The endosperm serves as a source of nutrition for a plant embryo in a seed, somewhat analogous to the yolk of an egg for an animal embryo. It is actually, in a sense, a separate organism, with a genetic code from the combination of two different cells than the gametes that created the embryo. The endosperm is of crucial economic and nutritional performance for the vast majority of humanity.
The endosperm is a type of energy and nutrient storage for a growing embryo in a plant seed. Since the seed does not start with leaves or roots, it can neither gather energy from the sun nor nutrients from the soil. Thus, the first part of its growth is fueled entirely by the endosperm. The endosperm is solely a feature of flowering plants, or angiosperms. Other types of plant use different strategies to start their seeds out in life.
Humans make use of the endosperms of various plants, and these plants constitute the majority of calories consumed by modern humans. The majority of any grain humans eat is endosperm tissue. Flour is made from ground wheat endosperm, and white rice is the pure starchy endosperm of rice seeds. All the edible tissue of a coconut, including the liquid at the center, is endosperm tissue.