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.
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:
- Root cap
- Region of cell division
- Region of elongation
- Region of maturation
We will discuss each region in greater detail.
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.
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.
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 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 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.
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.