Plants are eukaryotes, predominantly photosynthetic, that form the kingdom Plantae. Many are multicellular. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi. All current definitions exclude the fungi and some of the algae. By one definition, plants form the clade Viridiplantae (Latin for "green plants") which consists of the green algae and the embryophytes or land plants. The latter include hornworts, liverworts, mosses, lycophytes, ferns, conifers and other gymnosperms, and flowering plants. A definition based on genomes includes the Viridiplantae, along with the red algae and the glaucophytes, in the clade Archaeplastida.
Green plants, ranging in size from microscopic to some 100 metres tall, obtain most of their energy from sunlight, using chloroplasts derived from endosymbiosis with cyanobacteria. Chloroplasts perform photosynthesis, making sugars from carbon dioxide and water, using the pigment chlorophyll, which gives them their green colour. Some plants are parasitic, deriving their energy from other plants, and have lost the ability to produce normal amounts of chlorophyll or to photosynthesize. Plants are characterized by sexual reproduction and alternation of generations, but asexual reproduction is also common.
There are about 380,000 known species of plants, of which the majority, some 283,000, produce seeds. Green plants provide a substantial proportion of the world's molecular oxygen and are the basis of most of Earth's ecosystems. Grain, fruit, and vegetables are basic human foods and have been domesticated for millennia. Plants have many cultural and other uses, such as ornaments, building materials, writing materials, and, in great variety, they have been the source of medicines. The scientific study of plants is known as botany, a branch of biology.
All living things were traditionally placed into one of two groups, plants and animals.