorn-like structures are stems! Raspberry and rose prickles are extensions of their epidermis and are neither thorns nor spines. Other stems are modified for climbing, such as tendrils and ramblers.

Stems are vital to the human cause. They provide building materials, paper products, food and much, much more! Stop and think of how many things you encounter in a day that is either made of wood or plant fiber or a derivative product, chances are good the are a stem derivative.

Roots

Upon seed germination, the embryo root, called the radicle, grows and develops into the first root. The radicle may thicken into a taproot with many branching roots, or it may develop into many adventitious roots. The direct opposite of a taproot system is a fibrous root system. This develops out of the many adventitious roots. In diameter, the roots in a fibrous system are very fine. There are many mature plants that have a combination system, which means there is a main taproot with many branching fibrous roots attached.  Root hairs, or extensions of the epidermis as explained in the plant tissue tutorial, significantly increase the contact surface area of the root system. This allows for more exchange with the surrounding soil.

In general, most dicot plants (peas, carrots), or two seed-leaf plants, have taproot systems while monocot plants (corn, grasses), or one seed-leaf plants, have fibrous root systems. Additional differences between dicots and monocots will be discussed later on.

Root Structure

Historically, developing roots have been categorized into four zones of development. These are not strict zones, but rather regions of cells that gradually develop into those of the next region. The zones vary widely as far as extent and levels of development.

Regions of root development:

1. Root cap
2. Region of cell division
3. Region of elongation
4. Region of maturation

We will discuss each region in greater detail.

Root cap

In some plants the root cap is quite large and obvious, while in others it is nearly impossible to find. The root cap is made of parenchyma cells that form a thimble shape, as a covering for the tip of each root. The cap serves several functions. The main function being protection as the delicate root tip pushes through soil particles. In the outer cells of the root cap, the golgi bodies secrete a slimy substance that lodges in the walls and eventually pass to the outside. As the cells slough off, replaced from the inside, they form a slimy lubricant that aids root tip movement through the soil.  In addition, to aiding movement, the slime is a supportive medium for beneficial bacteria.

The root cap serves in an additional capacity in determining root growth direction. As the root cap has a life span of about one week, it can serve for some interesting experiments. Whether the cap sloughs off or is cut off, the root will grow in random directions, as opposed to downward, until a new root cap is formed.  This lends support to the notion that the root cap functions in the perception of gravity. On the sides of the root cap amyloplasts, or plastids containing starch grains, collect facing the direction of gravitational force. In documented experiments, when the root is tipped horizontally from it’s vertical growing position, the amyloplasts will reshift themselves to the “bottom” of the cells in which they are found. In a short time or 30 minutes to a few hours the root will resume growing downward. While the exact nature of this gravitational response, or gravitropism, is not fully known, there is some evidence that the calcium ions found in amyloplasts does influence the distribution of growth hormones in plant cells.

The Region of Cell Division

The region of cell division is the next zone in from the root cap. The root cap arises from the cells in this zone. This inverted cup-shaped region is composed of an apical meristem at it’s edges. The cells divide every 12 to 36 hours at the tip of the meristem, while the ones at the base of the meristem may divide once every 200 to 500 hours. Interestingly enough, the divisions are rhythmic and peak usually twice a day around noon and midnight. In the interim the cells are not usually dividing. Most of the cells in this region are cube shaped with fairly large nuclei and few, if any, small vacuoles.  As in stems as well, the apical meristem in the roots will subdivide and give rise to three meristematic areas: the protoderm, which gives rise to the epidermis; just to the inside of the protoderm, the ground meristem, which produces parenchyma cells of the cortex; and the solid looking cylinder in the center of the root, the procambium, which produces primary xylem and phloem. The central pith tissue is found in many monocots, such as grasses, but is generally not seen in mature dicot plants due to compression by the vascular cylinder.

The Region of Elongation

This region is merged with the upper (toward the soil surface), region of the root apical meristem. It is in this region that the cells become several times their original length, and somewhat wider.  The tiny vacuoles in each cell will merge and become one or two large vacuoles. In their final state, the enlarged vacuoles will account for up to 90% or more of the cellular volume. As only the root cap and apical meristem are actually moving through the soil, no further increase in cell size occurs above the region of elongation. While the elongated portions of the root generally remain stationary for the rest of their life, if a cambium is present there may be secondary growth and an increase in root girth.

The Region of Maturation

The region of maturation is sometimes also called the region of differentiation or root-hair zone.  In this region, cells mature into the various types of primary tissues. Recall that root hairs are extensions of the epidermis that serve to increase surface area and aid in absorption of water and soil nutrients. If the region of maturation is examined carefully, it would be noted that the cuticle is very thin on the root hairs and epidermal cells of roots. It is understood that any significant amount of fatty substance would interfere with the ability to absorb water, as fats are hydrophobic—or water repelling.  A root in cross-section would have an epidermis, cortex, endodermis, xylem, phloem and a pericycle. The cortex is tissue at the immediate inside of the epidermis that functions in storing food. Generally, the cortex is many cells thick and similar to the cortex of stems, with the exception of the presence of a root endodermis at the inner boundary.  In stems, an endodermis is quite rare, while in roots only three species of plants are known to lack a root endodermis. The endodermis is a cylinder formed by a single layer of tightly arranged cells. The primary walls of these cells contain suberin. The waterproof suberin forms bands, called Casparian strips, around the cell walls perpendicular to the root’s surface. The barrier that is formed forces all water and dissolved substances entering and leaving the central tissue core to pass through the plasma membrane or their plasmodesmata. This entire structure serves to regulate the types of minerals absorbed and transported by the root to the stems. 

Next to the inside of the endodermis is a cylinder of parenchyma cells called the pericycle. The pericycle is generally one cell wide, however, it can extend for several cells depending on the plant.  It is a vital tissue, as the pericycle is the point of origin for the lateral branch roots, and if it is a dicot, part of the vascular cambium.   The cells in the pericycle retain their ability to divide even after they have matured. Primary xylem, which contains water conducting cells, forms at the core of the root and may or may not have observable ‘branches’ which extend like an ‘x’ to the pericycle. Primary phloem, which contains the food conducting cells, fills in the spaces between the branches of xylem.  Any branch roots will usually arise in the pericycle opposite the xylem branches.

Most plants produce a fibrous root system, a taproot system, or most commonly a combination of both. However, some plants have roots with modifications that allow specific functions in addition to the absorption of water and minerals in solution.

Food-Storage Roots

In certain plants the roots, or part of the root system, is enlarged in order to store large quantities of starch and other carbohydrates. Sweet potatoes and yams, for example, have extra cambial cells that develop in the xylem portion of branch roots. The cambial cells produce numerous parenchyma cells that cause the organs to swell. Starches are then stored in the swollen areas of the root. Carrots, beets and turnips have storage organs that are actually a combination of root and stem. Approximately, the top two centimeters of a carrot are actually derived from the stem. Although, you likely will not be able to see the origin of the cells just by looking at a carrot. 

Water-Storage Roots

Plants that grow in particularly arid regions are known for growing structures used to retain water. Some plants in the Pumpkin Family produce huge water storing roots. The plant will then use the stored water in times or seasons of low precipitation. Some cultures will harvest the water storing root and use them for drinking water. Plants storing up to 159 pounds (72 kilograms) of water in a single major root have been found and documented.

To propagate means to produce more of oneself. Propagative root structures are a way for a plant to produce more of itself. Adventitious buds are buds that appear in unusual places. Many plants will produce these buds along the roots that grow near the surface of the ground. Suckers, or aerial stems with rootlets, will develop from these adventitious buds. The ‘new’ plant can be separated from the original plant and can grow independently. Some plants will produce propagative roots up to 30 feet or more away from the parent plant. This can be a nuisance for some people, while others may enjoy the propagative qualities of their cherry tree, strawberries or horseradish plants.

Pneumatophores

Pneumatophores are spongy roots that develop in most plants that grow in water. Swamps, marshes and coastal areas are good places to find plants with pneumatophores. These specialized roots account for the fact that water, even after having air bubbled through it, has less than one thirtieth of the amount of free oxygen that is found in air. Plants growing in water may require additional methods of obtaining oxygen for respiration. Pneumatophores fill that need by rising above the water surface and facilitating gas exchange.

There are many different kinds of aerial roots produced by a wide variety of plants. Orchids produce velamen roots, corn plants have prop roots, ivies have adventitious roots and vanilla orchids even have photosynthetic roots that can manufacture food. Banyan trees have aerial roots that grow down from the tree branches until they touch find the soil. In a nutshell, aerial roots are roots that are not covered by soil hence out in the air.  They can facilitate climbing and various types of support as demonstrated by ivies and creeper plants.

Contractile roots are roots that pull the plant deeper into the soil. Lily bulbs are a good example, as each bulb is pulled a little further into the soil as additional contractile roots are developed each year. When a region of stable temperature is reached, the contractile roots quit pulling. Dandelions also have contractile roots, and their presence is noticeable because the lower leaves may look like they are coming right out of the ground. In reality, the roots are pulling the stem downward. The actual mechanism of contraction involves the thickening and constriction of parenchyma cells. This causes the components of xylem to spiral into a corkscrew shape. The portion of the root that contracts may lose up to two-thirds of its length within weeks.

Tropical trees may have large buttress roots at the base of the trunk. These roots add stability to the tree and give an angular look to the lower visible portion of the trunk.

Some plants, such as dodders, broomrapes and pinedrops do not have chlorophyll. They will parasitize other plants and utilize their chlorophyll and food making abilities. The parasitic mechanism involves rootlike projections called haustoria (singular haustorium). These projections develop along stems that are in contact with the host. They will penetrate the outer tissues of the host plant, and will tap into the water and food conducting tissues (xylem and phloem). Other plants with chlorphyll, such as mistletoes, will also form haustoria in order to obtain water and dissolved minerals from host plants. They are capable of producing their own food, and thus are considered to be partially parasitic.

Mycorrhizae

Mycorrhizae are fungal roots found in many plants. These fungal associations are important for both the plant and for the fungal and are therefore considered to be mutualistic. Essentially, the fungus will have a greater capacity for absorbing phosphorus than root hairs alone. The fungus will also grow and increase the absorption of water and other nutrients. In return, the plant provides sugars and amino acids vital to the survival of the fungus. Plants with mycorrhizae generally have less root hairs than those without. Nearly all woody trees and shrubs found in forests have fungal associations in their root systems. However, it has been demonstrated that mycorrhizae are particularly susceptible to acid rain. This may have a direct impact on forest health and maintenance.  

Root Nodules

It is important to note that root nodules are not root knots, which are root swellings in response to worm invasions. Root nodules are beneficial bacterial colonies that are visible as small swellings in the root system. The bacteria aid the plant in fixing, or converting, atmospheric nitrogen in to a form that the plant can use. Root nodules are found extensively throughout the legume family. A nodule develops when a substance leaked into the soil by plant roots stimulates Rhizobium bacteria to produce another substance that coused root hairs to bend sharply. The bacterium may attach in the crook of the bend and then ‘invade’ the cell with a tubular infection thread. This thread does not penetrate the cell wall and plasma membrane. The thread, does however, grow through to the cortex which is stimulated to produce new cells that will become part of the housing for the bacterium. As the bacteria multiply and the colony grows, the nodule will swell. It is in the crook of root hairs that the nitrogen fixing takes place.

Soils

Where a plant grows and what resources are available to it is of vital importance to the life of a plant. The soil type and quality can be the difference between survival and termination for a plant. Soil is a very dynamic and complex portion of the earth’s crust. In some places the soil is only a few centimeters thick, while in others it is hundreds of feet deep. In the grand scheme of things, soil is vital not only to us as humans, but also to the existence of nearly all living organisms. Soil contributes to the plants that grow in it, just as the growing plants give back to the soil. It is a point of dynamic exchange between the living and non-living components of the earth.

The soil as what it is today, is a result of many factors coming together: climate, parent material, local topography, vegetation, living organisms and, of course, time.  All of the factors can be involved in various degrees, which is why there are many thousands of soil types. The solid bulk of soil consists of minerals and organic matter. In between the solid particles are pore spaces, which are filled with varying amounts of air and water. The pore sizes and how they are connected within the soil bulk, determine the quality of soil aeration. Aeration refers to how water and air are held within a soil sample.

In looking closer at soils, it is important to understand that there are general regions or horizons of soil development that are usually obvious in an undisturbed area. For example, if we found an undisturbed area, and dug down 3 to 6 feet (1 or 2 meters), we would likely find a soil profile (cross section of the horizons) of three integrated horizons. The composition and stage of development will obviously vary widely depending on where the soil profile is taken. The top horizon is called the A horizon or topsoil. This horizon is usually 4 to 8 inches on average, again depending on where your sample is from. The A horizon is further subdivided into a darker upper portion, called the A1 horizon and a lighter lower portion A2 horizon. The A1 horizon contains the majority of the organic material out of the three integrated horizons. The next horizon is the B horizon, or the subsoil. This is usually 1 or 2 feet deep on average. The subsoil usually contains more clay, so less pore spaces, and is lighter in color than the topsoil.  The lowest horizon is called the C horizon and it could be 4 inches to 10 feet deep or it may not be present. The C horizon is called the soil parent material and it extends down to bedrock.

Soil Parent Material

As a quick review, soil development is first predicated on the formation of parent material. Parent material accumulates via the weathering of igneous, sedimentary and metamorphic rocks. Igneous rocks are ones from volcanic activity, sedimentary rocks are formed from glacial deposits, water or wind action, and metamorphic rocks are ones formed from the other two types after undergoing extreme pressures and heat deep within the earth. These three types of rocks would require separate tutorials of their own for complete understanding. However, for now, just realize that these three rock types are the basis for the development of soil.

Climate

The climate is a globally varying feature and weighs heavily upon the weathering of rocks for soil parent material. For example, in desert or arid regions, there is little rain weathering and the soils are poorly developed. In contrast, areas of high liquid precipitation result in well developed soils. In areas where the temperature ranges widely, rocks may split and crack and cause rock breakdown. It is important to understand that the climate plays a direct role not only in the ensuring water resources to plants, but also the soil resources to plants.

Organic Composition and Living Organisms

In addition to nematode activity, the bacteria and fungi present in the soil decompose all sorts of organic material, such as leaves, dead roots, and animal carcasses.  Living organisms and their organs, such as roots, produce carbon dioxide, which combines with water and forms an acid. The acidic nature results in a higher dissolving rate for the minerals present in the soil. Bacteria, fungi, nematodes, birds, ants and other burrowing insects all serve as composters for topsoil. As these organisms alter the soil through their activities, they add to it with their wastes and the decomposition of their bodies when they die. The organic composition of soil depends on external factors, such as location and water resources. If an area is constantly wet, and oxygen is limited or lacking in the soil, the microorganism activity may be quite low and the organic content may be as high as 90%. Thus the organic content in that soil will be quite high. On average, topsoil might be composed of 25% air, 25% water, 48% minerals and 2% organic material. Furthermore, with the exception of legumes and a few other plants, almost all of the nitrogen needed by growing plants, and most of the phosphorus and sulphur come from the decomposition of organic matter. As the decomposition progresses, the acid content increases which in turn increases the breakdown of minerals which will be carried into solution into growing plants.

Local Area Topography

Topography refers to the surface features of an area. For example, if an area is steep soil that is weathered from parent material may quickly wash away or erode through the actions of wind, ice and water.  On the other hand, if an area is flat and poorly drained, water may pool in slight depressions after it rains. This lack of drainage could result in interrupting the activities of microorganisms in the soil. Thus soil development is stopped. An ideal set of surface features would be ones that allow for soil drainage without erosion.

Mineral Composition and Soil Texture

The size of the individual soil particles is called soil texture. The three general designations are sand, silt and clay. Generally, sands and gravels are composed of many small particles bound together chemically or by a matrix of cementing material.  Silt is composed of finer particles that are usually too small to see without a hand lens or microscope. Clay particles are even smaller yet. They are visible individually with a electron microscope as even a powerful light microscope will not render them visible.  Clay particles are individually called micelles. These particles are negatively charged, sheet-like and held together by chemical bonds. Because of their negative nature, they attract, trade or capture positively charged ions. The water that adheres tightly to the particle surface acts as a binding agent and a lubricant. This lends to the plastic nature of clay. Clay is also a colloid, meaning it is a suspension of particles that are larger than molecules, yet do not settle out of a fluid medium. The fluid medium is generally water.  These three particles in various balances comprise the bulk of soil.

Soils are sometimes referred to as heavy and light. Light soils have a low clay content and a high sand content; while heavy soils have a high clay content and a low sand content. Clay soils have a high water content as they do not allow water to pass through between the particles, recall that water is a binding agent in clay. Sandy, or coarse-textured, soils do not retain much water. Organic matter and clay store more plant nutrients in the form of ions than sand and silt.  The smaller clay and organic particles have a greater total surface area for the attachment of ions compared to an equal volume of sand and silt particles.

Soil Structure

Soil structure refers to aggregates. Aggregates are formed as a result of the arrangement and grouping of soil particles. Sand and gravel aggregates do not demonstrate cohesion very well. In most good agricultural soils, aggregates that stick together form readily. The structure of soil, aggregates, forms when colloidal particles clump together. The clumping is usually a result of the activities of soil organisms and temperature changes resulting in freezing and thawing. Without a coating of organic matter, the individual granules will continue to clump until they are clods.

Good agricultural soils are granular with many pore spaces, occupying between 40% and 60% of the total soil volume.  Recall how important pore spaces are to aeration and drainage. Air and water are trapped within these pores and if the pores are numerous, but small and poorly connected, as in clay soils, the restricted movement does not allow for good drainage or aeration. Furthermore, when the pores are full of water and air is not able to move in, plant roots suffer as there is not enough oxygen for root growth.  Sandy soils on the other hand have their problems as well. For one, the large pores allow gravity to drain water out of the soil as soon as the pores are filled. As the water drains, the pores fill with air, which in turns speeds up nitrogen release by microorganisms. Much of the nitrogen released is lost as plants cannot utilize nitrogen that quickly. 

Contrary to popular habit, watering can severely damage plants. Mainly, too much water leaches minerals and slows mineralizations processes with anaerobic conditions (due to the reduction or lack of free oxygen).  Overwatering can slow the release of nitrogen, which interferes with plant growth and speed up the breakdown of nitrates.

Water in the Soil

There are three forms of water occurring in soil.

1. Hygroscopic water is unavailable to plants, because it is physically bound to soil particles.
2. Gravitational water is the water that drains from pore spaces after a rain. Plant grow can be affected if drainage is poor.
3. Capillary water is the main source of water for plant needs.  This is the water held in soil pore spaces against the force of gravity.  Both structure and organic material composition of the soil enable the soil to hold water in this manner. Vegetation density and type and the location of underground water tables are the determinants of how much capillary water is available to the plant.

Following rain or irrigation, gravity drains water away from the soil. The field capacity of the soil is the amount of water remaining after such a draining.  This characteristic is mainly determined by the texture of the soil, however, structure and organic material content also play a role.  When soil is at or near field capacity plants will readily absorb water. In the process of soil drying, the film of water around each soil particle becomes thinner and more tightly bound to the particle. As the water film binds to the particle, the likelihood of the water entering the root is lessened. A point of no return is eventually reached, if water is not added to the plant. This point is called the permanent wilting point and refers to the soil moisture. It is at this point that the plant is unable to absorb water at a rate sufficient for its needs. The plant wilts and will die. The permanent wilting point in a clay soil is reached when the water content drops below 15%. In sandy soils this point can be as low as 4%. The soil water between field capacity an the permanent wilting point is called available water.

Earlier we mentioned that the breakdown of minerals lends to the acidity of the soil. It is important to understand that the pH, acidity or alkalinity, of soil affects both the soil and the plant in a variety of ways.  Soil that is not balanced, that is either too acidic or too alkaline, may be toxic to roots. However, normally an unbalanced soil pH will affect nutrient availability before they will affect the plant directly. In general, when soil is acidic the nitrogen fixing abilities of plants are affected. Alkalinity will affect the availability of minerals such as copper, iron and manganese. In areas of high precipitation, soil is usually acidic because alkaline components are leached from the topsoil. Many agricultural operations will counterbalance any soil imbalances by adding lime (compounds of calcium or magnesium) to balance acidic soil, or by adding sulphur to alkaline soils. Bacteria in the soil will convert the sulphur to sulphuric acid, which will lower the overall pH which will make the soil more acidic. Nitrogen based fertilizers may have the same effect on alkaline soils.

It is a fine balance to have, but soil quality directly influences plant quality.

Soil Mineral Components as Classified by U.S.D.A.

Mineral

Stones

Gravel

Very Coarse Sand

Coarse Sand

Medium Sand

Fine Sand

Very Fine Sand

Silt

Clay

Diameter (range in mm)

> 76 mm

76.0 mm-2.0 mm

2.0 mm-1.0 mm

1.0 mm-0.5 mm

0.5 mm-0.25 mm

0.25 mm-0.10 mm

0.10 mm-0.05 mm

0.05 mm-0.002 mm

Comments

Do not support plant growth, affect permeability and erosion rates

 

 

 

Makes soil feel gritty

 

 

Need lens or microscope to see fine particles

May absorb water, swell and later shrink causing the soil to crack as it dries

Leaves

Leaves are highly efficient solar energy converters. They capture light energy and through the process of photosynthesis they are able to trap energy in the form of sugar molecules that are constructed from carbon dioxide and water (both found in the atmosphere). All the energy required by living organisms is ultimately dependent upon photosynthesis. Leaves are able to twist on their petioles, stalks, in order to maximize sun exposure and photosynthetic activity. Leaves are covered with a thin layer of epidermal cells which permit light to the interior of the leaf, yet protect the cells from physical damage. In addition to photosynthesis leaves are involved in other vital plant functions. Respiration is a metabolic process which produces waste products. These products are deposited outside the plant when the leaves are shed. Leaves are also important to the movement of water absorbed by the roots and transported throughout the plant. The water that reaches the leaves mostly evaporates off into the atmosphere via transpiration.  Leaves are complex plant organs upon which life depends. We will look into all of these processes in more detail and see just how vital leaves are to sustaining plant and animal life.

Leaf Arrangements and Types

There are over 275,000 different kinds of plants and most of them can be distinguished from each other by their leaves alone. As mentioned in the last tutorial, leaves originate as primordial in the buds regardless of their ultimate size and shape.  When all is said and done, leaves usually consist of a stalk, the petiole and a flattened blade, the lamina, which has a network of veins also known as the vascular bundles. Some leaves have a pair of appendages called stipules at the base of their petiole. In some cases, there is no petiole or stalk, and these leaves are called sessile. Deciduous trees generally lose their leaves once a year, after the growing season. Evergreens, or conifers, usually are only functional for two to seven years.

The overall arrangement of leaves with respect to the plant stem is called phyllotaxy.  Leaves may be arranged in an alternate, opposite pattern if they are attached at the same node, or a whorled pattern if three or more are attached at a node. The leaf itself may be a simple leaf, which has an undivided blade; or a compound leaf, in which the blade is divided into leaflets in various ways. A pinnately compound leaf has leaflets in pairs along a central stalk—called the rachis.  A palmately compound leaf has all its leaflets attached at the same point on the end of the petiole. The leaflets of a pinnatley compound leaf are sometimes subdivided into even smaller leaflets which makes a bipinnately compound leaf.  The venation, or arrangement of vascular bundles, in a leaf blade or a leaflet may be either pinnate or palmate. A pinnately veined leaf has a main vein called the midrib with secondary veins branching out from it. However, in a palmately veined leaf, several veins branch out from the base of the blade—rather than from a central midrib. Monocot plants generally have leaves with parallel venation as compared with dicots, which have branching and diverging veins. The Ginkgo tree is special in that it has no midrib or other large veins. The veins fork evenly and progressively from the base of the blade out to the opposite margin of the leaf. This arrangement is called dichotomous (branching) venation.

In cross section there are three major regions to see in the inside of a leaf: epidermis, mesophyll and veins—or vascular bundles. The epidermal layer is one cell thick and covers the entire surface of the leaf. On the lower surface of the leaf blade, the epidermis is interrupted by stomata. Which will be discussed shortly. From the top, the epidermal cells look like jigsaw puzzle pieces fit tightly together.  The guard cells in the lower epidermal layer contain chloroplasts, but otherwise the epidermal cells do not have any chloroplasts and function as primary protection for the cells beneath. Most leaves have a thin covering of waxy cuticle.

Stomata

Stomata distinguish the lower epidermis from the upper epidermis. The upper epidermis is generally uninterrupted, but the lower epidermis is perforated by numerous tiny pores called stomata. The stomata (stoma singular) are very numerous and facilitate gas exchange between the interior of the leaf and the environment. Each stoma is regulated by a pair of sausage-shaped guard cells. They, as mentioned earlier, are the only cells in the epidermis with chloroplasts for photosynthesis. The photosynthetic products in the guard cells provide the energy for the functioning of the cells. The walls of the guard cells are thickened, except for the side adjacent to the pore. The cells will expand or contract with changes in the amount of water in the cells, hence the need for energy as the water is moved into and out of the guard cells. When the guard cells are full of water the stoma pore is open and when the water is evacuated the pore is closed. 

Mesophyll and Veins

The majority of photosynthesis takes place in the mesophyll between the upper and lower epidermis layers. Usually the two layers of mesophyll can be distinguished from each other. The upper region is made of cells that look like short posts in two rows. These cells are parenchyma cells and make up the palisade mesophyll tissue. It is this tissue that contains more than 80% of the chloroplasts in the leaf. The lower layer of mesophyll, the spongy mesophyll tissue, is composed of loosely arranged parenchyma cells with abundant air space. The lower layer also contains many chloroplasts and its loose structure allows for movement of air in from the stomata. For future reference, parenchyma tissue containing numerous chloroplasts is called chlorenchyma tissue. It is also found in the outer parts of cortex in the stems of herbaceous plants. However, in the leaf, the surfaces of the mesophyll that come into contact with the air are moist. The stomata will close if the internal moisture drops below a certain level in order to reduce drying inside the leaf.

The skeleton of a leaf are the veins, or vascular bundles. They are of various sizes and as described in the leaf arrangement section, are scattered throughout the leaf and are organized distinctly in different types of leaves.  The veins are surrounded by a jacket of fibers called the bundle sheath. The sugars produced in the mesophyll are transported via the veins throughout the plant—specifically in solution in the phloem.  In dicots, the veins run in all directions. In monocots, the veins are parallel and are not scattered. In addition, monocots do not have mesophyll differentiated into two layers. Instead, some will have large thin-walled buliform cells surrounding the main vein. The thin-walled cells are sensitive to water conditions and will collapse in dry conditions which causes the leaf blade to fold or roll which reduces transpiration (water loss).

Depending on the conditions where a particular plant lives, it may or may not require some specialized adaptations in order to accommodate various environmental factors: humidity, temperature, light, water, and soil conditions for example. We will look briefly at ten types of specialized leaves. I would suggest further research if you are interested in more detail.

 

1. Shade Leaves—In some plants, leaves with barely noticeable or unnoticeable modifications will occur right alongside those that are unmodified. Leaves in the shade tens to be thinner and have fewer hairs than those on the same tree exposed to direct light. In addition, they  are generally larger and have less defined mesophyll layers and reduced numbers of chloroplasts than their better lit counterparts.
2. Leaves of Arid Regions—In growing environments with extremely arid conditions, the plants will generally have thicker more leathery leaves. Their stomata are usually reduced in number and are sunken into the leaf surface in special depressions. Some may have succulent leaves or no leaves at all—where the stem takes over photosynthetic responsibilities—or they may have dense hairy coverings. In areas where the soil freezes and water resources are limited, pine trees may have modifications similar to desert plants. Including sunken stomata, thicker cuticle and a hypodermis (thick walled cells) beneath the epidermis. The compass plant is a unique example of growth set up directionally—East and West—in order to reduce moisture loss.
3. Tendrils—Many plants have modified leaf structures called tendrils that aid in climbing or supporting the plant’s weight. Tendrils are very sensitive to contact and can be readily redirected based on touch and solid contact.  Tendrils become coiled like springs and when contact with a support structure is made, the tip not only coils around it but the tip direction reverses. It needs to be noted that not all tendrils are modified leaves, tendrils of the grapevine, for example, are modified extensions of the stem tissue.
4. Spines, Thorns and Prickles—Desert plants have leaves modified as spines. Water loss is correlated to surface area, so the decrease in leaf surface area consequently decreases water loss to the outside. In plants with spines, photosynthesis is generally conducted by the stem tissue. The tissue is made of sclerenchyma cells and replaces any ‘normal’ leaf tissues. The modifications arising in the axils of leaves are stem modifications not leaf spines, but thorns. Recall, that the prickles of roses and raspberries are not leaves or stems, but outgrowths of the epidermal or cortex just beneath the prickle.
5. Storage Leaves—Succulent leaves are leaves modified to retain and store water. Water storage is permitted because of the thin-walled, non-chloroplast parenchyma cells just beneath the epidermis and to the interior of the chlorenchyma tissue. The vacuoles in the non-photosynthetic cells store the extra water resources. There are plants with succulent leaves that have a special photosynthetic process. We will look at these in a later tutorial. The fleshy leaves of onions and lily bulbs store large amounts of carbohydrates which are utilized by the plant in the next growing season.
6. Flower Pot Leaves—the leaves of some plants, such as the Dischidia plant from tropical Australasia, develop odd pouches that become the symbiotic homes of ant colonies. The colonies carry in soil particles and add nitrogenous wastes, which the leaves collect moisture through the condensation of water vapor via the stomata. The area is a rich medium for the adventitious roots that grow down into the soil contained in the pouch—hence the flower pot function of the modified leaf.
7. Window Leaves—There are at least three members of the Carpetweed family in the Kalahari desert with unique adaptations to the sandy growing environment. These plants have leaves shaped like ice cream cones. The leaves are buried in the sand, leaving the transparent dime-sized tip of the leaf exposed at the surface. The transparent surface is covered with a thick epidermis and cuticle and has virtually no stomata. This arrangement allows light nearly direct access to the mesophyll with chloroplasts inside. The plant, for the most part, is buried and away from drying winds and abrasive blowing sands. There are other examples of succulent plants with window leaves.
8. Reproductive Leaves—Walking fern leaves produce new plants at their tips. Air plants, a succulent, have little notches along their leaf margins where new plant are produced with leaves and roots of their own. The baby plants will produce even if the parent leaf is separated from the rest of the plant.
9. Floral Leaves (Bracts)—Bracts are found at the bases of flowers and are sometimes mistaken as petals. They compensate for small flowers or absent petals. The poinsettia ‘flower’ is really composed of bracts. The center cluster of tiny flowers is the main event, while the bracts do all the attracting.
10. Insect-Trapping Leaves—These plants are always attention grabbers and have intrigued folks for centuries. Plants that trap insects usually occur in swampy areas and bogs of tropical and temperate regions. Generally, the soil is lacking some vital ingredient for life and the plants utilize trapped insects and small organisms to fill the gap. The captured prizes are dissolved and absorbed by the plant. However, if insects are not available (i.e. a laboratory situation) the plants will develop if nutrients are given instead. The following four plants represent the four main mechanisms of capture.

 

Autumn Changes in Leaf Color

As leaf cells break down after the growing season is over, the leaves tend to turn some shade of brown or tan due to a reaction between leaf proteins and tannins stored in the cell vacuoles. Prior to going completely tan or brown, the leaves usually demonstrate a wide variety of colors as they go through various stages of degeneration. In the chloroplasts of mature leaves are several groups of pigments such as green chlorophylls and carotenoids including yellow carotenes and pale yellox xanthophylls. These pigments play various roles in photosynthesis. The green chlorophylls are usually found in higher concentrations and during the season of active growth they are able to mask the other pigments. As the chlorophylls break down during the fall, the other colors become apparent. The breakdown of chlorophyll is not completely understood, however, it appears to be tied to the gradual reduction in day length. Anthocyanin, a common red pigment and betacyanin a second red color may also accumulate in the cell vacuoles as fall progresses. Anthocyanins are red if the cell sap is slightly acidic and blue if the sap is more alkali (basic). Betacyanins are restricted to several plant families, including cacti and beets. While some trees demonstrate brilliant fall displays of chlorophyll breakdown, others such as birch trees have a single shade of color in their fall leaves.

Abscission

Deciduous trees and plants, the ones who lose their leaves once a year have different cycles depending on where they are at in the world. In temperate climates, the leaves generally drop in the fall in preparation for winter and new growth comes in the spring. In tropical regions, the cycle follows the cycles of wet and dry seasons. Evergreen trees do shed their leaves, however not all at once or even annually. Abscission is the process in which leaves shed; whether deciduous or evergreen.

At the base of the petiole, stalk, of each leaf there is an abscission zone. Changes that take place in this region ultimately result in the drop of leaves. Hormonal changes take place as the leaf ages and two layers of cells become differentiated. (In young leaves hormones prevent these cells from differentiating.) The cell layer closest to the stem becomes the protective layer  which is usually several cells deep and suberized, or coated with a fatty suberin substance. The other layer, the separation layer, forms on the leaf side of things. The cells swell and become like jelly. The pectins in the middle lamella of the cells in the separation layer are broken down by enzymes until an external event causes the leaf to fall: this could include the force of gravity overcoming the strength of the strands of xylem holding the leaf to the petiole, thus breaking it off at the gelatinous zone, wind, rain, animals etc. The pectin breakdown begins in response to environmental conditions such as dropping temperature, lack of adequate water, decreasing day lengths, changing light intensities, or damage to the leaf.

Importance to humans

Leaves are vital to humans. Not just for food but many medicines come from plant leaves. Tobacco products come from leaves, as do some hemp products and other textile fibers. Cocaine and aspirin are from leaves as are some insecticides. Aloe vera for the relief of burns—even x-ray burns will respond to aloe vera. Leaves are also used in floral arrangements and other products of aesthetic value. Bottom line: leaves, like stems are of great value to humans.

Fruits Flowers and Seeds

Flowers, Fruits and Seeds

Flowering plants grow in a wide variety of habitats and environments. They can go from germination of a seed to a mature plant producing new seeds in as little as a month or as long as 150 years. Plants that complete their life cycle in a single season are called annuals; while biennials take two years; and perennials may take several to many years to go from germinated seed to producing new seeds. There are two major classes of flowering plants, monocots and dicots—which have been mentioned previously in the leaves and stems tutorials. In order to keep these two classes separate in our minds, let’s take a moment and outline some of the differences between them.

Differences between monocots and dicots

Monocots:

1. One seed leaf—cotyledon
2. Flower components in threes or multiples of three
3. Leaf veins are parallel
4. No vascular or cork cambiums
5. Vascular bundles are scattered throughout the stem
6. One aperture (thin spot) in pollen grains

Dicots:

1. Two seed leaves—cotyledon
2. Flower components in four or fives or multiples of fours or fives
3. Leaf veins are branching and networked
4. Vascular cambium present, usually cork cambium present
5. Vascular bundles are arranged in a ring in the stem
6. Three apertures in pollen grains

Dicots account for slightly under three quarters of all flowering plants. Nearly all flowering trees and shrubs are dicots as well as many annual plants. Monocots include bulb producing plants, grasses, orchids and palms. They are primarily herbaceous, meaning no secondary woody growth.

Structure of Flowers

There are all sorts of flower shapes, sizes, colors and arrangements, however there are a few features that are central to all flowers regardless of their form. A flower starts as an embryonic primordium that develops into a bud and is situation as a specialized branch at the end of a stalk called the peduncle. The receptacle is a small pad-like swollen area on the very top of the peduncle. This serves as the platform for the flower parts. Whorls, which are three or more plant parts, are attached to the receptacle. The sepals are the outermost whorl and are usually green. Sometimes they are confused with leaves. They are usually three to five in number and are collectively referred to as the calyx. The second whorl of flower parts are the petals and are collectively referred to as the corolla. The corolla is usually extra-showy in order to attract pollinators. In wind-pollinated plants the corolla may be missing to maximize pollen exposure to the female flower parts. Just as the sepals in the calyx, the petals in the corolla may be fused together or separate individual units.  Nestled inside the two outer whorls are the sexual organs of the flower. The stamens entail the male structures: a semi rigid filament with a sac called the anther dangling from the tip. Pollen grains develop in the anthers (a process which we will discuss in further detail in a later tutorial). Most anthers have slits or pores on the sides to accommodate pollen release. The female organs are collectively referred to as the pistil and includes: a ‘landing pad’ at the top called the stigma, a slender stalk like style that leads down to the swollen base called the ovary. The ovary is what will develop/ripen into a fruit.

As you might have guessed, there are names for the different ways that the flower parts are arranged with respect to the ovary. The ovary is said to be superior if the calyx and corolla are attached to the receptacle at the base of the ovary. However, if the receptacle grows up and around the ovary and the calyx and corolla are attached above it, then the ovary is said to be inferior.

Inside the ovary is an egg-shaped ovule which is held in place within the ovary by means of a short stalk. The ovule is what develops into a seed. Fruits have seeds.

Some flowers are produced all alone, while others are produced in clusters called inflorescences. An inflorescence is characterized by one peduncle with many little stalks serving individual flowers. The little stalks, in this case, are called pedicels and each stalk services one flower.

Fruits

A fruit is a mature, or ripened, ovary that usually contains seeds. In contrast, a vegetable can consist of leaves (lettuce, cabbage), leaf petioles (celery), specialized leaves (onions), stems (white potato), stems and roots (beets), flowers and their peduncles (broccoli), flower buds (globe artichokes) and or other parts of the plant. A fruit is by definition just the ovary part of a flower, therefore all fruits come exclusively from flowering plants.

Fruit Regions

A fruit, ripened ovary, has three major regions that are sometimes difficult to distinguish from each other. The outer layer, sometimes referred to as the skin, is actually called the exocarp. The mesocarp is the fleshy portion which is usually eaten when consuming fruit. The endocarp is the innermost boundary around the seed. Sometimes the endocarp is hard and stony such as a peach pit that surrounds the seed. The endocarp can also be papery as in apples, where it is barely visible in cross section. All three of these regions; the exocarp, mesocarp and endocarp, are collectively called the pericarp. The pericarp can be quite thin, as is the case with dry fruits.

Some fruits have flower parts modified or fused to the ovary at maturity. Fruits are classified according to features at maturity: fleshy, dry, split exposing seeds, non-splitting, one ovary or multiple ovaries. We will go through these various classifications and see what examples fall into the various categories. 

Kinds of Fruits

Fleshy fruits—these fruits have a mesocarp that is at least partially fleshy at maturity.

Simple Fleshy Fruits—Fruits develop from a flower with a single pistil. The ovary may be simple, meaning derived from one modified leaf called a carpel, or compound. The ovary also may be superior or inferior and may develop into a fruit with or without other flower parts integrated.

Drupe— Drupes are simple fleshy fruits with one seed encased in a stony pit. Usually the ovary is a superior ovary with one ovule. The stone fruits—cherries, peaches, olives, apricots and almonds—are examples. Although, not readily recognized as a fleshy fruit, coconuts are drupes. The husk is the mesocarp and exocarp which is generally removed before making it to the market. The pit with the watery seed endosperm is what we see piled up at the store.

Berry—These develop from a compound ovary and usually contain multipleseeds. It is difficult to distinguish the three regions. This group is broken down further into three types of berries:

True berries are fruits with a thin skin and a pericarp. They are generally soft at maturity and usually have multiple seeds although dates and avocados are notable exceptions. Some berries have incorporated flower parts which can be seen in remnant as scars. Examples are tomatoes, grapes, peppers, blueberries, cranberries, bananas and eggplants. Note that botanically speaking raspberries, strawberries and blackberries are not berries.

Pepos are berry fruits with thick rinds. They have multiple seeds and include pumpkins, watermelons, cantaloupes and squashes.

Finally, the hesperidium is a leathery skinned berry that contains oils. Saclike outgrowths of the inner ovary wall become filled with juice as the ovary matures. All members of the Citrus Family produce hesperidium fruits.

Pome—The majority of the flesh in pomes comes from the swollen receptacle that grows up and around the ovary (inferior ovary). The seeds are encased by a leathery or papery endocarp. Apples are good examples and the apple core is the ovary with seeds, and the rest if overgrowth of receptacle. (Sometimes these fruits that are derived from more than the ovary are called accessory fruits.) Examples of pomes include: apples, pears and quinces.

Aggregate fruit—These fruits come from a single flower with multiple pistils. The individual pistils start as tiny drupes or other fruitlets, but at maturity they cluster on a single receptacle. Examples are strawberries, raspberries and blackberries.

Multiple fruit—Several of many flowers in a single inflorescence will develop into a multiple fruit. The flowers develop separately into fruitlets on their own receptacles, but at maturity they will cluster together and develop into a larger single fruit. Pineapples and figs are good examples, although the fig develops from a unique “outside in” inflorescence.

Dry fruits—Mesocarp is dry at maturity.

Dry fruits that split at maturity: distinguished by the way in which they split.

Follicle—Splits along one side, or one seam. Ex: larkspur, milkweed Legume—Splits along two sides or seams. Ex: Peas, beans, kudzu, peanuts, carob

Silique—split along two sides or seams, the difference from legumes is that the seeds are carried on a central partition which is exposed upon splitting. Ex: Mustard Family, including broccoli, cabbage, radish, watercress

Capsule—Most common of splitting dry fruit. Composed of two carpels and split in a variety of ways: along carpel partitions, through carpel cavities, pores or via a cap that pops off to release seeds. Ex: irises, poppies, orchids, violets and snapdragons.

Dry fruits that do not split at maturity: the single seed is more or less united with the pericarp.

Achene—seed is attached to the pericarp (husk in this case) only at the bottom, and can be separated easily. Ex: sunflower seeds (husk, plus edible seed constitutes the achene), buttercup and buckwheat.

Nut—the pericarp of nuts are generally harder than the achenes, although they are otherwise quite similar in structure. Nuts develop with a cup or cluster or bracts at their base. Ex: hazelnuts, hickory nuts and chestnuts. Note that botanically speaking most things called ‘nuts’ are not nuts such as peanuts (legume), coconuts, almonds, walnuts, pecans (all drupes) Brazil nuts (capsule) and pistachios (drupes). Yet another misnomer commonly accepted.

Grain—Grains are all of the Grass Family and feature a pericarp that cannot be separated from the seed. Ex. Corn, wheat, barley, rice, and oats. The grains are also called caryopses.

Samara—pericarp extends as a wing or membrane which aids in dispersal. Usually samaras are produced in pairs, although elms and ash trees produce them singly. These are the ‘helicopters’ that I am certain we all have played with at one point or another. Ex: maple trees

Schizocarp—schizocarps are the twin fruits, as at maturity the fruit dries out and breaks into two one-seeded segments. Ex: carrots, dill, parsley and anise.

Fruit and Seed Dispersal

There are a variety of methods that will get seeds from the ovary to a fertile spot to begin germinating and growing. Not all methods will work for every plant and some plants are very method specific.

Dispersal by Wind

The wind can carry light seeds for miles and most seeds and fruits relying on wind dispersal have specialized adaptations. The samaras with their wings and membranes are highly ideal fruits for wind dispersal. Some fruits are too large to be carried in the air, but can be rolled along by the wind. Cottony or woolly hair type adaptations as in the Willow Family, enable better transfer of seeds via the wind. Tumbleweed plants break off and blow along in the wind, all the while dispersing seeds as it bumps along.

Dispersal by Animals

There are so many adaptations for the dispersal of seeds by animals that it would take a volume or two to discuss them all. Birds can carry seeds in the mud that they pick up on their feet. Seeds pass through digestive tracts and are deposited randomly by animals. Ants carry collect and carry seeds. Some seeds will not germinate unless they have passed through the acidic environment of a digestive tract. Fur and feathers can trap seeds and some seeds have burrowing type screws or hooks to ensure getting caught on something and carried along.

Dispersal by Water

Some fruits contain trapped air and are thus adapted to dispersal by water. Some pericarps are thick and spongy enough to absorb water slowly and will thus protect the tiny embryo held within. Saltwater dispersed plants generally have these type pericarps and survival requires washing up on a beach somewhere before the saltwater reaches the inside of the seed.

Other Dispersal Mechanisms

Some fruits mechanically eject fruits, some at a violent velocity. Humans are another method of dispersal whether intentionally or not. Most countries have regulations with regards to bringing fruits and seeds into the country that may harm native species and cultivated crops.

Seeds

We have been talking about seeds but haven’t really mentioned what a seed is made of and how it becomes a mature plant.

Seed Structure

First of all dicot and monocot seeds are different. Recall that a dicot has two seed leaves in the plant embryo, while a monocot has one seed leaf. These seed leaves are called cotyledons. The cotyledons are the food storage organs and will also serve as the first leaves of the growing plant. If you look at a kidney bean—a dicot—you will notice a small white scar on the inner concave edge of the seed, which is called the hilum. The hilum is where the ovule was attached to the ovary wall—analogous to a belly button in a human. The cotyledons are attached to a tiny embryo plant contains the undeveloped leaves and meristematic tissue at one end.  The embryo shoot is called the plumule and the cotyledons are attached just beneath the plumule. Above the cotyledons is the stem portion of the axis, and is called the epicotyl. The portion below the cotyledon attachment is called the hypocotyls. The plant embryo is tiny and it will be difficult to see where the stem ends and the root begins. The embryonic root is called the radicle.  In some monocots, the radicle and plumule are enclosed for added protection. The tubular sheathing structures are called the coleoptile for the plumule and the coleorhiza for the radicle. At some point the embryonic shoot and root will overtake the protective structures, and the sheathing will cease growing.

Germination

Germination is the start of the growing process for a plant embryo. There are a host of internal and external factors that have to be in place in order for germination to occur. Most seeds require some period of dormancy before they will germinate. This can come about by either physiological or mechanical methods or both. Some seeds can break dormancy by scarification which involves artificially cracking the seed coat. In nature, seeds may require a period of freezing and thawing in order to crack the seed coat, or passage through an acidic digestive tract. In most woody plants in temperate regions, a cold period is required before growth will commence. Some plants will absorb vast amounts of water which instigates the activity of enzymes before germination begins. When the seed is water logged oxygen may be reduced and anaerobic respiration may occur until the seed coat cracks and oxygen is admitted to the embryo. In most cases, temperature is vital to germination. Light roles in germination vary depending on the kind of plants involved. Some lettuce seeds, for example, will not germinate in the dark, whereas some seeds such as the California poppy will only germinate in the dark.

Water in Plants

The movement of molecules, specifically water and any solutes, is vital to understand in light of plant processes.  This will be more or less a quick review of several guiding principles of water motion in reference to plants.

Molecular Movement

1. Diffusion—Diffusion is the net movement of molecules or ions from an area of higher concentration to an area of lower concentration. Think of it as a rebalancing. The molecules or ions are said to be moving along a diffusion gradient. If molecules or ions moving in the opposite direction are said to be moving against a diffusion gradient. Diffusion will continue until a state of equilibrium is reached. Rates of diffusion are affected by temperature and the density of the involved molecules among other things.  In the leaves, water diffuses out via the stomata into the atmosphere.
2. Osmosis—Osmosis in plant cells is basically the diffusion of molecules through a semipermeable, or differentially permeable, membrane from a region of higher solute concentration to a region of lower solute concentration.  The application of pressure can prevent osmosis from occurring. Plant physiologists like to describe osmosis more precisely in terms of potentials. Osmotic potential is the minimum pressure required to prevent fluid from moving as a result of osmosis. Fluid will enter the cell via osmosis until the osmotic potential is balanced by the cell wall resistance to expansion. Any water gained by osmosis may help keep a plant cell rigid or turgid. The turgor pressure that develops against the cell walls as a result of water entering the cell’s vacuole. This pressure is also referred to as the pressure potential. The crunch when you bite into a celery stick is as a result of the violation of the cell’s turgor pressure. The osmotic potential and pressure potential combined make up the water potential of a plant cell. If there are two cells next to each other of different water potentials, water will move from the cell with the higher water potential to the cell with the lower water potential.  Water enters plant cells from the environment via osmosis. Water moves because the overall water potential in the soil is higher than the water potential in the roots and plant parts. If the soil is desiccated then there will be no net movement into the plant cells and the plant will die.
3. Plasmolysis—Plasmolysis is the loss of water via osmosis and accompanying shrinkage of the protoplasm away from the cell wall. When this occurs, the cell is said to be plasmolyzed. This process can be reversed if the cell is placed in fresh water and the cell is allowed to regain its turgor pressure. However, as with anything living, there is a point of no return and permanent or fatal damage to the cell can occur.
4. Imbibition—Imbibition is the swelling of tissues, alive or dead, to several times their original volume. This is a result of the electrical charges on materials in suspension (colloidal) such as minerals, cellulose and starches attracting highly polar water molecules which then move into the cell. This swelling process is the initial step in the germination of seeds.
5. Active Transport—Active transport is the energy assisted movement of substances against a diffusion or electrical gradient. This process requires enzymes and a ‘proton-pump’ embedded in the plasma membrane. The pumps are energized by ATP molecules—a cellular energy storage molecule.

Water and its Movement Through the Plant

Roughly 90% of the water that enters a plant is lost via transpiration. Transpiration is the loss of water vapor through the leaves, just to refresh you. In addition, less than 5% of the water entering the plant is lost through the cuticle. Water is vital to plant life, not just for turgor pressure reasons, but much of the cellular activities occur in the presence of water molecules and the internal temperature of the plant is regulated by water. Recall that the xylem pathways go from the smallest part of the youngest roots all the way up the plant and out to the tip of the smallest and newest leaf. This internal plumbing system, paired with phloem and its nutrient transportation system, maintains the water needs and resources in the plant. The issue of the processes by which water is raised through columns—of considerable height at times—has been studied and debated for years in botany circles. The end result is the cohesion-tension theory.

The Cohesion-Tension Theory

Polar water molecules adhere to the walls of xylem tracheids and vessels and cohere to each other which allows an overall tension and form ‘columns’ of water in the plant. The columns of water move from root to shoot and the water content of the soil supplies the ‘columns’ with water that enters the roots via osmosis. The difference between the water potentials of the soil and the air around the stomata are capable of producing enough force to transport water through the plant—from bottom to top and thus goes the cycle.

Transport of Food Substances (Organic Solutes) in Solution

Phloem is responsible for transporting food substance throughout the plant. As with water movement in plants, the movement of organic solutes in plants has been studied and debated for years. The currently accepted hypothesis is the pressure-flow hypothesis for the translocation of solutes.

The Pressure-Flow Hypothesis

This is essentially a source and sink hypothesis. Food substances that are in solution flow from a source, which is generally where water is taken up by osmosis (roots; food storage tissues, such as root cortex or rhizomes; and food producing tissues such as mesophyll in leaves), and the food substances are then given up at a destination or a sink where the food resources will be utilized in growth. The idea is that the organic solutes are moved along concentration gradients existing between sources and sinks.

At the source, phloem-loading occurs and sugars are moved by active transport into the sieve tubes of the smallest veins. The overall water potential in the sieve tube drops and then water enters the phloem cells via osmosis. The resulting turgor pressure from the movement of the water is enough to drive the solution through the sieve network to the sink . The sugar is unloaded at the sink via active transport and water then exits the ends of the sieve tubes. The pressure drops as the water exits, which causes a mass flow from the higher pressure at the source to the now lowered pressure at the sink. Much of the water that exits the sieve tubes will diffuse back into the xylem where it can be recirculated, transpired once it reaches the source. In a nutshell the mass flow is caused by drops in turgor pressure at the sink as the sugar molecules are removed. This generates the next push of materials toward the sink.

Regulation of Transpiration

It is the responsibility of the stomata to regulate transpiration and gas exchange via the actions of the guard cells. The pores of the stomata are closed when turgor pressure in the guard cells is low, and they are open when turgor pressure is high. Changes occur when light intensity, carbon dioxide concentration or water concentration change. The guard cells of the stomata use energy to take up potassium ions from adjacent epidermal cells.The uptake opens the stomata because water potential in the stomata drops and water moves into the guard cells and increases turgor pressure. When the potassium ions are released, the water then leaves the cells as the water potential shifts again. There is evidence that stomata will close with water stresses, but there also seems to be some indication that hormones are involved cause a loss of potassium ions from the guard cells and thus a pore closure.

Most plants keep their stomata open during the day and close them at night. However, there are plants that do the opposite and open their stomata during the night when overall water stress is lower. These plants have a specialized form of photosynthesis called CAM photosynthesis since the standard source of carbon dioxide is shut off as the stomata are closed during daylight hours. There are desert plants that are able to store carbon dioxide in their vacuoles in the form of organic acids that are converted back into carbon dioxide during the daytime for standard photosynthetic processes. As mentioned earlier, there are also adaptations such as sunken stomata which reduce the loss of water. Submerged or partially submerged plants generally do not have stomata on the underwater portions of their leaves.

High humidity will reduce transpiration rates while low humidity accelerates the process. There is a direct correlation between temperature and water movement out of the leaf. At high temperatures the rate of transpiration increases, while the opposite occurs at lower temperatures.

Mineral Requirements for Growth

Many external factors will affect growth rates and quality. The minerals available in the local soil is one such source of external input. Essential plant elements include: carbon, hydrogen, oxygen, phosphorus, potassium, nitrogen, sulphur, calcium, iron, magnesium sodium, chlorine, copper, manganese, cobalt, zinc, molybdenum, and boron to name the most common. Other minerals are required but they vary greatly from plant to plant. For example some algae need large amounts of iodine and silicon, while some loco weed species need selenium—which is poisonous to cattle on its own.

Macronutrients and Micronutrients

When any of these elements are lacking in the soil and the deficiencies are not compensated for by adding fertilizer compounds of compost the plant will demonstrate characteristic symptoms of mineral deficiencies. Most commercial fertilizers are some ratio of nitrogen, phosphorus and potassium and thus are able to compensate for a wide variety of issues.  As an example of uses for the essential element in plants we will look at a few elements and how they are utilized:

As you can see by scanning through the list, all of these elements are involved to one degree or another in vital life sustaining processes!

Plant Metabolism

Introduction

Plants are responsible for incredible feats of molecular transformation. The processes are always being studied, but there are a few basic things that are well understood at this point in history. We will be looking at photosynthesis and respiration in some detail, as always, if you have additional questions please post them on the forum.

Photosynthesis

Photosynthesis is the process by which light energy is captured, converted and stored in simple sugar molecule. This process occurs in chloroplasts and other parts of green organisms.  It is a backbone process, in the sense that all life on earth depends on it’s functioning. The following equation sums up the process:

6CO₂ + 12 H₂O + light energy -> C₆H₁₂O₆ + 6O₂ +6H₂O

carbon        water                              glucose     oxygen   water

As you see from the equation, this process is vital to us as humans, because it transforms carbon dioxide into oxygen—which we enjoy with every breath!

Carbon Dioxide (CO₂)

The earth’s atmosphere contains approximately 79% nitrogen, 20% oxygen and the remaining 1% is a mixture of less common gases—including 0.039% carbon dioxide. Carbon dioxide in the atmosphere reaches plant mesophyll via the stomata. The carbon dioxide dissolves on the thin film of water that covers the outside of cells. The carbon dioxide then diffuses through the cell wall into the cytoplasm in order to reach the chloroplasts. The oceans hold a large reservoir of carbon dioxide, which keeps the atmospheric levels essentially constant. Although there are some indicators that the atmospheric levels of CO₂ are rising and adding to the global warming issue. That is a whole other topic though.

Water

Water is plentiful on earth, however, it may or may not be plentiful at the location of each individual plant. Therefore, plants will close their stomata, if need be, which reduces the CO₂ supply to the mesophyll. Not even 1% of the water that is absorbed by plants is used in photosynthesis, the remainder is either transpired or incorporated into protoplasm, vacuoles or other cell materials. The water utilized in photosynthesis is the source of oxygen released as a photosynthetic byproduct.

Light

Light has a dual nature, in that it exhibits properties of both waves and particles. The energy from the sun comes to earth in various wavelengths, the longest being radio waves and the shortest are gamma rays.  Approximately 40% of the radiant energy the earth receives from the sun is visible light. Visible light ranges from red, 780 nanometers to violet, 390 nanometers. The violet to blue and red to orange ranges are the most often used in photosynthesis. Most light in the green range is reflected. Of the visible light that reaches a leaf, approximately 80% is absorbed. Light intensity varies widely. Time of day, temperature, season of year, altitude, latitude and other atmospheric conditions all play roles in the intensity of the radiant energy that will reach the earth and it’s organisms. High intensity light isn’t necessarily a beneficial thing for plants. In high intensity light, photorespiration may occur, which is a type of respiration that uses oxygen and releases carbon dioxide but differs from standard aerobic respiration in the pathways that it utilizes.

Chlorophyll

A few things to know about chlorophyll before we get into the nitty gritty of photosynthesis and respiration. There are more than one type of chlorophyll, however, they all have one atom of magnesium in the center. In some ways the chlorophyll is quite analogous to the heme structure in hemoglobin (the iron containing pigment that carries oxygen in blood). Chlorophyll has a long lipid tail that anchors the molecule in the lipid layers of the thylakoid membranes—recall that thylakoids are coin-like discs in stacks within the stroma of the chloroplasts.  The chloroplasts of most plants contain two types of chlorophyll imbedded in the thylakoid membranes. The formula for bluish-green chlorophyll a is C₅₅H₇₂MgN₄O₅ and the formula for yellow-green chlorophyll b is C₅₅H₇₀MgN₄O₆. In general, most a chloroplast has about three times as much chlorophyll a than b. The main role of chlorophyll b is to broaden the spectrum of light available for photosynthesis: chlorophyll b absorbs light energy and transfers the energy to a chlorophyll a molecule. Other pigments are contained in chlorophyll c, d, and e and take the place for chlorophyll b in some cases. Note that all the chlorophyll molecules are related to each other and differ only slightly in molecular structure.  Light-harvesting complexes contain 250 to 400 pigment molecules and are referred to as a photosynthetic unit. There are countless numbers of these units spread throughout the grana of a chloroplast. In the chloroplasts of green plants, two types of these harvesting units operate together in order to bring about the first phase of photosynthesis.

The photosynthetic process occurs in two successive processes: the light reactions and the carbon-fixing reactions.

1. The light reactions

The light reactions involve light striking the chlorophyll molecules embedded in the thylakoids of chloroplasts. The subsequent reaction results in the conversion of some light energy to chemical energy. In the light reactions, water molecules are split apart into hydrogen ions and electrons and oxygen gas is released. In addition, ATP (adenosine triphosphate) molecules are created and the hydrogen ions derived from the water molecules are involved in “loading” NADP which carries the hydrogen as NADPH. NADPH is integral in providing the hydrogen ions used in the second series of major photosynthetic reactions: the carbon-fixing reactions.

1. The carbon-fixing reactions

The carbon-fixing reactions used to be called the dark reactions because light does not play a direct role in their functioning. The reactions take place in series outside of the grana in the stroma of the chloroplast. These reactions only occur if the end products of the light reactions are available for use. Depending on the plant involved, the carbon-fixing reactions may develop or progress in different ways. The most common type of carbon-fixing reactions in plants is the process called the Calvin cycle. In the Calvin cycle, carbon dioxide from the atmosphere is combined with a 5-carbon sugar—RuBP, or ribulose bisphosphate. The combined molecules are converted via several steps into a 6-carbon sugar, such as glucose. The ATP and NADPH molecules from the light reactions provide the energy and resources for the reactions. Some of the sugars produced are further combined into polysaccharides (strings of simple sugars) or are stored as starch within the plant. There are other variations, including the 4-carbon pathway which is usually found in desert plants (C₄ plants).

Before getting into respiration let’s take a closer look at what happens in both the light reactions and the carbon-fixing reactions.

Nitty-gritty of Light Reactions

Einstein called the discrete particles of light photons.  Particles (photons) and waves are both currently accepted aspects of light. The quantum (energy) of photons is different depending on what kind of light they are in. Longer wavelength light has lower photon energies, while light with shorter wavelengths have higher photon energies. As mentioned earlier, every pigment color has a different distinctive pattern of light absorption—called the pigment’s absorption spectrum. The energy levels of some of the pigment’s electrons are raised when the pigment absorbs light. If energy is emitted immediately upon absorption, the effect is called fluorescence. The red part of light does this characteristically, as demonstrated when chlorophyll is placed in light it will appear red. If the absorbed energy is emitted as light after a delay, then the effect is called phosphorescence. The energy may be converted to heat or stored, as in photosynthesis within chemical bonds.

Oxidation-reduction reactions

OIL RIG, a cute little mnemonic device to remember that oxidation is loss and reduction is gain. Perhaps better put, oxidation results in the net loss of an electron or electrons, while reduction results in a net gain of an electron or electrons. The electrons come from  compounds within the process or donated in from previous processes. These types of chemical reactions are found scattered throughout the processes within photosynthesis and respiration.

Photosystems

The two types of photosynthetic units in most chloroplasts are what constitute photosystem I and photosystem II.

1. Photosystem I contains photosynthetic units with 200 or more molecules of chlorophyll a, small amounts of chlorophyll b, protein saddled carotenoid pigment, and a pair of specialized reaction-center molecules of chlorophyll called P₇₀₀. All pigments in a photosystem are capable of absorbing photons, however, only the reaction-center molecules can really utilize the light energy.  The other pigments aren’t worthless in the system, as they act sort of like an antenna in gathering and passing light energy along to the reaction-center. Iron-sulphur complexed proteins initially receive electrons from P₇₀₀ and serve as primary electron acceptors for the unit.
2. Photosystem II contains chlorophyll a, protein saddled beta-carotene, a small amount of chlorophyll b and special pair of reaction-center molecules of chlorophyll a otherwise called, P₆₈₀. The photosystem has a primary electron acceptor called pheophytin or Pheo.

For the record, the 680 and 700 in the names of the reaction-center molecules stands for the peaks in the absorption spectra of light waves of 680 nm and 700 nm.

Photolysis

A photon of light strikes the photosystem II reaction-center, the P₆₈₀ molecule to be exact near the inner surface of a thylakoid membrane. The received light energy excites an electron (boosts it to a higher energy level) which is an unstable reaction and thus most of the energy is lost to heat. Up to four photons at a time can strike the P680 molecule, however, it can only accept one electron at a time.  The molecule of pheophytin picks up the excited electron, which then crosses the thylakoid membrane and is passed along to another acceptor called plastoquinone or Pq near the outside surface of the thylakoid membrane. Protein Z extracts electrons from water and replaces the ones lost by the P680 molecule. Protein Z contains manganese which is required in order to split water molecules. Simultaneously, as two water molecules are split and molecule of oxygen and four protons are produced. This enzyme-mediated water splitting process is called photolysis.

Photophosphorylation

Pq, the acceptor molecule, releases the excited electron into the care of an electron transport system that is sort of like a downhill bucket brigade. The transport system moves electrons extracted from water temporarily to a high-energy storage molecule called nicotinamide adenine dinucleotide phosphate (NADP⁺). NADP⁺ is an electron acceptor for photosystem. The transport chain is essentially iron-containing pigments, cytochromes, a copper containing protein called plastocyanin and other electron transferring molecules. As electrons are passed through the chain and protons are being shuffled through a coupling factor, ATP molecules are assembled from ADP and phosphate in a process called photophosphorylation.

A similar series of events occurs in photosystem I. After a photon of light strikes a P₇₀₀ molecule, the resulting excited electron is passed along to an iron-sulphur molecule Fe-S which in turn passes it to another acceptor molecule ferrodoxin, (Fd). The ferrodoxin molecule releases the electron to a carrier molecule called flavin adenine dinucleotide (FAD) and then eventually on to NADP⁺. A reduction occurs and NADP+ becomes NADPH.  Electrons from photosystem II and the activities of the electron transport system replace any electrons removed from the P₇₀₀ molecule. Because the electrons move in one direction, the movement of electrons from water to photosystem II to photosystem I to NADP⁺ are said to be part of noncyclic electron flow. Any ATP that is produced are designated noncyclic phosphorylation.

It should be noted that photosystem I can operate independently of photosystem II. When this occurs, the electrons boosted from P₇₀₀ reaction-center molecules (photosystem I) are passed through an intermediary acceptor molecule called P₄₃₀ and then on to the electron transport chain. This is rather then to the ferrodoxin and NADP⁺. After being passed through the electron transport chain, the electron is dumped back into the reaction-center of photosystem I. This process demonstrates cyclic electron flow and any ATP generated by cyclic electron flow is termed cyclic phosphorylation. Note, that no water molecules are split and no NADPH or oxygen is produced.

Chemiosmosis

Earlier we mentioned in passing a coupling factor. The enzyme necessary for mediation of the splitting of water molecules is on the inside of the thylakoid membrane. As a result of this, a proton gradient forms across the membrane and the movement of these protons is thought to be a source of energy for generating ATP. The motion is thought to be similar to molecular movement during osmosis and has hence been termed chemiosmosis. As the protons move across the membrane, they are assisted in crossing by protein channels called ATPase or coupling factor.  Because of the proton movement, ADP and phosphate combine which produces ATP.

Nitty-gritty of Carbon-Fixing Reactions

Both ATP and NADPH are important products of the light reactions and both of them play roles in the synthesis of carbohydrates from atmospheric carbon dioxide. Although the carbon-fixing reactions do not require daylight, they generally are conducted during daylight hours as there is some indication that some of the enzymes required for the processes in carbon-fixing may require some level of light.  These reactions take place in the stroma of the chloroplast.

Three known mechanisms of converting carbon dioxide to sugar.

1. The Calvin Cycle or the 3-carbon pathway—The Calvin cycle is the most common of the three mechanisms and has four main results:
   1. With the assistance of the enzyme rubisco (RuBP carboxylase), six molecules of atmospheric carbon dioxide combine with six molecules of ribulose 1, 5-bisphosphate (RuBP)
   2. The result of the first step is six unstable 6-carbon complexes, which immediately split into two 3-carbon molecules of 3-phosphoglyceric acid or 3PGA. This is the first stable compound in photosynthesis.
   3. NADPH and ATP from the light-reactions, supply the energy required to convert the 3PGA to 12 molecules of glyceraldehydes 3-phosphate (GA3P), which is a 3-carbon sugar phosphate complex.
   4. Finally, of the 12 molecules formed; 10 are restructured into six 5-carbon molecules of RuBP—the sugar that the process started with.

 The sugars produced can either add to an increase in the sugar content (carbohydrate content) of the plant or they can be used in pathways that lead to the production of lipids and amino acids.

1. 4-Carbon Pathway—C₄ plants: These plants use a 4-carbon molecule called oxaloacetic acid in place of the 3-carbon 3-phosphoglyceric acid used in step two of the Calvin cycle. Oxaloacetic acid is produced from a 3-carbon compound PEP—phosphoenolpyruvate and carbon dioxide. This process is enzyme mediated and occurs in the mesophyll cells of the leaf. Some species will convert the resulting oxaloacetic acid to aspartic, malic or other acids.

Note that the acids do not substitute for 3PGA. The 4-carbon acids migrate to the bundle sheaths surrounding the vascular bundles, where they are further converted to pyruvic acid and carbon dioxide. In returning to the mesophyll cells and interacting with ATP molecules, the pyruvic acids molecules are able to produce additional PEP. In the bundle sheath cells, the carbon dioxide formed converts into 3PGA and other molecules, by combining with RuBP. The other molecules formed are similar to the other ones formed in the Calvin cycle. The C₄ cycle furnishes carbon dioxide to the Calvin cycle in a more roundabout way than the C₃ pathway, but there is an advantage to this extra pathway. The extra pathway greatly reduces photorespiration in C₄ plants, and this is a good thing because photorespiration is in direct competition with the Calvin cycle and even takes place in the light while the Calvin cycle is functioning. During photorespiration, RuBP reacts with oxygen to create carbon dioxide; in contrast, during photosynthesis RuBP and carbon dioxide are used to form carbohydrates. C₄ plants are able to pick up carbon dioxide in very low concentrations via the mesophyll cells. The Calvin cycle occurs in the bundle sheath where carbon dioxide is readily available. Because of the location, the enzyme rubisco will be in a prime spot to catalyze the reaction between RuBP and carbon dioxide, rather than oxygen. As a result C₄ plants have photosynthetic rates that are two to three times higher than C₃ plants. There are a few other characteristic features of C₄ plants worth noting:

1. C₄ plants have two types of chloroplasts and an alternate pathway for using carbon dioxide. C₃ plants only have one type of chloroplast and one pathway. Chloroplasts with starch grains and are large with very little grana, and sometimes none, in the bundle sheath cells. In the mesophyll, the small, but numerous chloroplasts have no starch grains and contain highly developed grana.
2. PEP carboxylase is found in high concentration in the mesophyll cells which permits ready conversion of carbon dioxide to carbohydrate at lower concentrations than does rubisco (in bundle sheath cells) of the Calvin cycle.
3. The temperature ranges for C₄ plants are much higher than C₃ plants which enables C₄ plants to live well in conditions that would likely kill a C₃ plant.

1. CAM Photosynthesis—Crassulacean acid metabolism is a modified photosynthetic system that is somewhat similar to C₄ photosynthesis in that 4-carbon compounds are produced during the carbon-fixing reactions. CAM plants accumulate malic acid in their chlorenchyma tissues at night, which is converted back to carbon dioxide during the day. In the daytime, malic acid diffuses out of the vacuoles and is converted to carbon dioxide for use in the Calvin cycle. PEP carboxylase is responsible for converting the carbon dioxide plus PEP to malic acid at night. This modification allows for a greater amount of carbon dioxide to be converted to carbohydrate during the day than would be otherwise converted given the conditions CAM plants generally grow in. CAM plants generally close their stomata during the day in order to reduce water loss. There are more than 20 families that contain CAM plants, including cacti, stonecrops, orchids, bromeliads and many succulents growing in regions of high light intensity. There are some succulents that do not have CAM photosynthetic capabilities, as well as non-succulents that do have the ability. 

There are great resources available that go into even greater detail on these reactions. If you are interested in these titles, please don’t be afraid to ask on the forum for direction.

Respiration

Respiration is the group of processes that utilizes the energy that is stored through the photosynthetic processes. The steps in respiration are small enzyme-mediated steps tha release tiny amounts of immediately available energy, the energy released is usually stored in ATP molecules which allow for even more efficient use of an organism’s energy. Respiration occurs in the mitochondria and cytoplasm of cells.

There are several forms of respiration: aerobic—which requires oxygen, anaerobic—which occurs in the absence of oxygen, and fermentation—which also occurs in the absence of oxygen.

Aerobic respiration is the most common form of respiration and cannot be completed without oxygen gas. The controlled release of energy is the main event in aerobic respiration.

Certain types of bacteria and other organisms without oxygen gas carry on anaerobic respiration and fermentation. Compared to aerobic respiration the amount of energy released is quite small. The main difference between aerobic respiration and fermentation is in the way hydrogen is released and combined with other substances. Two very common forms of fermentation are summed up by the following equations:

C₆H₁₂O₆ -> (with enzymes)-> 2C₂H₅OH + 2CO₂ + energy (ATP)

glucose                                       ethyl alcohol   carbon dioxide

C₆H₁₂O₆ -> (with enzymes) -> 2C₃H₆O₃ + energy (ATP)

glucose                                         lactic acid

Note the first equation is particularly valuable to the brewing industry.

Major Steps in Respiration:

Glycolysis—the first step does not require oxygen gas (O₂) and takes place in the cytoplasm. The glycolytic phase is subdivided into three main steps and several smaller ones. Each step is mediated by an enzyme. A small amount of energy is released and hydrogen atoms are removed from compounds derived from glucose. The main gist of the steps are:

A.     the glucose molecules goes through several steps and becomes a double phosphorylated fructose molecule.

B.     The 6-carbon fructose molecule is split into two 3-carbon fragments, each with a phosphate, GA3P

C.     Hydorgen, energy and water are removed from the GA3P molecules leaving pyruvic acid.

Glycolysis requires two molecules of ATP to get the process started. In the processes, four ATP molecules are created, with a net gain of 2 ATP molecules at the end of glycolysis. The hydrogen ions and electrons that are released are held by an acceptor molecule called NAD—nicotinamide adenine dinucleotide.  The overall end products of gylcolysis is: 2-ATP molecules, 2-NADH molecules, and pyruvic acid.

The next step depends on the kind of respiration involved—aerobic, true anaerobic or fermentation.

Aerobic Respiration (with oxygen present)

1. The Krebs Cycle (or citric acid cycle)—The Krebs cycle takes place in the fluid matrix of the cristae compartments of the mitochondria. It is called the citric acid cycle because of all the intermediate acids in the cycle. The pyruvic acid product of glycolysis is restructured, some of the CO₂ is lost and becomes acetyl CoA which then dumps into the Krebs cycle. During the restructuring of pyruvic acid, a molecule of NADH is produced.  The Krebs cycle removes energy, CO₂ and hydrogen from acetyl CoA via enzyme mediated reactions of organic acids.

The hydrogen removed during the Krebs cycle is picked up by FAD and NAD acceptor molecules. The end result of the metabolizing of two acetyl CoA molecules in the Krebs cycle is: 2-ATP molecule, oxaloacetic acid (to further drive the cycle), 6-NADH₂ molecules, 2-FADH₂ molecules and 2CO₂ molecules.

The NAD and FAD molecules and the hydrogens that they carry will be dumped into the next step in respiration in order to extract the energy from the molecules.

1. The Electron Transport Chain—The electron transport chain is a bit like a bucket brigade in that the chain passes the molecules along until the job is done. Energy is released as the hydrogen and electrons from the NAD⁺ and FAD⁺ carrier molecules is dumped into the system. When the electrons reach the end of the chain they pick up an oxygen and water is released. ATP is produced by oxidative phosphorylation during the action of the electron transport chain. This occurs essentially like chemiosmosis.

As a whole, from glycolysis to finish aerobic respiration yields the following:

Glycolysis:
  4-molecules of ATP
 +2-molecules of NADH—which yields 4-ATP in the ETS
 ------------
  8-molecules of ATP net
  -2 ATP molecules to start the glycolysis process
 -------------
  6- ATP molecules
 Conversion of pyruvic acid to acetyl CoA:
  2-molecules of NADH—yields 6 ATP in the ETS

Krebs Cycle:
  2-molecules of ATP
 2-molecules of FADH₂—which yields 4-ATP in the ETS
 6-molecules of NADH₂—which yields 18-ATP in the ETS
 ------------
Total ATP yield: 36

The 36 resulting ATP molecules represent approximately 39% of the energy in a molecule of glucose. Compared to each other, aerobic respiration is about six times as efficient as anaerobic respiration.

Anaerobic respiration and fermentation result in a net gain of 2-ATP molecules from glycolysis. It should be noted, that the by-products of these processes, lactic acid and alcohol, will eventually kill the organism if the products are not digested.

Factors regulating rate of respiration

Temperature—To a point, the higher the temperature the faster respiration occurs. At some temperature, enzymes will become inactivated, although there are thermophilic (heat-loving) organisms that do quite well in high-temperature environments. Energy from sugar is released faster as the rate of respiration increases which results in a net weight loss. Plants offset the weight loss by increasing photosynthetic production of sugar. Note that during respiration, some of the energy is lost as heat, which results in an overall increase in organism temperature—not necessarily detectible by human hands. 

Water—Enzymes generally operate in the presence of water, and reduced water in a plant will reduce the rate of respiration. Seeds usually have a water content of less than 10%, while mature living cells usually are in excess of 90% water. Seeds keep better if they are kept dry as the respiration rate remains quite low. However, if a seed comes into contact with water and via imbibition swells, the respiration rate will skyrocket. The temperature could increase to the point of killing the seeds. Spontaneous combustion can occur from the respiration generated heat when a fungi or bacterium is permitted to grow on wet seeds. Kind of a neat little trivia fact to tuck away.

Oxygen—Oxygen is an important regulator of respiration. If oxygen is drastically reduced, respiration may drop off to the point of retarding growth or death.  Low oxygen concentrations can lead to the onset of fermentation processes.

Assimilation and Digestion

Assimilation is the conversion of the sugar produced by photosynthesis to fats, proteins, complex carbohydrates and other substances. While digestion is the breakdown of large insoluble molecules by hydrolysis to smaller soluble forms that can be transported to various parts of the plant.

Summary of key differences between photosynthesis and respiration:

Photosynthesis

Energy stored in sugar molecules

Carbon dioxide and water used

Increases weight

Requires light

Occurs in chlorophyll

In green organisms, produces oxygen

With light energy, produces ATP

Respiration

Energy released from sugar molecules

Carbon dioxide and water released

Decreases weight

Can occur in light or darkness

Occurs in all living cells

Uses oxygen (aerobic respiration)

With energy released from sugar, produces ATP.

Growth and Plant Hormones

Growth

All living organisms begin in the same form: as a single cell. That cell will divide and the resulting cells will continue dividing and differentiate into cells with various roles to carry out within the organism. This is life and plants are no different. Plant growth can be determinate or indeterminate, meaning some plants will have a cycle of growth then a cessation of growth, breakdown of tissues and then death (think of a radish plant or a tomato plant) while others (think of a giant cedar tree) will grow and remain active for hundreds of years. A tomato plant is fairly predictable and is said to have determinate growth, while the cedar tree has indeterminate growing potential. Development refers to the growth and differentiation of cells into tissues, organs and organ systems. This again all begins with a single cell.

Plant Growth Regulators and Enzymes

Genetic information directs the synthesis and development of enzymes which are critical in all metabolic process within the plant. Most enzymes are proteins in some form or another, are produced in very minute quantities and are produced on site—meaning they are not transported from one part of the organism to another. Genetic information also regulates the production of hormones, which will be addressed shortly. The major difference is that hormones are transported from one part of the plant to another as needed. Vitamins vital in the activation of enzymes and are produced in the cytoplasm and membranes of plant cells. Animals and humans utilize plants in order to provide some vitamin resources. In general, hormone and vitamin effects are similar and are difficult to distinguish in plants, and both are referred to in general as plant growth regulators.

Plant Hormones

The growth and development of a plant are influenced by genetic factors, external environmental factors, and chemical hormones inside the plant. Plants respond to many environmental factors such as light, gravity, water, inorganic nutrients, and temperature.

Groups of Hormones

Plant hormones are chemical messengers that affect a plant's ability to respond to its environment. Hormones are organic compounds that are effective at very low concentration; they are usually synthesized in one part of the plant and are transported to another location.  They interact with specific target tissues to cause physiological responses, such as growth or fruit ripening. Each response is often the result of two or more hormones acting together.

Because hormones stimulate or inhibit plant growth, many botanists also refer to them as plant growth regulators. Many hormones can be synthesized in the laboratory, increasing the quantity of hormones available for commercial applications. Botanists recognize five major groups of hormones: auxins, gibberellins, ethylene, cytokinins, and abscisic acid.

Auxins

Auxins are hormones involved in plant-cell elongation, apical dominance, and rooting. A well known natural auxin is indoleacetic acid, or IAA which is produced in the apical meristem of the shoot. Developing seeds produce IAA, which stimulates the development of a fleshy fruit.  For example, the removal of seeds from a strawberry prevents the fruit from enlarging. The application of IAA after removing the seeds causes the fruit to enlarge normally.  IAA is produced in actively growing shoot tips and developing fruit, and it is involved in elongation. Before a cell can elongate, the cell wall must become less rigid so that it can expand. IAA triggers an increase in the plasticity, or stretchability, of cell walls, allowing elongation to occur.

Synthetic Auxins

Chemists have synthesized several inexpensive compounds similar in structure to IAA. Synthetic auxins, like naphthalene acetic acid, of NAA, are used extensively to promote root formation on stem and leaf cuttings. Gardeners often spray auxins on tomato plants to increase the number of fruits on each plant. When NAA is sprayed on young fruits of apple and olive trees, some of the fruits drop off so that the remaining fruits grow larger. When NAA is sprayed directly on maturing fruits, such as apples, pears and citrus fruits, several weeks before they are ready        to be picked; NAA prevents the fruits from dropping off the trees before they are mature. The fact that auxins can have opposite effects, causing fruit to drop or preventing fruit from dropping, illustrates an important point. The effects of a hormone on a plant often depend on the stage of the plant's development.

NAA is used to prevent the undesirable sprouting of stems from the base of ornamental trees. As previously discussed, stems contain a lateral bud at the base of each leaf. IN many stems, these buds fail to sprout as long as the plant's shoot tip is still intact.  The inhibition of lateral buds by the presence of the shoot tip is called apical dominance. If the shoot tip of a plant is removed, the lateral buds       begin to grow. If IAA or NAA is applied to the cut tip of the stem, the lateral buds remain dormant.  This adaptation is manipulated to cultivate beautiful ornamental trees. NAA is used commercially to prevent buds from sprouting on potatoes during storage.

Another important synthetic auxin is 2,4-D, which is an herbicide, or weed killer. It selectively kills dicots, such as dandelions and pigweed, without injuring    monocots, such as lawn grasses and cereal crops. Given our major dependence on             cereals for food; 2,4-D has been of great value to agriculture. A mixture of 2, 4-D         and another auxin, called Agent Orange, was used to destroy foliage in the jungles of Vietnam. A non-auxin contaminant in Agent Orange has caused severe health problems in many people who were exposed to it.

Gibberellins

In the 1920's scientists in Japan discovered that a substance produced by the fungus Gibberella caused fungus-infected plants to grow abnormally tall. The substance, named gibberellin, was later found to be produced in small quantities by plants themselves. It has many effects on a plant, but primarily stimulates elongation growth. Spraying a plant with gibberellins will usually cause the plant to grow to a larger than expected height, i.e. greater than normal.

Like auxins, gibberellins are a class of hormones that have important commercial applications. Almost all seedless grapes are sprayed with gibberellins to increase the size of the fruit and the distance between fruits on the stems.  Beer makers use gibberellins to increase the alcohol content of beer by increasing the amount of sugar produced in the malting process. Gibberellins are also used to treat seeds of some food crops because they will break seed dormancy and promote uniform germination.

Ethylene

The hormone ethylene is responsible for the ripening of fruits. Unlike the other four classes of plant hormones, ethylene is a gas at room temperature. Ethylene gas diffuses easily through the air from one plant to another. The saying "One bad apple spoils the barrel" has its basis in the effects of ethylene gas.  One rotting apple will produce ethylene gas, which stimulates nearby apples to ripen and eventually spoil because of over ripening.

Ethylene is usually applied in a solution of ethephon, a synthetic chemical that breaks down and releases ethylene gas. It is used to ripen bananas, honeydew melons and tomatoes. Oranges, lemons, and grapefruits often remain green when they are ripe. Although the fruit tastes good, consumers often will not buy them, because oranges are supposed to be orange, right? The application of ethylene to green citrus fruit causes the development of desirable citrus colors, such as orange and yellow.  In some plant species, ethylene promotes abscission, which is the detachment of leaves, flowers, or fruits from a plant. Cherries and walnuts are harvested with mechanical tree shakers. Ethylene  treatment increases the number of fruits that fall to the ground when the trees are shaken. Leaf abscission is also an adaptive advantage for the plant. Dead, damaged or infected leaves drop to the ground rather than shading healthy leaves or spreading disease. The plant can minimize water loss in the winter, when the water in the plant is often frozen.

Cytokinins

Cytokinins promote cell division in plants. Produced in the developing shoots, roots, fruits and seeds of a plant, cytokinins are very important in the culturing of plant tissues in the laboratory.  A high ratio of auxins to cytokinins in a tissue-culture medium stimulates root formation. A low ratio promotes shoot formation. Cytokinins are also used to promote lateral bud growth in flowering plants.

Abscisic Acid

Abscisic acid, or ABA, generally inhibits other hormones, such as the auxin IAA. It was originally thought to promote abscission, hence its name. Botanists now know that ethylene in the main abscission hormone. ABA helps to bring about dormancy in a plant's buds and maintains dormancy in its seeds. ABA causes the closure of a plant's stomata in response to drought. Water stressed leaves produce large amounts of ABA, which triggers potassium ions to be transported out of the guard cells. This causes stomata to close, and water is held in the leaf. It is too costly to synthesize ABA for commercial agriculture use.

Other Growth Regulators

Many growth regulators are widely used on ornamental plants. These substances do not fit into any of the five classes of hormones. For example, utility companies all over the country often apply growth retardants, chemicals that prevent plant growth, to trees in order to prevent them from interfering with overhead utility lines. If is less expensive to apply these chemicals than to prune the trees, not to mention safer for the utility workers. Also, azalea growers sometimes apply a chemical to the terminal buds rather than hand-pruning them. Scientists are still searching for a hormone to slow the growth of lawn grass so that it doesn't have to be mowed so often.

Plant movements

Plants appear immobile because they are usually rooted in one place. However, time lapse photography reveals that parts of plants frequently move. Most plants move too slowly for the passerby to notice. Plants move in response to several environmental stimuli such as: light, gravity and mechanical disturbances. These movements fall into two groups: tropisms and nastic movements.

Tropisms

A tropism is a plant movement that is determined by the direction of an environmental stimulus. Movement toward an environmental stimulus is called a positive tropism, and movement away from a stimulus is called a negative tropism. Each kind of tropism is named for its stimulus. For example, a plant movement in response to light coming from one particular direction is called a phototropism. The shoot tips of a plant that grow toward the light source are positively phototropic.

Phototropism

Phototropism, as mentioned, is illustrated by the movement of sprouts in relation to light source direction. Light causes the hormone auxin to move tot he shaded side of the shoot. The auxin causes the cells on the shaded side to elongate more than the cells on the illuminated side. As a result, the shoot bends toward the light and exhibits positive phototropism. In some plant stems, phototropism is not caused by auxin presence or movement. In these instances, light causes the production of a growth inhibitor on the illuminated side of the shoot. Negative phototropism is sometimes seen in vines that climb on flat walls where coiling tendrils have nothing to coil around. These vines have stem tips that grow away from the light, or better put, toward the wall. This brings adventitious roots or adhesive discs in contact with the wall on which they can cling and climb.

Solar tracking is the motion of leaves or flowers as the follow the suns' movement across the sky. By continuously facing toward a light source, moving or not, the plant maximizes the light available for photosynthesis.

Thigmotropism

Thigmotropism is a plant growth response to touching a solid object. Tendrils and stems of vines, such as morning glories, coil when they touch an object. Thigmotropism allows some vines to climb other plants or objects, thus increasing its chance of intercepting light for photosynthesis. It is thought that an auxin and ethylene are involved in this response.

Gravitropism

Gravitropism is a plant growth response to gravity. A root usually grows downward and a stem usually grows upward; that is, roots are positively gravitropic and stems are negatively gravitropic. Like phototropism, gravitropism appears to be regulated by auxins. One hypothesis proposes that when a seedling is placed horizontally, auxins accumulate along the lower sides of the root and the stem. This concentration of auxins stimulates cell elongation along the lower side of the stem, and the stem grows upward. A similar concentration of auxins inhibits cell elongation in the lower side of the root, and thus the root grows downward.

Chemotropism

Chemotropism is a plant growth response to a chemical. After a flower is pollinated, a pollen tube grows down through the stigma and style and enters the ovule through the micropyle. The growth of the pollen tube in response to chemicals produced by the ovule is an excellent example of chemotropism.

Nastic Movements

Plant movements that occur in response to environmental stimuli, but that are independent of the direction of the stimuli are called nastic movements. These movements are regulated by changes in water pressure in certain plant cells.

Thigmonastic Movements

Thigmonastic movements are a type of nastic movements that occur in response to touching or shaking a plant. Many thigmonasties involve rapid plant movements, such as the closing of the leaf trap of a Venus flytrap plant or the folding of a plant's leaves in response to being touched. Some leaves of sensitive plants will fold within a few seconds after being touched. This movement is caused by the rapid loss of turgor pressure (water pressure) in certain cells, a process similar to that which occurs in guard cells in order to close stomata. Physical stimulation of the plant leaf causes potassium ions to be pumped out of the cells at the base of the leaflets and petioles. Water then moves out of the cells by osmosis. As the cells shrink, the plant leaves move. It is believed that the folding of a plant's leaves in response to touch is to discourage insect feeding.

In addition, thigmonastic movements help prevent water loss in plants. When the wind blows across a plant, the rate of transpiration is increased. If the leaves of a plant fold in response to the "touch" of the wind, water loss is reduced.

Nyctinastic Movements

Nyctinastic movements are plant movements in response to the daily cycle of light and dark. Nyctinastic movements involve the same type of osmotic mechanism as thigmonastic movements, but the changes in turgor pressure are more gradual. Nyctinastic movements occur in many plants. Examples of plants that demonstrate these movements include honeylocust trees, silk trees and bean plants. The prayer plant gets its name from the fact that its leaf blades are vertical at night, resembling praying hands. During the day, however, the leaf blades are positioned horizontally. Carolus Linnaeus planted a "flower clock" made of different species of plants with nyctinastic movements. The movements of each plant species occurred at a specific time of day when the light was right for the plant.

Seasonal Responses

In nontropical areas, plant responses are strongly influenced by seasonal changes. For example, many trees shed their leaves in the fall, and most plants flower only at certain times of the year. Plants are able to sense seasonal changes. Although temperature changes are involved in some case and to certain degrees, plants mark the seasons primarily by sensing changes in night length.

Photoperiodism

A plant's response to changes in the length of days and nights is called photoperiodism. Photoperiodism affects many plant processes, including the formation of storage organs and bud dormancy. However, the most studied photoperiodic process is flowering. Some plants require a particular night length to flower. In other species, a particular night length merely makes a plant flower sooner than it otherwise would.

Critical Night Length

It has been discovered that the important factor in flowering is the amount of darkness, or night length, that a plant receives. Each plant species has its own specific requirements for darkness, called the critical night length. Although it is now understood that night length, and not day length, regulates flowering, the terms short-day plant and long-day plant are still used. A short-day plant flowers when the days are short and the nights are long. Conversely, a long-day plant flowers when the days are long and the nights are short compared to the requirements of another plant.

Responding to Day Length and Night Length

Plants can be divided into three groups, depending on their response tot he photoperiod, which again acts a season indicator.

One group, called day-neutral plants (DNPs) are not affected by day length. Examples of DNPs for flowering include tomatoes, dandelions, roses, corn, cotton and beans.

Short-day plants (SDPs) flower in the spring of fall, when the day length is short. For example ragweed flowers when the days are shorter than 14 hours and poinsettias flower when the days are shorter than 12 hours. Chrysanthemums, goldenrods, and soybeans are SDPs for flowering.

Long-day plants (LDPs) flower when the days are long, usually in summer. For example, wheat flowers only when the days are longer than 10 hours. Radishes, asters, petunias, and beets are LDPs for flowering.

Phytochrome Regulation in Plants

Plants monitor changes in day length with a bluish, light-sensitive protein pigment called phytochrome. Phytochrome exists in two forms, based on the wavelength of the light that it absorbs. It is generally produced in meristematic tissues in very minute amounts. The two stable forms can be converted to each other by absorbing light. P_red (P_r) which absorbs red light and P_far-red (P_fr) which absorbs far-red light. In the daylight more P_r is converted to P_fr (the active form) than vice versa. P_fr will convert back to P_r over several hours in the dark where it would be stable indefinitely. The conversion in light is almost instantaneous. The phytochrome mechanism is what transforms the crook in the hypocotyls of the emerging seedling into a straight stalk. Stem elongation appears to be inhibited by P_fr.  However, if light levels are low, the shaded stems of a tree for example, more far-red light will reach them and cause the conversion to P_r which lowers inhibition and allows the stems to grow longer and out from under the shade.

The interconversion abilities of phytochrome:

Vernalization

Vernalization is the low-temperature stimulation of flowering. Vernalization is important for fall-sown grain crops, such as winter wheat, barley and rye. For example, wheat seeds are sown in the fall and survive the winter as small seedlings. Exposure to cold weather causes the plants to flower in the early spring, and an early crop is produced. If the same wheat is sown in the spring, it will take about two months longer to produce a crop. Thus, cold temperatures are not absolutely required for most crops, but they do expedite flowering. Farmers often use vernalization to grow and harvest their crops before a summer drought sets in and stunts growth.

A biennial plant is a plant that lives for two years, usually producing flowers and seeds during the second year. Biennial plants, such as carrots, beets, celery and foxglove, survive their first winter as short plants. In the spring their flowering stem elongates rapidly, a process called bolting. Most biennials must receive cold weather to vernalize before they flower during the second year. They will then die after flowering. Treating a biennial with gibberellin is sometimes a substitute for cold temperatures in vernalization, and will stimulate the plant to grow.

Fall colors

Some tree leaves are noted for their spectacular fall color display. The changing fall colors are caused primarily by a photoperiodic response but also by a temperature response. As nights become longer in the fall, leaves stop producing chlorophyll. As the chlorophyll chemically degrades, it is not replaced. Other leaf pigments, the carotenoids, become visible and the green/orange splotches become more visible as the green chlorophyll turns orange. Carotenoids include the orange carotenes and the yellow xanthophylls. Anthocyanins produce the deep red and purplish-red colors in the fall display.

Meiosis and alternation of generations

Review of Mitosis: Cell Cycle

The cell cycle contains the process in which cells are either dividing or in between divisions. Cells that are not actively dividing are said to be in interphase, which has three distinct periods of intense activity that precedes the division of the nucleus, or mitosis. The division of the rest of the cell occurs as an end result of mitosis and this process occurs in regions of active cell division, called meristems. Meristems will be looked at in the plant tissue tutorial.

Mitosis is a process within the cell cycle that is divided into four phases which we will sum up here:

1. Prophase—the chromosomes and their usual two-stranded nature becomes apparent, the nuclear envelope breaks down.
2. Metaphase—the chromosomes become aligned at the equator of the cell. A spindle composed of spindle fibers is developed and some attach to the chromosomes at their centromere.
3. Anaphase—the sister chromatids of each chromosome, that is now called the daughter chromosomes, separate lengthwise and each group of daughter chromosomes migrates to the opposite ends of the cell.
4. Telophase—the groups of daughter chromosomes are grouped within a developing nuclear envelope which makes them separate nuclei. A wall forms between the two sets of daughter chromosomes thus creating two daughter cells.

In plants, as the cell wall is developing, droplets or vesicles of pectin merge forming a cell plate that eventually will become the middle lamella of the new cell wall.

The key feature of mitosis is that the daughter cells have the same chromosome number and are otherwise identical to the parent cell.

Meiosis Introduction

Mitosis, as just reviewed above, is a process by which a cell can reproduce itself and the number of chromosomes and the nature of the DNA will be identical to the original parent cell. Very few species will grow or live indefinitely, so there must be some way to ensure the continuity of the species. Reproduction is the only way a species can be perpetuated, without perpetuation the species will become extinct. Reproduction can occur in several ways as vegetative propagation, such as in the development of runners in strawberry plants, or by special cells called vegetative spores which are products of mitosis. In these processes, the ‘offspring’ have identical cells and identical chromosomes to the parent cells and thus the processes are called asexual reproduction—a means without, so without sex reproduction. Most plants, however, will undergo sexual reproduction which involves the production and recombining of sex cells called gametes. In flowering and cone-bearing plants this involves the production of seeds. The gametes produced are male and female, and are called sperm cells and egg cells, correspondingly. When the gametes combine together, the cells fuse and form a single cell called a zygote. It is the zygote that will go on to become the plant embryo and eventually a mature, adult plant.

However, in thinking about this process, what would happen if both gametes had the same number of chromosomes as the rest of the cells in the organism? When they fused to become a zygote, they would have two times the number of chromosomes as the rest of the cells in the organism. The number of chromosomes would increase exponentially through the generations if this occurred. This is where meiosis comes in to play. Meiosis is the process by which gametes, sex cells, are formed. It is unique because gametes have exactly half of the total number of chromosomes as the rest of the cells in the parent organism. When two gametes, each with half the number of chromosomes, get together they are able to restore the chromosome number to the same as the rest of the cells in the parent organism. When the zygote develops into a plant embryo and eventually a mature plant, it will have the exact number of chromosome specific to the species. Note that the processes and steps in meiosis are very similar to mitosis, so make certain you have a good understanding of mitosis so that you will be able to compare the two processes.

Before we get into the nitty-gritty of meiosis, keep in mind that all living cells have two sets of chromosomes—one from a male and one set from a female parent. The genes in the chromosomes may control the same characteristics but in contrasting ways—for example: genes for plant height, genes for plant color, genes for fruit color, etc—the female gamete might code for short plants, while the male gamete might code for tall plants. That is more of a genetics topic though. But you should know that the chromosomes that code for the same characteristics are called homologous chromosomes.

Phases of Meiosis:

The end result of one round of meiosis will be four cells with half the number of chromosomes as the parent cell. The daughter cells are rarely, if ever, identical to each other or the parent cell depending on the organism involved. There are two successive divisions in meiosis, which in plants occur without a pause. Mitosis takes roughly 24 hours, while meiosis takes up to two weeks. In some organisms, meiosis takes weeks or years depending on the organism.

Division I –Reduction division—the chromosome number is reduced to half the parent cell chromosome number. End result of division one is two cells.

Prophase I—Main features:

1. Chromosomes coil, becoming shorter and thicker, the two-stranded nature becomes apparent, two strands are called a chromatid and chromosomes are aligned in pairs. Each pair of chromosomes has four chromatids and they have a centromere attached in the center holding the four strands together.
2. Nucleolus disassociates and nuclear envelope dissolves.
3. Segments of the closely associated pairs of chromatids may be exchanged with each other (between the pair members) this is called crossing-over. Each chromatid contains the original amount of DNA but now may have “traded” genetic material.
4. The chromosomes separate. Some spindle fibers are forming and some are attaching to the centromeres of the chromosomes. The fibers extend from each pole of the cell.

Metaphase I—Main features:

1. In pairs, the chromosomes align at the equator of the cell, with the centromeres and spindle fibers apparent.
2. The two chromatids, from each chromosome, function as a single unit.

Anaphase I—Main features:

1. One entire chromosome, consisting of two chromatids, migrates from the equator to a pole. The chromosomes do not separate from each other and retain both chromatids when the reach their pole. At each pole, there will be half the chromosome number. If crossing over occurred in prophase then the chromosomes will consist of original DNA and DNA from a homologous chromosome—now at the opposite pole.
2. The centromere remains intact in each pair of chromatids.

Telophase I—Main features:

1. What occurs in this step, depends on the species involved, as they may revert to interphase or proceed directly to division II.
2. If they revert to interphase, they will only do so partially and the chromosomes will become longer and thinner.
3. nuclear envelopes will not form, but the nucleoli will generally recluster.
4. Telophase is over when the original cell becomes two cells or two nuclei.

Division II—Equational division—the chromosome number stays the same, the cells replicate and result in four cells. The events closely resemble the events in mitosis, except that there is no duplication of DNA during the interphase that may or may not occur between the two divisions.

Prophase II—Main features:

Chromosomes of both nuclei become shorter and thicker. The two-stranded nature becomes apparent once again.

Metaphase II—Main features:

1. Chromosomes align their centromeres along the equator.
2. Spindle fibers form and attach to each centromere, extending from one pole to the other.

Anaphase II—Main features:

The centromeres and chromatids of each chromosome separate and begin their migration to the opposite poles.

Telophase II—Main features:

1. the coils of chromatids—now called chromosomes again—relax and the chromosomes become longer and thinner.
2. Nuclear envelopes and nucleoli reform for each group of chromosomes.
3. New cell walls form between the four groups of chromosomes.
4. Each set of chromosomes in the four new cells, has exactly half of the chromosome number of the original number.

Alternation of Generations

One member of every original pair of chromosomes end up in each resulting cell, which means each cell has one set of chromosomes. None of the daughter cells is identical to the parent cell. A cell with one set of chromosomes is called haploid and a cell with two sets is called diploid. In sum, a diploid cell undergoes meiosis and results in four haploid cells.

Sex cells, or gametes, of an organism are haploid and when a zygote is formed, the zygote is diploid. This is always true, no matter how many chromosomes an organism might have. In many places, you will find it stated that an organism has n number of chromosomes. In this case, an organism will have 2n chromosomes in its diploid cells and 1n chromosomes in its haploid cells. Alternation of generations refers to a plants life cycle including sexual reproduction that is characterized by alternating between a diploid (2n) sporophyte phase and a haploid (1n) gametophyte phase.

Bryophytes

Bryophytes are essentially nonvascular plants, meaning they do not have xylem or phloem. The habitations of bryophytes are widely varied and include bare rocks in the scorching sun to frozen alpine slopes. Bryophytes include mosses, liverworts and hornworts. These groups of plants require external water, usually in the form of dew or rain. Some bryophytes grow exclusively in dark, damp environments in order to provide moisture. Water is essential for bryophyte reproductive activities.  Most mosses have water-conducting cells called hydroids in the centers of their stems, and some even have food conducting cells called leptoids. These cells are not nearly as efficient as xylem and phloem and generally, bryophytes are not very tall plants. The lack of vascular tissue leaves the plant body very soft and pliable. Bryophytes are good nest-making material for birds.

The alternation of generations in bryophytes is quite obvious as the gametophyte generation is the ‘leafy’ plant that is generally visible. The sporophyte generation that produces spores is located at the tips of the ‘leafy’ gametophyte generation. The sporophyte generally looks like a slender stalk with a cap on top. While the lifecycles of all bryophytes are similar and even their chromosome number and habituation have similarities; they care divided into three distinct divisions based on the few differences in structure and reproduction.

Three divisions:

1. Division Hepaticophyta—Liverworts
Wort means plant or herb, and in ancient times herbalists thought that some bryophytes—specifically the ones that look like liver lobes—were useful in treating liver ailments. Although, the belief was discounted the name stuck.

Structure—About 20% of the liverworts have a flattened, somewhat leaf like body called a thallus (plural thalli). The other 80%, which aren’t as common in nature, are ‘leafy’ and look more like mosses. Mosses are more complex than liverworts. They have one-celled rhizoids on their lower surfaces. The rhizoids look like tiny roots and anchor the plants to surfaces and soil particles. The thalli are the gametophyte generation and develop almost directly from spores. They have smoother upper surfaces and the cell wall corners are thickened.

Thalloid liverworts—Marchantia is the best-known species of thalloid liverwort. It can usually be found on damp soil after a fire. Marchantia can reproduce asexually and sexually. Asexual reproduction is accomplished through gemmae, which are tiny lens-shaped pieces of tissue that detach from the thallus. Gemmae cups are produced along the upper surface of the gametophyte. Raindrops splash onto the cups and the cups detach and may splash up to 3 feet away from the ‘parent’ gametophyte.  Lunularic acid inhibits the Gemmae from growing, as soon as it is out of the cup, though, the inhibition is removed and each may develop into a new thallus.  Sexual reproduction in the Marchantia involves the interaction between spores on separate male and female gametophytes. The gametangia are formed on gametophores, or umbrella like structure on long stalks. The stalks are positioned between the grooves of the thallus. The male gametophore is shaped like a disc with a scalloped edge, while the female gametophore looks like the hub and spokes of a wheel. the male gametangia are called antheridia, contain numerous sperms and are produced in rows just beneath the upper surface of the antheridiophore. The female gametangia are flask-like and each one contains a single egg. They are produced in rows and hang with their neck downward from the archegoniophore. Rain will splash and release the flagellated sperm cells. The stalks of the archegoniophores may not be finished growing at the time of fertilization. The zygote, fertilized egg, will develop into a multicellular embryo (a.k.a. immature sporophyte) which is anchored from the tissues of the archegoniophore by a knoblike foot. The foot is connected to the sporophyte (also referred to as the capsule, out of which develops the various type of tissues) by a short, thick stalk called the seta. Inside the capsule, spore mother cells undergo meiosis which results in haploid spores. Some capsule cells do not undergo meiosis, but remain diploid and develop into elaters, which are long and pointy and responsive to changes in humidity. The elaters will twists and untwist rapidly in order to disperse the spores. The young sporophyte will be protected until maturity by a caplike tissue called the calyptra. There are other variations of this cycle in the other thalloid forms. In some floating or amphibious liverworts, the spores are freed only as the thallus decays.

“leafy” liverworts—These liverworts are usually found in tropical jungles of fog belts. They have two rows of partially overlapping ‘leaves’ in which the cells contain distinctive oil bodies. They are of course, not true leaves. They do have folds and lobes that collect water and usually house tiny animals. The male and female gametangia produce antheridia and archegonia in cuplike structures. When the sporophytes mature they will germinate and produce a protonema which has photosynthetic cells and will develop into a new gametophyte plant.

2. Division Anthocertophyta

Hornworts—The mature sporophytes of a hornwort look like miniature green cattle horns. Their gametophyte generation looks similar to filmy versions of thalloid liverworts. Hornworts are rare in artic regions and are usually found in moist, shady areas, although some do grow on trees. They have between one and eight chloroplasts—usually just one—and the chloroplasts have pyrenoids similar to green algae.

Asexual reproduction—They can reproduce asexually by fragmentation or by developing lobes that are separate from the main part of the thallus.

Sexual reproduction—Hornworts also reproduce sexually and the plants can be unisexual, like mosses and liverworts, or they can be bisexual, meaning the archegonia and antheridia are on the same plant.  The sporophytes are distinctive in form and contain numerous stomata. They do not have stalks, setae, and look instead like tiny broom handles rising out of the gametophyte generation. Meristematic tissue at the base of the sporophyte continually increases the length of the sporophyte until conditions are favorable and the axis (central core) undergoes meiosis to produce spores. The sporophyte tip will split and the spores will be dispersed as the horn peels into ribbon like segments.

3. Division Bryophyta—Mosses

Subclasses—There are three subclasses of mosses: peat mosses, true mosses and rock mosses. They share similar reproductive and life cycles and they are all distinct from other plant organisms. Moss ‘leaves’ do not have mesophyll, stomata or veins and all the ‘leaf’ cells are haploid. The cells in the ‘leaf’ do contain numerous lens-shaped chloroplasts, except in the midrib. Mosses have transparent water-storage cells that chloroplasts, yet aid in absorbing water and thus providing moisture to the avascular plant. Mosses also have root like rhizoids which do provide for some water absorption. However, most of the water for the plant travels up the moss surface by way of capillarity.

Asexual reproduction—This reproductive process does not rely on a sexual cycle and its alternation of generations, instead it has been demonstrated that moss fragments can produce protonemata which will ‘bud’ and develop into gametophyte mosses.

Sexual reproduction—Mosses produce gametangia on the same plant or on separate plants. Commonly, the gametangia are produced on the same plant. The multicellular antheridia and archegonia are produced at the tips of ‘leafy’ shoots. The individual archegonium has a cavity, the venter with a single egg, and a neck through which the sperm gains access to the egg. Sperm cells are produced in the antheridia. Upon fertilization, the zygote develops into an embryo that remains attached to the gametophyte by an embedded foot. The embryo develops into a sporophyte with a capsule and a seta, a stalk.  The gametophyte produces a calyptra which partially covers the capsule. Inside the capsule, spore mother cells undergo meiosis and produce spores which are then released through the teeth of the peristome located at the tip of the capsule.  Until spore maturity, the peristome is protected by an operculum, which will fall off at maturity. After the spores germinate, protonemata will ‘bud’ and develop into gametophyte mosses.

Vascular plants: ferns and relatives

Vascular plants—Ferns and relatives

These plants are seedless plants, but unlike the bryophytes, they do have vascular tissue (xylem and phloem). Because of the presence of vascular tissue, the leaves of ferns are their relatives are better organized than the mosses and liverworts.

Four divisions:

1. Division Psilotophyta

Overview:

The sporophytes in this division have neither true leaves nor roots, their stems and rhizomes fork evenly.

Whisk ferns—Whisk ferns are the simplest of the vascular plants. They consist of evenly forking stems with small protuberances called enations. They lack leaves and roots. These ferns have a central vascular cylinder composed of xylem and phloem.

Reproduction—Whisk ferns reproduce via gametangia. Spores will germinate into tiny saprophytic gametophytes on the surfaces which antheridia and archegonia are scattered. The resulting zygote will develop a foot and a rhizome. An upright stem is produced when the foot separates from the rhizome.

2. Division Lycophyta

Overview:

These plants have stems that are covered with photosynthetic microphylls. Microphylls are leaves with a single vein and a trace that is not associated with a leaf gap.

Club mosses –there are two types of club mosses that still have living representatives. Ground pines will develop sporangia in the axils of sporophylls. Each gametophyte may be capable of producing several sporophytes. The second type of club moss include the spike mosses. These plants are heterosporous and have a ligule, or little tongue, on each microphyll. The microspores will develop into male gametophytes with antheridia (sperm producing); while the megaspores will develop into female gametophytes with archegonia (egg producing).  The biggest difference between the ground pines and the spike mosses is the presence of the ligule (spike mosses) and the spike mosses produce two types of spores and gametophytes (heterospory).

Quillworts

Quillworts are found partially submerged in water for at least part of the year. Their microphylls (leaves) look somewhat like porcupine quills although they lack the rigidity of an actual porcupine quills. The microphylls arise from a corm like base. The corm base has a cambium that will remain active for many years.

Club moss spores have many uses including flash powder, medicine, talcum powder, as well as ornamental uses and novelty items.

3. Division Sphenophyta

Overview:

The plants in this division have ribbed stems that contain silica deposits in the epidermal cells. Their scale-like microphylls lack chlorophyll. However, the silica in the stems makes these plants useful for scouring. Horsetails and scouring rushes occur in both the branched and unbranched forms. Either way, they are jointed stems with small whorls of scale-like leaves at the base of the plant.

Horsetails, or Equisetum, are hollow in the center of their stem which contains cylinders of carinal and vallecular canals. The hollow stems arise from extensively branching rhizomes just beneath the surface of the soil. Some types of horsetails have non-photosynthetic stems. They reproduce via strobili, cones, that are produced in the spring in all species. Horsetail spores have ribbon like elaters that are sensitive to humidity. An interesting note about horsetails; usually equal numbers of male and female gametophytes are produced by these plants, however, the female gametophytes may become bisexual and the development of more that one sporophyte from a gametophyte is common.

As mentioned, horsetails have been used for scouring. They can be eaten if the silica is removed, however, they really don’t make for a feast. Plants from this division have also been used for medicinal purposes including as a diuretic, treatment for tuberculosis and treating venereal diseases. Horsetails have been used as an ingredient in shampoos and metal polish.

4. Division Pterophyta

Ferns are the most common and best recognized examples of the vascular seedless plants. The leaves of ferns have megaphylls—or leaves with more than one vein and a leaf trace/leaf gap association—which are large and usually subdivided into many lobes. Hence, the ‘finger’ look to a fern plant. Fern fronds, as the leaves are called, typically are dissected and feathery in appearance. However, realize that they do vary quite a bit as far as external structure and form goes. Fern fronds start as croziers or fiddleheads which unfurl into the main leaf form. On the underside of the frond, patches of sporangia can be found. These sporangia are usually in sori, which are clusters; and are sometimes covered by a clear flap called the indusium. This setup allows for the maximum protection of the sporangia, but allows for distribution. Individual sporangia have an ‘spring-loaded’ annulus which allows for mature spores to be catapulted out of the sporangium. The gametophytes of ferns are called prothalli and develop after the spores germinate. Prothalli contain both archegonia and antheridia and only one zygote develops into a sporophyte.

Ferns are used for ornamental purposes as well as stuffing materials for bedding in tropical regions. Fern fronds are used in weaving baskets and hats, brewing some types of ale and numerous folk medicine practices. 

Seed Plants

Seed plants:

There are two main subdivisions of seed plants—the ones without covered seeds, the gymnosperms, and the ones with covered seeds, the angiosperms. In this tutorial we will briefly look at both types of plants. 

Gymnosperms—naked seed plants

Gymnosperms are plants that do not flower and do not bear their seeds in an enclosure such as a fruit. The seeds are produced on the surface of the sporophylls or similar structures until they are dispersed. The sporophylls are usually arranged in a spiral on the female strobili (cones) which develop at the same time as the smaller male strobili. The male strobili produce the pollen which will fertilize the ovules in the female cones.  The ovule contains a nutritious nucellus which is itself enclosed in several layers of integument. The integument layers will eventually become the seed coat, after fertilization and further development of the embryo takes place.

Gymnosperms are classified into one division and three subdivisions: division Pinophyta  with subdivision Cycadicae which includes the palm like cycads;  subdivision Pinicae which includes conifers and class Ginkgoatae the Ginko trees; and subdivision Gneticae which includes the gnetophytes.

1. subdivision Pinicae: Conifers—pine trees and evergreens to the layman.
structure and form—For the sake of discussion we will look at Pines, which are the largest genus of conifers. Pine needles are their leaf structures. They are usually arranged in clusters or bundles of two to five leaves (needles), although some species have as few as one or as many as eight leaves in a cluster, the clusters are sometimes referred to as fascicles. Each needle is covered with a thick cuticle over the epidermal layer and a layer of thick-walled cells just beneath the epidermis called the hypodermis. The stomata on the epidermal surface are sunken and are surrounded by an endodermis. The mesophyll cells do not have the wide air spaces as broadleaf and flowering plant leaves. Resin, and resin canals develop noticeably throughout the mesophyll cells. The canals are tubes in which resin is secreted. Resin is both aromatic and antiseptic and helps to prevent fungal infections and deter insect attacks. Some conifers produce resin in response to injury. The fascicles, needle clusters, will fall off every two to five years after maturing. They do not, however, fall off all at once and unless diseased, will not look bare like other flowering trees. The secondary xylem, wood, in conifers varies in hardness. Most gymnosperm wood consists of tracheids and has no vessel members or fibers as do flowering trees. Therefore the wood lacks thick walled cells. Conifer wood is considered to be softwood, while the wood of broadleaf trees is considered to be hardwood. The xylem rings in conifers  are often fairly wide as a result of rapid growth. Both vertical and horizontal resin canals can be found throughout the wood. Pine phloem lacks companion cells, but has albuminous cells that perform similar function for the phloem. The roots of pine trees are always found in association with mycorrhizal fungi.  The fungi perform functions for the roots, which enable normal growth. Pine trees can be found in all types of environments and ones of opposite extremes.

Reproduction—There are two kinds of spores produced by pine trees. The microspores are produced in the smaller male strobili which develop at the tips of lower branches. At the base of the male cone, the microsporangia develop in pairs and give rise to four-celled pollen grains. Millions of pollen grains are produced per cone. The female cones are larger and produce the megaspores. They are formed in ovules at the bases of female cone scales. A pore, called the micropyle, allows for access by the sperm (pollen grain) to the ovule. The pore is formed by the overlapped layers of integument protecting the ovule. Each megaspore develops into a female gametophyte. The archegonia is contained in the mature female gametophyte. Prior to the maturity of the archegonia, pollen grains will become lodged in the cone scales in sticky pollination drops. The pollen grains develop a long pollen tube that digests its way down to the developing archegonia. Upon arrival at the archegonia, two of the original four cells in the pollen grain will migrate into the tube. One sperm will unite with the egg cell to form a zygote. The zygote will continue developing and will become a seed embryo with a membranous wing formed from part of the cone scale. The seed embryo is ready for distribution and upon landing will germinate and become a new tree.

Class Ginkoatae

Ginkgo trees have small fan-shaped leaves with veins that evenly fork. They have similar reproductive cycles to that of the conifers with the exception that the edible seeds are encased in a fleshy covering. The covering smells like rancid butter at seed maturity.

2. subdivision Cycadicae

Cycads—These plants look like little palm trees with unbranched trunks and large crowns of pinnately divided leaves. Their strobili and cones are quite similar to those of conifers, however, their sperms have numerous flagella—much unlike conifers.

3. subdivision Gneticae

Gnetophytes—These plants have vessels in their xylem. Most of the species are in the genus Ephedra and have jointed stems and leaves that are nothing more than scales. Sometimes the plants in this genus are called joint firs, as they look like jointed sticks. The plants in this subdivision are adapted to unusually dry environments. They produce tiny leaves in groups of twos and threes, which turn brown as soon as they appear.  Male and female strobili may occur on the same plant.

Relevance to humans

Conifers are sources for paper products and lumber materials. The resin from conifers has historically been used as sealing pitch, turpentine, floor waxes, printer’s ink, perfumes, menthol manufacture and rosin for musical instruments. Ginko leaves are used medicinally as are plants from the genus Ephedra. Arrowroot starch was once purified from a cycad species. Teas have been made from conifers.

Angiosperms—flowering seed plants (covered seed plants)

Angiosperms are plants that have seeds encased in a protective covering. That covering is the ovary which is part of the flower structure and distinguishes angiosperms from gymnosperms, the other seed plants. So it can be said that angiosperms are also flowering plants. There is one division of angiosperms, Magnoliophyta, which is divided into two classes: monocots and dicots. Angiosperms, like gymnosperms, are heterosporus, which means they produce two types of spores and their sporophytes are more dominant than those of gymnosperms.  At maturity, the female gametophytes are reduced to a few cells and are completely enclosed within sporophyte tissue; while the male gametophytes consist of a binucleate cell with a tube nucleus which forms a pollen tube much like the one formed in gymnosperm pollination.

Development of gametophytes

Angiosperm gametophytes develop in separate structures, sometimes on the same plant. The embryo sac, or female gametophyte, develops in the ovule which is surrounded by integuments. The integuments will later become the seed coat after fertilization has occurred. The male gametophytes, pollen grains, develop in the anthers.

Pollination—The process of transferring pollen grains from the anther to a stigma is called pollination. This may occur via wind, insects, birds or other agents. Flowers may be geared toward certain pollinators. For example, flowers pollinated by bees are usually sweet and fragrant. Their colors are usually blue and yellow. Beetles are attracted to flowers with strong odors and dull colors or white flowers. There are fly-pollinated flowers that smell like rotting meat. Moths are drawn to pale yellow or white flowers. Birds, akin to bee preferences, like sweet nectar and fragrant flowers with bright colors. Orchids have unusual pollination mechanisms, and in some they literally grab an insect and manually attach two pollen packets to the hind end of the insect before releasing it.

Fertilization and seed development—The pollen grain contains two sperm nuclei (formed from a generative nucleus that splits) and one tube nucleus. The tube nucleus forms a pollen tube that grows through the stigma, the style and into the ovary via the micropyle. Upon arrival at the ovary, one sperm nuclei will fertilize the egg and form a zygote. The other sperm nuclei will fuse with the polar nuclei of the egg in order to form a 3n endosperm nucleus. This will be conserved as food for the plant embryo or may become part of the seed. Some species will end up with 5n, 9n, or 15n endosperm tissue, depending on how the embryo sac develops.

Parthenocarpy—Some fruits can develop without the development or fusion of gametes, but with the otherwise standard structures being involved. These fruits are called parthenocarpic fruits. Not all ‘seedless’ fruits are parthenocarpic.

Classification trends in flowering plants

Specializations in flowering plants include an overall efficiency in plant growth, by reducing the number of parts; fusion of parts; the appearance of compound pistils that are composed of several carpels; various positioning of the ovary with respect to the receptacle including inferior and superior ovaries; irregular flowers and unisexual flowers.  A monoecious species of flower has both male and female flowers on the same plant, while a dioecious flower would have separate plants for male and female flowers. All of these are characteristics that can be looked at in the classification and categorization of flowering plants.

Relevance to humans

Beyond ornamental uses, flowering plants constitute much of what we eat, parts of the clothes we wear, the wood in our homes and furniture and the medicines we consume. Flowering plants are everywhere and thus have a million uses. All fruit comes from flowering plants, obviously, and think of how many just in the edible category there is, not to mention all of those that aren’t for eating.  Stop and think for a minute on the plants that you encounter in daily life, chances are good they came from a flowering plant.

http://www.biology-online.org/11/1_plant_cells_vs_animal_cells.htm