green plants can convert solar energy intro useable forms of chemical energy and humans derive all of their conversion. Dependence on green plants for our food supply means dependence on seeds because seed are used to reproduce crops and are frequently harvested for food and feed.Plant species cultivated by humankind are used in many different ways, for some, fresh fruits are harvested, in other cases, roots or tubers represent the economic yield.The entire above-ground plant is harvested in some vegetable or forage crops wheras immature fruits or seed represent the economic yield for some vegetable crops.But the crops plant making the most important contribution, by far, to world's food supply are those harvested at maturity for their seed.