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.
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.