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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 (see mitosis).
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 liverworts and mosses. 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 (see Polypodiophyta), the cone-bearing gymnosperms (see Pinophyta), and the angiosperms, or true flowering plants (see Magnoliophyta). 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.
Importance of Plants
The Plantae share the characteristics of multicellularity, cellulose cell walls, and photosynthesis using chlorophylls a and b (except for a few plants that are secondarily heterotrophic). Most plants are also structurally differentiated, usually having organs specialized for anchorage, support, and photosynthesis. Tissue specialization for photosynthetic, conducting, and covering functions is also characteristic. Plants have a sporic (rather than gametic or zygotic) life cycle that involves both sporophytic and gametophytic phases, although the latter is evolutionarily reduced in the majority of species. Reproduction is sexual, but diversification of breeding systems is a prominent feature of many plant groups. See Photosynthesis, Reproduction (plant)
A conservative estimate of the number of described species of plants is 250,000. There are possibly two or three times that many species as yet undiscovered, primarily in the Southern Hemisphere. Plants are categorized into nonvascular and vascular groups, and the latter into seedless vascular plants and seed plants. The nonvascular plants include the liverworts, hornworts, and mosses. The vascular plants without seeds are the ground pines, horsetails, ferns, and whisk ferns; seed plants include cycads, ginkgos, conifers, gnetophytes, and flowering plants. Each of these groups constitutes a division in botanical nomenclature, which is equivalent to a phylum in the zoological system. See Plant taxonomy