Endosperm is specialised storage tissue providing nutrients for the embryo when the seed germinates. It is also a source of carbohydrate for humans. In popcorn, Z. mays var. everta, the outer part of the endosperm is hard but the centre is soft. Sweetcorn, Z. mays var. It is grown in the US and Europe as a vegetable, as well as in Mexico and South Africa where it is used in brewing beer.
Sugars enter the developing seed and are converted to starch.
As the grain matures, excess water is removed leaving a hard, dry starch. In sweetcorn the process is not completed by the time the plant is harvested, leaving the grain moist and sweet. In popcorn the centre of the endosperm is still rich in water at the time of harvest, but the outside is hard. On heating, the water in the centre turns to steam and blows the grain open. No other crop has received such intensive genetic study, and hundreds of different varieties are grown throughout the world.