Key Concepts
An organism that belongs to the kingdom Plantae in biological classification. A plant (see illustration) is a member of the taxonomic kingdom Plantae and is generally distinguished by the presence of chlorophyll, a rigid cell wall, and abundant, persistent, active embryonic tissue. It also lacks the power of locomotion. In the past, the kingdom Plantae comprised any organism that was not considered to be an animal; this meant that all fungi and algae, and even bacteria, were classified as members of the kingdom Plantae. Today, in the most commonly accepted modern classifications (the five- and six-kingdom systems), fungi, bacteria, and some algae belong to separate kingdoms rather than to the plant kingdom. However, depending on the exact classification scheme, certain algae are still considered to belong to the kingdom Plantae. In general, the study of plants is called plant biology or botany. See also: Algae; Biological classification; Botany; Cell walls (plant); Chlorophyll; Fungi; Plant kingdom; Plant taxonomy
Characteristics
Members of the kingdom 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 also: Cellulose; Photosynthesis; Plant anatomy; Plant breeding; Plant cell; Plant evolution; Plant physiology; Plant reproduction; Plant tissue systems
Diversity
A conservative estimate of the number of described species of plants is at least 390,000. There are also many thousands of species as yet undiscovered, including vast numbers of unknown species in the Southern Hemisphere. In general, the known species of plants are categorized into nonvascular and vascular groups, and the latter group is divided 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 (angiosperms). Each of these groups constitutes a division in botanical nomenclature, which is equivalent to a phylum in the zoological system. See also: Plant phylogeny; Speciation; Species concept; Taxonomic categories
Function
Plants are essential to the survival of virtually all living things. Through photosynthesis, plants provide all of the food for human and animal consumption. Plants also are the source of countless and diverse products, such as building materials, textile fibers, gums, resins, waxes, rubber, perfumes, dyes, and tanning materials. Most of the drugs that are used in modern medicine were originally derived from plants (although many drugs also came from fungi). Plants also provide habitats for wildlife and birds, offer shelter, and contribute to soil-building processes. Plants of the Carboniferous era provided the energy that is now used as oil, coal, and gas. Because of the importance of plants to the existence of humans, the preservation and study of plants are essential components of environmental studies. See also: Carboniferous; Soil ecology
