Plants are multicellular, mostly photosynthetic eukaryotes that also have cell walls composed of cellulose, do not have a central nervous system, are generally non-motile, and reproduce sexually, often by alternation of phases of a single generation (Alternation of generations). This kingdom includes familiar organisms such as trees, shrubs, herbs, and ferns. Over 350,000 species of plants have been estimated to exist. As of 2004, some 287,655 species had been identified, of which 258,650 are flowering plants.
In addition to plants' central ecological role—photosynthesis and carbon fixation by plants is the ultimate source of energy and organic material for nearly all ecosystems, and plants are primary producers of atmospheric oxygen—plants provide to humans vital nutritional and economic values. Indeed, the human diet is centered on plants, whether directly via grains, fruits, vegetables, legumes, and so forth, or indirectly via animals that consume or pollinate plants. Plants also provide valuable products, such as lumber, paper, and medicines. Beyond these external values, plants also touch upon the inner nature of people by providing aesthetic value and joy, such as their use in landscaping, decoration, and works of art, as well as via the smells and sights of flowers and the rich tastes of fruits.
Aristotle divided all living things between plants, which generally do not move or have sensory organs, and animalsexhibiting sensory movement and motility. In Carolus Linnaeus' system, these became the Kingdoms Vegetabilia (later Plantae) and Animalia. Since then, it has become clear that the Plantae as originally defined included several unrelated groups, and the fungi and several groups of algae were removed to new kingdoms. However, these are still often considered plants in many contexts. Indeed, any attempt to match "plant" with a single taxon is doomed to fail, because plant is a vaguely defined concept unrelated to the presumed phylogenic concepts on which modern taxonomy is based.