Plants
Plants are mainly multicellular, predominantly photosynthetic eukaryotes of
the kingdom Plantae. They form the cladeViridiplantae (Latin
for "green plants") that includes the flowering
plants, conifers and
other gymnosperms, ferns, clubmosses, hornworts, liverworts, mosses and
the green
algae, and excludes the red and brown
algae. Historically, plants were treated as one of two kingdoms including
all living things that were not animals, and
all algae and fungi were
treated as plants. However, all current definitions of Plantae exclude the
fungi and some algae, as well as the prokaryotes (the archaea and bacteria).
Green plants
have cell
walls containing cellulose and 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 secondarily parasitic or mycotrophic and
may lose the ability to produce normal amounts of chlorophyll or to
photosynthesize. Plants are characterized by sexual reproduction and alternation of generations,
although asexual reproduction is also common.
There are
about 300–315 thousand species of plants, of which the great majority, some
260–290 thousand, are seed plants (see the table below).[5] Green
plants provide a substantial proportion of the world's molecular oxygen[6] and
are the basis of most of Earth's ecosystems, especially on land. Plants that
produce grain, fruit and vegetables form
humankind's basic foodstuffs, 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 drugs. The scientific
study of plants is known as botany, a branch of biology.