More than 250,000 known species belong to the kingdom Plantae, including trees, vegetables, mosses, flowers and ferns. All plant life on the planet belongs to this kingdom, including every fruit, nut, seed and grain humans and other animals rely on for food.
All plants fall into one of two categories: terrestrial and aquatic. It is thought that all plants originated from a species of green algae that existed approximately 1.6 billion years ago.
One feature that all plants share in common to some extent is their green color, which is the result of a green chemical inside the plant called chlorophyll. Chlorophyll makes plants unique, as this substance allows plants to capture energy from sunlight, which they then use to produce energy in a process known as photosynthesis.
Plants also need water and carbon dioxide to perform photosynthesis; they usually take in these substances through microscopic holes on their leaves. During the process of photosynthesis, one of the byproducts is oxygen, which the plants then release into the atmosphere through these same small holes. As a result, plants are the primary source of the oxygen that humans and other animals breathe, and the planet would otherwise be uninhabitable without it.