It is well known from light microscope studies of potato starch that the granules formed in a constant environment (of light and temperature) have a ring formation indistinguishable from that of granules formed under field conditions. Electron microscope studies have confirmed that normal potato starch granules have a fine shell structure not usually resolved by the light microscope, and also that shells do not develop in barley granules grown in a constant environment. The paper presented here reports a further study of the dependence of shell formation on environment. Potatoes were grown in a constant environment and starch granules from the newly formed tubers were examined in the light microscope, and in the electron microscope after corrosion by acid. No difference between these granules and normal granules was observed; both wide (light microscope) rings and fine lamellae developed in both granules. Parallel studies were made on wheat starch granules. In this case, shells were not differentiated in granules that developed in a constant environment, but they could be produced at will by imposing a dark period. Thus, shell formation in potato granules must be controlled by an endogenous rhythm, whereas in wheat granules it must be controlled by external environment.