Endosperm is largely a form of starch that acts as a food source to keep seeds alive during their dormant state, after plant fertilization, but before the seeds have a chance to germinate. It is a form of plant tissue that surrounds a seed's embryo, and is produced by virtually all types of flowering plants. Seeds can exist for long periods of time as viable offspring of a plant as long as they have sufficient endosperm to fuel very slow metabolic processes for the embryo. Much of the human diet that relies on ground seed, such as that obtained from grains, is based on the nutritional value of endosperm in the grain. Corn endosperm, for example, is either ground into flour, or consumed directly in the form of popcorn.
Seed endosperm is a direct result of the fertilization process in plants. As a male gamete fuses with two female nuclei in the embryo sac, it produces endosperm which has a triploid nature, meaning that it has a complete set of three chromosomes. This gives it a diversity of genetic and nutritional value where it contains essential protein, fats, and starch that both plants and animals rely on for health. The endosperm is divided up into three regions: the aleurone, which is a thin border region that helps the seed to break down starch for growth as it germinates; the transfer layer, which serves as the interface with the plant itself for absorbing nutritional elements; and the bulk of the seed, which is the internal starchy layer.
Plants use endosperm at different rates, with some vegetables such as peas and beans entirely consuming it to reach maturity, and others, such as wheat endosperm and coconuts, retaining it longer, which makes them valuable food sources for the human diet. The process of producing and utilizing endosperm to keep seed viable can be so efficient that some rare cases of stored seed have remained capable of germinating for extremely long periods of time. Seed from a date palm plant carbon-dated to 2,000 years ago was germinated in Israel in 2005, which came from the palace of Herod the Great who ruled Judea in the region of Israel from the year 37 to 4 BCE. Seeds of a type of waterlily or lotus found buried in a layer of peat in Japan have also been proven to be viable. Most of those tested germinated after being removed from museum exhibits, and were carbon-dated to being between 830 and 1,250 years old.