The outermost layer is formed by the fusion of the fruit-wall and seed-coat. The endosperm constitutes the upper one-third to the three-quarters of the grain. The endosperm has thin outer aleurone layer and and an inner part containing starch grains. The aleurone layer consists of proteins and fats.
A shield-shaped single cotyledon is known as the scutellum.It is separated form the endosperm by the epithelial layer. The embryo is embedded in the scutellum. Its plumule is covered by a sheath called the coleoptile and the radicle by the coleorhiz.
The External Structure
The whole seed is covered by a single layer, the seed-coat which is functioning to reduce evaporation from the seed and to protect it against diseases and mechanical injuries. Near the middle of the convex edge of the seed, there is a large oval scar called the hilium where food and water from the fruit-wall is supplied to the young seed.