plant, any organism of the plant kingdom, as opposed to one of the animal animal, any member of the animal kingdom (kingdom Animalia), as distinguished from organisms of the plant kingdom (kingdom Plantae) and the kingdoms Fungi , Protista , and Monera in the five-kingdom system of classification.
..... Click the link for more information. kingdom or of the kingdoms Fungi Fungi (fŭn`jī), kingdom of heterotrophic single-celled, multinucleated, or multicellular organisms, including yeasts, molds, and
..... Click the link for more information. , Protista Protista (prōtĭs`tə) or Protoctista
..... Click the link for more information. , or Monera Monera, taxonomic kingdom that comprises the prokaryotes ( bacteria and cyanobacteria ). Prokaryotes are single-celled organisms that lack a membrane-bound nucleus and usually lack membrane-bound organelles (mitochondria, chloroplasts; see cell , in biology).
..... Click the link for more information. 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 cellulose, chief constituent of the cell walls of plants. Chemically, it is a carbohydrate that is a high molecular weight polysaccharide. Raw cotton is composed of 91% pure cellulose; other important natural sources are flax, hemp, jute, straw, and wood.
..... Click the link for more information. . 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 cell, in biology, the unit of structure and function of which all plants and animals are composed. The cell is the smallest unit in the living organism that is capable of integrating the essential life processes. There are many unicellular organisms, e.g.
..... Click the link for more information. and in certain details of reproduction (see mitosis interphase the chromosomes are dispersed in the nucleus and appear as a network of long, thin threads or filaments, called the chromatin. At some point before prophase begins, the chromosomes replicate themselves to form pairs of identical sister chromosomes, or chromatids; the
..... Click the link for more information. ).

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 taxonomy, the study of the relationships of organisms, which includes collection, preservation, and study of specimens, and analysis of data provided by various areas of biological research.
..... Click the link for more information. 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 reproduction, capacity of all living systems to give rise to new systems similar to themselves. The term reproduction may refer to this power of self-duplication of a single cell or a multicellular animal or plant organism.
..... Click the link for more information. ). 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 algae (ăl`jē) [plural of Lat. alga=seaweed], a large and diverse group of primarily aquatic plantlike organisms.
..... Click the link for more information. and fungi (once considered plants). The subkingdom Embryobionta is composed of two groups: the bryophytes (liverwort liverwort, any plant of the class Marchantiopsida. Mosses and liverworts together comprise the division Bryophyta , primitive green land plants (see moss ; plant ); some of the earliest land plants resembled modern liverworts.
..... Click the link for more information. and moss 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.
..... Click the link for more information. ), division Bryophyta Bryophyta (brī`əfī'tə, brī'əfī`tə)
..... Click the link for more information. , 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 Psilotophyta (sīlō'tŏf`ətə), division of vascular plants consisting of only two genera, Psilotum and
..... Click the link for more information. , with only three living species; the Lycopodiophyta Lycopodiophyta (lī'kōpō'dēŏf`ətə)
..... Click the link for more information. (club mosses); and the Equisetophyta Equisetophyta (ĕk'wəsətŏf`ətə)
..... Click the link for more information. (horsetails). All the plants of a fourth subdivision, the Rhyniophyta Rhyniophyta (rī'nēŏf`ətə), division of plants known only from fossils, of which the genus Rhynia
..... Click the link for more information. , are extinct. The remaining divisions include the dominant vegetation of the earth today: the ferns (see Polypodiophyta Polypodiophyta (pŏl'ēpō'dēŏf`ətə)
..... Click the link for more information. ), the cone-bearing gymnosperms (see Pinophyta Pinophyta (pī'nŏf`ətə), division of the plant kingdom consisting of those organisms commonly called gymnosperms.
..... Click the link for more information. ), and the angiosperms, or true flowering plants (see Magnoliophyta Magnoliophyta (măg'nōlēŏf`ətə)
..... Click the link for more information. ). 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 conifer (kŏn`ĭfûr) [Lat.,=cone-bearing], tree or shrub of the order Coniferales, e.g.
..... Click the link for more information. , the ginkgo ginkgo (gĭng`kō) or maidenhair tree, tall, slender, picturesque deciduous tree (Ginkgo biloba
..... Click the link for more information. , and the cycad cycad (sī`kăd), any plant of the order Cycadales, tropical and subtropical palmlike evergreens.
..... Click the link for more information. . The angiosperms are separated into the monocotyledonous plants—usually with one cotyledon per seed seed, fertilized and ripened ovule, consisting of the plant embryo, varying amounts of stored food material, and a protective outer seed coat. Seeds are frequently confused with the fruit enclosing them in flowering plants as in the grains and nuts.
..... Click the link for more information. , scattered vascular bundles in the stem stem, supporting structure of a plant, serving also to conduct and to store food materials. The stems of herbaceous and of woody plants differ: those of herbaceous plants are usually green and pliant and are covered by a thin epidermis instead of by the bark of woody
..... Click the link for more information. , little or no cambium cambium (kăm`bēəm), thin layer of generative tissue lying between the bark and the wood of a stem, most active in woody plants.
..... Click the link for more information. , and parallel veins in the leaf leaf, chief food-manufacturing organ of a plant, a lateral outgrowth of the growing point of stem. The typical leaf consists of a stalk (the petiole) and a blade—the thin, flat, expanded portion (needlelike in most conifers) that is normally green in color
..... Click the link for more information. —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 fruit, matured ovary of the pistil of a flower, containing the seed . After the egg nucleus, or ovum, has been fertilized (see fertilization ) and the embryo plantlet begins to form, the surrounding ovule (see pistil ) develops into a seed and the ovary wall
..... Click the link for more information. .

Importance of Plants

Plants are essential to the balance of nature and in people's lives. Green plants, i.e., those possessing chlorophyll chlorophyll (klôr`əfĭl')
..... Click the link for more information. , manufacture their own food and give off oxygen in the process called photosynthesis photosynthesis (fō'tōsĭn`thəsĭs)
..... Click the link for more information. , 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 wood, botanically, the xylem tissue that forms the bulk of the stem of a woody plant. Xylem conducts sap upward from the roots to the leaves, stores food in the form of complex carbohydrates, and provides support; it is made up of various types of cells specialized
..... Click the link for more information. 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 botany, science devoted to the study of plants. Botany, microbiology, and zoology together compose the science of biology . Humanity's earliest concern with plants was with their practical uses, i.e., for fuel, clothing, shelter, and, particularly, food and drugs.
..... Click the link for more information. ; the study of their relationship to their environment and of their distribution is plant ecology ecology, study of the relationships of organisms to their physical environment and to one another. The study of an individual organism or a single species is termed autecology; the study of groups of organisms is called synecology.
..... Click the link for more information. . The cultivation of plants for food and for decoration is horticulture horticulture [Lat. hortus=garden], science and art of gardening and of cultivating fruits, vegetables, flowers, and ornamental plants. Horticulture generally refers to small-scale gardening, and agriculture to the growing of field crops, usually on a large
..... Click the link for more information. . For specific approaches to the study of plants and animals, see biology biology, the science that deals with living things. It is broadly divided into zoology , the study of animal life, and botany , the study of plant life. Subdivisions of each of these sciences include cytology (the study of cells), histology (the study of tissues),
..... Click the link for more information. .

plant

Any organism in the kingdom Plantae, consisting of multicellular, eukaryotic life forms (see eukaryote) with six fundamental characteristics: photosynthesis as the almost exclusive mode of nutrition, essentially unlimited growth at meristems, cells that contain cellulose in their walls and are therefore somewhat rigid, the absence of organs of movement, the absence of sensory and nervous systems, and life histories that show alternation of generations. No definition of the kingdom completely excludes all nonplant organisms or even includes all plants. Many plants, for example, are not green and thus do not produce their own food by photosynthesis, being instead parasitic on other living plants (see parasitism). Others obtain their food from dead organic matter. Many animals possess plantlike characteristics, such as a lack of mobility (e.g., sponges) or the presence of a plantlike growth form (e.g., some corals and bryozoans), but in general such animals lack other plant characteristics. Some past classification systems (see taxonomy) placed difficult groups such as protozoans, bacteria, algae, slime molds, and fungi (see fungus) in the plant kingdom, but structural and functional differences between these organisms and plants have convinced most scientists to classify them elsewhere.

A building or complex or a designated area. See outside plant and inside plant.

Plant

An organism that belongs to the Kingdom Plantae (plant kingdom) in biological classification. The study of plants is called botany. See Botany, Classification, biological

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