[plant, plahnt]
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 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 (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

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. For specific approaches to the study of plants and animals, see biology.

African plant of genus Chlorophytum (lily family). This popular houseplant has long, narrow, grassy green-and-white-striped leaves. Periodically a flower stem emerges, and tiny white flowers (not always produced) are replaced by young plantlets, which can then be detached and rooted.

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Either of two plants in the pea family (see legume) that close up their leaves and droop when touched. This unusually quick response is due to rapid water release from specialized cells at the bases of leaftsalks. The more common plant is Mimosa pudica (see mimosa). Spiny and shrubby, with fernlike leaves and small, globular, mauve flower puffs, it grows about 1 ft (30 cm) high as a widespread tropical weed and a greenhouse curiosity. Wild sensitive plant (Cassia nictitans) is less sensitive to touch; a larger plant, 20 in. (50 cm) high, it is native to the eastern U.S. and the West Indies.

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or spermatophyte

Any of the flowering plants (angiosperms) and the conifers and related plants (gymnosperms). Seed plants share many features with ferns, including the presence of vascular tissue (see xylem and phloem), but unlike ferns, they have stems that branch sideways and vascular tissue that is arranged in strands (bundles) around the core. Seed plants have generally more complex plant bodies and reproduce via seeds. As the main dispersal unit of seed plants, the seed represents a significant improvement over the spore, with its limited capacity for survival. Seed plants also differ from ferns in having gametophytes that are reduced in size and are embedded in the sporophytes (and thus are less vulnerable to environmental stress). Another land-based adaptation of seed plants is pollen dispersed by wind or animals. The dispersal of pollen, in addition to dispersal of seeds, promotes genetic recombination and distribution of the species over a wide geographic area.

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or India rubber plant

Tropical tree (Ficus elastica) of the mulberry family. The rubber plant is large in its native Southeast Asia and other warm areas; elsewhere it is commonly grown indoors as a potted plant. The plant has large, thick, oblong leaves and pairs of figlike fruits along its branches. The milky sap, or latex, was once an important source of an inferior natural rubber. Young plants available in the florist's trade are durable and grow well under less-than-ideal indoor conditions. Some cultivated varieties have broader, darker green leaves; others are variegated. Seealso rubber tree.

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Any of various viruses that can cause plant disease (e.g., the tobacco mosaic virus). Plant viruses are economically important because many of them infect crop and ornamental plants. Numerous plant viruses are rodlike and can be extracted readily from plant tissue and crystallized. Most lack the fatty membrane found in many animal viruses, and all contain RNA. Plant viruses are transmitted in various ways, most importantly through insect bites, mainly by aphids and plant hoppers. Symptoms of virus infection include colour changes, dwarfing, and tissue distortion. The appearance of streaks of colour in certain tulips is caused by a virus. Seealso reovirus.

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Any of several species of sapsucking, soft-bodied insects (order Homoptera) that are about the size of a pinhead, with tubelike projections on the abdomen. Serious plant pests, they stunt plant growth, produce plant galls, transmit plant viral diseases, and deform leaves, buds, and flowers. Ants may take care of aphids, protecting them from weather and natural enemies and transferring them from wilted to healthy plants. The ants in turn obtain honeydew, a sweet product excreted by aphids, which the ants retrieve by “milking” the aphids (stroking their abdomens).

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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.

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Yellow pitcher plant (Sarracenia flava).

Any carnivorous plant with pitcher-, trumpet-, or urn-shaped leaves. Several families include pitcher plants: Nepenthaceae (Old World pitcher plants), Cephalotaceae, Asclepiadaceae (milkweed family), and especially Sarraceniaceae (New World pitcher plants, particularly those in the eastern North American genus Sarracenia). Pitcher plants inhabit bogs, swamps, wet or sandy meadows, or savannas where the soils are water-saturated, acidic, and deficient in nitrates or phosphates. Their unusual tubular leaves have a series of nectar-secreting glands that extend from the lip down into the interior and attract insects. Once in the plant, the prey tumbles down into a liquid pool and drowns, after which an enzyme secreted within the leaf digests it, releasing nitrates and other nutrients, which supplement the meager nutrient supply of bogs. Most pitcher plants produce pitcher-shaped, insect-catching leaves in the spring and tubeless leaves in the fall. Their flowers are showy and have an agreeable scent.

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or oyster plant or vegetable oyster

Flower of goatsbeard (Tragopogon pratensis)

Biennial herbaceous plant (Tragopogon porrifolius) of the composite family, native to the Mediterranean. The thick white taproot is cooked as a vegetable and tastes somewhat like oysters. The plant has purple flowers and narrow leaves whose bases usually clasp the stem. Goatsbeard, or meadow salsify (T. pratensis), is a weedy European species, naturalized in North America, that has a large yellow flower head. It is occasionally cultivated as an ornamental, and its leaves, flowers, and roots are sometimes eaten in salads.

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Any of about 400 diverse species of plants specially adapted for capturing insects and other tiny animals by ingenious pitfalls and traps and for digesting the nitrogen-rich animal proteins to obtain nutrients. These adaptations are thought to enable such plants to survive under otherwise marginal or hostile environmental conditions. The conspicuous trapping mechanism (a leaf modification) draws the prey's attention to the plant. More than half the species belong to the family Lentibulariaceae, most being bladderworts. The remainder belong to several families composed of the pitcher plants, sundews, and flytraps (see Venus's-flytrap). Most are found in damp heaths, bogs, swamps, and muddy or sandy shores where water is abundant and where nitrogenous materials are often scarce or unavailable because of acid or other unfavourable soil conditions. The smallest Drosera species are often hidden among the moss of a sphagnum bog; most carnivorous plants are small herbaceous perennials. Some become large shrubby vines.

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Any shrub or herb in the genus Indigofera of the pea family (see legume). Most occur in warm climates and are silky or hairy. The leaves are usually divided into smaller leaflets. Small rose, purple, or white flowers are borne in spikes or clusters. The fruit is a pod. Some species, particularly I. sumatrana and I. arrecta, were once an important source of indigo dye, a deep navy blue.

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Plant adapted for growing indoors, commonly a member of a species that flourishes naturally only in warm climates. Two factors contribute to the success of the huge number of species grown as houseplants: they must be easy to care for, and they must be able to tolerate the fairly low levels of light and humidity found in most homes. Houseplants are selected for their foliage or flowers or both. Aspects of a houseplant's environment that must be managed include light, temperature and humidity, soil, water, and nutrients.

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Nongreen herbaceous plant (Monotropa uniflora) that is saprophytic (living on the remains of dead plants). Clusters grow in moist, shady, wooded areas of North America and Asia. The entire plant is white or grayish, occasionally pink, and turns black as it dries out. A single odourless, cup-shaped flower droops from the tip of a stalk 6–10 in. (15–25 cm) tall. The leaves, which lack chlorophyll and do not perform photosynthesis, are small scales. The name reflects the resemblance of this plant to a miniature Indian peace pipe with its stem stuck in the ground.

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Castor-oil plant (Ricinus communis).

Large plant (Ricinus communis) of the spurge family, probably native to Africa and naturalized throughout the tropics. It is grown commercially for the pharmaceutical and industrial uses of its oil and for use in landscape gardening because of its handsome, giant, fanlike leaves. The bristly, spined, bronze-to-red clusters of fruits are attractive but are often removed before they mature because of the poison concentrated in their mottled, beanlike seeds. There are hundreds of natural forms and many horticultural varieties of this species.

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Any plant that grows upon or is attached to another plant or object merely for physical support. Epiphytes are found mostly in the tropics and are also known as air plants because they have no attachment to the ground or other obvious nutrient source. They obtain water and minerals from rain and from debris on the supporting plants. Orchids, ferns, and members of the pineapple family are common tropical epiphytes. Lichens, mosses, liverworts, and algae are epiphytes of temperate regions.

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