Plants are mainly multicellular, predominantly photosynthetic eukaryotes of the kingdom Plantae. They form the clade Viridiplantae (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). Green plants provide a substantial proportion of the world's molecular oxygen 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.
All living things were traditionally placed into one of two groups, plants and animals. This classification may date from Aristotle (384 BC – 322 BC), who made the distincton between plants, which generally do not move, and animals, which often are mobile to catch their food. Much later, when Linnaeus (1707–1778) created the basis of the modern system of scientific classification, these two groups became the kingdoms Vegetabilia (later Metaphyta or Plantae) and Animalia (also called Metazoa). Since then, it has become clear that the plant kingdom as originally defined included several unrelated groups, and the fungi and several groups of algae were removed to new kingdoms. However, these organisms are still often considered plants, particularly in popular contexts.
The term "plant" generally implies the possession of the following traits: multicellularity, possession of cell walls containing cellulose and the ability to carry out photosynthesis with primary chloroplasts.
When the name Plantae or plant is applied to a specific group of organisms or taxon, it usually refers to one of four concepts. From least to most inclusive, these four groupings are:
Another way of looking at the relationships between the different groups that have been called "plants" is through a cladogram, which shows their evolutionary relationships. These are not yet completely settled, but one accepted relationship between the three groups described above is shown below. Those which have been called "plants" are in bold.