plant
plant, any organism of the plant kingdom, as opposed to one of the animal kingdom or of the kingdoms Fungi, Protista, or Monera in the five-kingdom system of classification. (A more recent system, suggested by genetic sequencing studies, places plants with animals and some other forms in an overarching group, the eukarya, to distinguish them from the prokaryotic bacteria and archaea, or ancient bacteria.) A plant may be microscopic in size and simple in structure, as are certain one-celled algae, or a gigantic, many-celled complex system, such as a tree.
Plants are generally distinguished from animals in that they possess chlorophyll, are usually fixed in one place, have no nervous system or sensory organs and hence respond slowly to stimuli, and have rigid supporting cell walls containing cellulose. In addition, plants grow continually throughout life and have no maximum size or characteristic form in the adult, as do animals. In higher plants the meristem tissues in the root and stem tips, in the buds, and in the cambium are areas of active growth. Plants also differ from animals in the internal structure of the cell and in certain details of reproduction
There are exceptions to these basic differences: some unicellular plants (e.g., Euglena) and plant reproductive cells are motile; certain plants (e.g., Mimosa pudica, the sensitive plant) respond quickly to stimuli; and some lower plants do not have cellulose cell walls, while the animal tunicates (e.g., the sea squirt) do produce a celluloselike substance.
The Plant Kingdom
The systems of classification of the plant kingdom vary in naming and placing the larger categories (even the divisions) because there is little reliable fossil evidence, as there is in the case of animals, to establish the true evolutionary relationships of and distances between these groups. However, comparisons of nucleic acid sequences in plants are now serving to clarify such relationships among plants as well as other organisms.
A widely held view of plant evolution is that the ancestors of land plants were primitive algae that made their way from the ocean to freshwater, where they inhabited alternately wet-and-dry shoreline environments, eventually giving rise to such later forms as the mosses and ferns. From some remote fern ancestor, in turn, arose the seed plants.
The plant kingdom traditionally was divided into two large groups, or subkingdoms, based chiefly on reproductive structure. These are the thallophytes (subkingdom Thallobionta), which do not form embryos, and the embryophytes (subkingdom Embryobionta), which do. All embryophytes and most thallophytes have a life cycle in which there are two alternating generations (see reproduction). The plant form of the thallophytes is an undifferentiated thallus lacking true roots, stems, and leaves. The subkingdom Thallobionta is composed of more than 10 divisions of algae and fungi (once considered plants). The subkingdom Embryobionta is composed of two groups: the bryophytes (liverwort and moss), division Bryophyta, which have no vascular tissues, and a group consisting of seven divisions of plants that do have vascular tissues. The Bryophyta, like other nonvascular plants, are simple in structure and lack true roots, stems, and leaves; they therefore usually live in moist places or in water.
The vascular plants have true roots, stems, and leaves and a well-developed vascular system composed of xylem and phloem for transporting water and food throughout the plant; they are therefore able to inhabit land. Three of the divisions of the vascular plants are currently represented by only a very few species. They are the Psilotophyta, with only three living species; the Lycopodiophyta (club mosses); and the Equisetophyta (horsetails). All the plants of a fourth subdivision, the Rhyniophyta, are extinct. The remaining divisions include the dominant vegetation of the earth today: the ferns , the cone-bearing gymnosperms and the angiosperms, or true flowering plants The latter two classes, because they both bear seeds, are often collectively called spermatophytes, or seed plants.
The gymnosperms are all woody perennial plants and include several orders, of which most important are the conifer, the ginkgo, and the cycad. The angiosperms are separated into the monocotyledonous plants—usually with one cotyledon per seed, scattered vascular bundles in the stem, little or no cambium, and parallel veins in the leaf—and the dicotyledonous plants—which as a rule have two cotyledons per seed, cylindrical vascular bundles in a regular pattern, a cambium, and net-veined leaves. There are some 50,000 species of monocotyledon, including the grasses (e.g., bamboo and such cereals as corn, rice, and wheat), cattails, lilies, bananas, and orchids. The dicotyledons contain nearly 200,000 species of plant, from tiny herbs to great trees; this enormously varied group includes the majority of plants cultivated as ornamentals and for vegetables and fruit.
liverwort
any plant of the class Marchantiopsida. Mosses and liverworts together comprise the division Bryophyta, primitive green land plants, some of the earliest land plants resembled modern liverworts. In contrast to mosses, most liverworts grow prostrate and consist of a flattened, branching (but undifferentiated) green structure, the thallus; other liverworts produce leafy stems, which are flattened and usually prostrate. The ancients believed that liverworts could cure diseases of the liver, hence the name. They are also called hepatics, and the unrelated flowering plant hepatica is frequently called liverwort. Liverworts are classified in the division Bryophyta, class Marchantiopsida.
moss
any species of the class Bryopsida, in which the liverworts are sometimes included. Mosses and liverworts together comprise the division Bryophyta, the first green land plants to develop in the process of evolution. It is believed that they evolved from certain very primitive vascular plants and have not given rise to any other type of plant. Their rootlike rhizomes and leaflike processes lack the vascular structure (xylem and phloem) of the true roots, stems, and leaves found in higher plants. Although limited to moist habitats because they require water for fertilization, bryophytes are usually extremely hardy and grow everywhere except in the sea. Mosses, the more complex class structurally, usually grow vertically rather than horizontally, like the liverworts. The green moss plant visible to the naked eye, seldom over 6 in. (15.2 cm) in height, is the gametophyte generation (see reproduction). Except for the commercially valuable sphagnum or peat moss, mosses are of little direct importance to humans. They are of some value in soil formation and filling in of barren habitats (e.g., dried lakes) prior to the growth of higher plants and also provide food for certain animals. Unrelated plants sharing the name moss include the club moss, flowering moss, or pyxie (of the diapensia family), Irish moss, or carrageen (see algae), reindeer moss (a lichen), and Spanish moss. Mosses are classified in the division Bryophyta, class Bryopsida.
Psilotophyta
The division of vascular plants consisting of only two genera, Psilotum and Tmesipteris, with very few species. These plants are characterized by the lack of roots, and, in one species, leaves are lacking also. The green, photosynthetic stem is well-developed. Like higher plants, e.g., the angiosperms (Magnoliophyta), Psilotophyta has specialized conducting, or vascular, tissue (xylem and phloem). Psilotum, with only two species, is widespread in tropical and subtropical areas, whereas Tmesipteris species is restricted to Australia and neighboring islands. The spore-producing structures are produced in clusters in the axil of a leaflike at the end of a short lateral branch. The gametophyte plant, arising from germination of a spore, is small and colorless, and derives its nutrition through a specialized association with a fungus. Sexual structures on the gametophyte produce eggs and sperm. The motile sperm, with numerous flagella, are able to swim through a film of water to the egg. The fertilized egg, or zygote, first absorbs nourishment from the gametophyte, and later becomes photosynthetic and self-sustaining. The life cycle is very much like that of ferns.
Lycopodiophyta
The division of the plant kingdom consisting of the organisms commonly called club mosses and quillworts. As in other vascular plants, the sporophyte, or spore-producing phase, is the conspicuous generation, and the gametophyte, or gamete-producing phase, is minute. The living representatives are all rather small herbaceous plants, usually with branched stems and small leaves, but their fossil ancestors were trees. Like other vascular plants, the axes of this group have epidermis, cortex, and a central cylinder, or stele, of conducting tissue. The spore cases, or sporangia, are borne at the base of leaves, either scattered along the stem or clustered into a terminal cone or strobilus. At maturity, the sporangia split across the top, releasing great quantities of spores. The spores germinate to produce small, nongreen, fleshy gametophytes, which bear both sperm-producing antheridia and egg-producing archegonia. The motile sperms swim to the egg through a film of water. The fertilized egg, or zygote, gives rise to an embryo and eventually to a mature sporophyte. The order Lycopodiales includes the common genus Lycopodium, the larger of two genera (the other is Phylloglossum) belonging to this order and containing some 100 species. The order Selaginellales contains only one living genus, Selaginella, with perhaps 600 species, although fossil forms resembling Selaginella are known from deposits of the Carboniferous period. The order Isoetales (quillworts) contains the small genus Isoetes, which grows in shallow water in lakes, ponds, and marshy places. The plants have a grasslike appearance and are therefore often not readily identified. The order Lepidodendrales contains members known only from fossil specimens dating from the Upper Devonian to Permian times. Lepidodendron, the most common genus, was of tree size.
Equisetophyta
A small division of the plant kingdom consisting of the plants commonly called horsetails and scouring rushes. Equisetum, the only living genus in this division, is descended evolutionarily from tree-sized fossil plants. There are about 30 species, distributed in every continent except Australia and Antarctica and in every climate from the tropics to the arctic. The plants, which generally grow in moist places, have roots and ribbed green stems, the surface of which is impregnated with silica crystals. Their abrasive texture made them useful in former times for scouring, hence their common name. Most species have numerous whorled branches that lend the plant a plumed or feathery appearance, thus giving rise to their other common name, horsetail. The scalelike nonphotosynthetic leaves are joined together to form a fringed whorl that encircles the stem at regular intervals; the green stems and branches are the photosynthetic organs. The stem has no cambium or secondary growth. It consists of a silica-impregnated epidermis, a cortex, and a central structure called a stele that contains a ring of vascular bundles, consisting of xylem and phloem. The conspicuous plant form of Equisetum, which may be more than 3 ft (1 m) high in some species, represents the diploid sporophyte generation. A cone, or strobilus, at the apex of the sporophyte stem bears spore-producing structures. Upon germination, the spores produce a green, frilled, thumbnail-sized haploid plant form, the gametophyte; specialized structures on the mature gametophyte, the archegonia and antheridia, produce, respectively, eggs and sperms. As in mosses, the sperm swims to the egg through a film of water, attracted by specific chemical substances. A zygote, formed as the result of fertilization, develops into green sporophytes to complete the life cycle. The order Calamitales contains plants known only from fossil remains so abundant in coals and associated shales from the Carboniferous period that it is assumed that they formed a major part of the vegetation that later became compressed into coal. The plants of the genus Calamites may have reached a height of 100 ft (30 m).
Rhyniophyta
The division of plants known only from fossils, of which the genus Rhynia was perhaps the most important. These plants date from the Silurian and Devonian age. Relatively simple in structure, they resemble the Psilotophyta in many features, such as the lack of clearly developed roots. Like modern higher plants the Rhyniophyta had the specialized conducting tissues xylem and phloem. The Rhyniophyta are the most primitive group of vascular plants so far known and appear to be ancestral to most of the major divisions of vascular plants.
Importance of Plants
Plants are essential to the balance of nature and in people's lives. Green plants, i.e., those possessing chlorophyll, manufacture their own food and give off oxygen in the process called photosynthesis, in which water and carbon dioxide are combined by the energy of light. Plants are the ultimate source of food and metabolic energy for nearly all animals, which cannot manufacture their own food. Besides foods (e.g., grains, fruits, and vegetables), plant products vital to humans include wood and wood products, fibers, drugs, oils, latex, pigments, and resins. Coal and petroleum are fossil substances of plant origin. Thus plants provide people not only sustenance but shelter, clothing, medicines, fuels, and the raw materials from which innumerable other products are made.
Plant Studies
The scientific study of plants is called botany; the study of their relationship to their environment and of their distribution is plant ecology. The cultivation of plants for food and for decoration is horticulture.