Plants are predominantly photosynthetic eukaryotes of the
kingdom Plantae. Historically, the plant kingdom encompassed all living
things that were not animals, and included algae and fungi; however, all
current definitions of Plantae exclude the fungi and some algae, as well
as the prokaryotes (the archaea and bacteria). By one definition, plants
form the clade Viridiplantae (Latin name for "green plants"), a group that
includes the flowering plants, conifers and other gymnosperms, ferns and
their allies, hornworts, liverworts, mosses, and the green algae, but
excludes the red and brown algae.
Most plants are multicellular organisms. Green plants obtain most of their
energy from sunlight via photosynthesis by primary chloroplasts that are
derived from endosymbiosis with cyanobacteria. Their chloroplasts contain
chlorophylls a and b, which gives them their green color. Some plants are
parasitic or mycotrophic and have lost the ability to produce normal
amounts of chlorophyll or to photosynthesize, but still have flowers,
fruits, and seeds. Plants are characterized by sexual reproduction and
alternation of generations, although asexual reproduction is also common.
There are about 320,000 species of plants, of which the great majority,
some 260–290 thousand, produce seeds. Green plants provide a substantial
proportion of the world's molecular oxygen and are the basis of most of
Earth's ecosystems. Plants that produce grain, fruit and vegetables also
form basic human foods and have been domesticated for millennia. Plants
have many cultural and other uses, as ornaments, building materials,
writing material and, in great variety, they have been the source of
medicines and psychoactive drugs. The scientific study of plants is known
as botany, a branch of biology.