The outermost protein layer of the endosperm of grass fruits (e.g. cereal grains) is aleurone. At germination the embryo produces gibberellin, which stimulates the aleurone layer to synthesise enzymes, especially a-amylase. The latter causes hydrolysis of the starch in the endosperm. The enzymes are synthesised from the amino acids supplied by breakdown o f aleurone grains.