There are about 300–315 thousand species of plants, of which the great majority, some 260–290 thousand, are seed plants . Green plants provide most of the world's molecular oxygen and are the basis of most of the earth's ecologies, especially on land. Plants that produce grains, fruits and vegetables form humankind's basic foodstuffs, and have been domesticated for millennia. Plants play many roles in culture. They are used as ornaments and, until recently and in great variety, they have served as the source of most medicines and drugs. The scientific study of plants is known as botany, a branch of biology. (Wikipedia)