Endosperm

Endosperm is the albumin tissue produced in the seeds of most flowering plants around the time of fertilization. It surrounds the embryo and provides nutrition in the form of starch, though it can also contain oils and protein. This makes endosperm an important source of nutrition in human diet. For example, wheat endosperm is ground into flour for bread (the rest of the grain is included as well in whole wheat flour), while barley endosperm is the main source for beer production. Other examples for edible endosperm are coconut "meat", popcorn and banana.



Double fertilization
Endosperm is formed when the two sperm nuclei inside a pollen grain reach the interior of an embryo sac or female gametophyte. One sperm nucleus fertilizes the egg, forming a zygote, while the other sperm nucleus usually fuses with the two female polar nuclei at the center of the embryo sac, creating endosperm (double fertilization). Thus endosperm cells are usually triploid (containing three sets of chromosomes) but can vary widely from diploid (2n) to 15n. 

Endosperm formation
There are two different types of endosperm formation, the nuclear (or liquid endosperm) type, where formation of cell wall is delayed for a number of cell divisions, and the cellular, where cell wall formation is initiated instantly. The nuclear type is the most common one in angiosperms. Sweet corn is picked for eating at the tender liquid endosperm stage, before cell walls have formed and the sugars have been converted to starch. The 'milk' of the coconut is a liquid endosperm.

The role of endosperm in seed development
In some species (e.g. grains) the endosperm persists to the mature seed stage as a storage tissue, and in others it is absorbed during embryo development (e.g. common bean, Phaseolus vulgaris) and the function of storage tissue is performed by enlarged seed leaves (cotyledons). In certain species (e.g. corn, Zea mays) the storage function is distributed between both endosperm and the embryo. Some mature endosperm tissues store fats (e.g. castor bean, Ricinis communis) and others (including grains, such as wheat and corn) store mainly starches. The dust-like seeds of orchids have no endosperm. Orchid seedlings are mycoheterotrophic in their early development. In some other species, such as coffee, the endosperm also does not develop. Instead the nucellus produces a nutritive tissue termed perisperm.

Cereal grains
Cereal crops are grown for their palatable fruit (grains or caryopsis), which are primarily endosperm. In the caryopsis, the thin fruit wall is fused to the seed coat. Therefore, the nutritious part of the grain is the seed and its endosperm. In some cases (e.g. wheat, rice) the endosperm is selectively retained in food processing (as in white flour), and the embryo and seed coat removed. Endosperm thus has an important role within the human diet, worldwide.

The aleurone is a maternal tissue that is retained as part of the seed in many small grains. The aleurone functions for both storage and digestion. During germination it secretes the amylase enzyme that breaks down endosperm starch into sugars to nourish the growing seedling.