Starch is a polymeric carbohydrate consisting of a large number of glucose unitsjoined by glycosidic bonds.
Formation of starch grains is under genetic control, and distinctive grain shape, location and shape of the hilum region, and other traits produce diagnostic granules in many species, which have intimate connection of ancient plant diet.
Starch grains are composed of amylose and amylopectin, and grow by accretion of layers of these polymers around a nucleation point, or hilum. The layers are visible microscopically in many starches. The crossing point of the extinction cross is located at the hilum. These characteristics-presence of the hilum, layering, and the extinction cross-are typically sufficient to identify a particle as starch.