What is a plant?
Plants are photosynthetic living organisms encompassed by a predominately a cellulose-based cell wall.
Historically, plants encompassed all living things that were not animals such as algae and fungi. However, all current definitions of plant exclude the fungi and some algae, as well as the prokaryotes (the archaea and bacteria).
Plants encompass all flowering plants, conifers and other gymnosperms, ferns and their allies, hornworts, liverworts, mosses, and the green algae, but excludes the red and brown algae. The vast majority of plants are multicellular organisms.
Green plants obtain most of their energy from sunlight through the process of photosynthesis via chloroplasts that are derived from endosymbiosis with cyanobacteria.
Their chloroplasts contain chlorophylls a and b, which gives them their green colour. Green being the colour of light that plants reflect and not use for energy production. Plants instead use blue and red light for their growth.
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 also characterised by sexual reproduction and alternation of generations, although asexual reproduction (i.e. clones).
There are about 391,000 species of plants, of which the great majority, some 260–290 thousand, produce fruit and seeds.
Plants provide a substantial proportion of the world's oxygen that we breath, 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.