The problem of the structure of reserve starch granules in plant storage organs such as seeds, bulbs, tubers, and so on, is essentially a genetic problem. Each plant species produces its own type of starch granule, characterized by shape, size, chemical composition, and strength of intermolecular bonding. Although the starch granules in higher plants are produced in plastids (amyloplasts) , the characteristics mentioned are controlled by the nucleus of the cell.A characteristic of amyloplasts, whether colorless (as in potato) or green (as in Pellionia), is that they are dependent on an external supply of sugars for starch synthesis, in contrast to chloroplasts, which contain a photosynthetic apparatus. The small starch granules that are the final product of the chloroplasts of most plants have a shape that is dictated by the space between thylakoids and therefore show no genetic characteristics.In potatoes, 14C is seen to travel through the tuber, then enter first the cytoplasm and subsequently the amyloplasts.1 In corn, it was recently demonstrated that the sugar of transport, sucrose, is first hydrolyzed to glucose and fructose, which travel through the endosperm before being resynthesized to sucrose for its conversion to starch.' We do not know where this resynthesis occurs, but recent methods developed for the isolation of leucoplasts from roots 3 open the possibility of investigating the location of sucrose synthetase.In the small leucoplasts of roots, or the proplastids that will develop into chloroplasts, the reserve starch granules are deposited only temporarily; they remain small and do not show genetically determined characteristic features. In general, temporary starch deposits cannot be used as taxonomic characteristics.The conversion of sucrose into starch takes place in the stroma (the ground substance of the amyloplasts). The stroma must therefore contain the necessary enzymes. Most of these are soluble, and are not present in membrane structures. This is demonstrated by the following observations: ( 1 ) starch granules originate near the periphery of the plastid, but show the highest rate of growth where they meet the bulk of the stroma, that is, away from the enveloping double membrane; (2) the starch granule continues to grow for some time, even when the amyloplast is pushed into the vacuole; and (3) starch granules can develop in the absence of membrane structures inside the stroma, and such structures, when present, act as obstacles to the growth of the granule.GWe may therefore assume that all processes of starch synthesis take place in the stroma of the plastid. This stroma is a complicated aqueous mixture. It contains the enzymes and their substrates, sugars, proteins, fatty substances, and salts; all of these substances are known to have an influence on the formation of starch molecules and on their association into a paracrystalline pattern.' It probably has a high viscosity, and it is in this mixture that the
