Leaf Starch: Its Isolation and Some of Its Properties

Our knowledge of starch has been gained largely from a study of the properties and composition of this substance as found in the storage organs of plants, such as seeds, tubers, and roots. Although its presence in leaves as a product of photosynthesis, that is autochthonic starch, has been known for a long time, and its formation, dissolution, and some of its properties in situ have been studied with great care by means of the microscope, little is known concerning the chemical properties of starch of this origin. The presence (or absence) of, starch in leaves has usually been recognized by means of the iodine test, and some endeavors have been made for its quantitative estimation on the basis of hydrolysis by means of amylase preparations or of acid, though these methods are usually subject to serious error. In the leaves of many species, on illumination, starch appears with extraordinary rapidity, but we still have little definite information concerning the first sugar which is formed in photosynthesis and from which presumably this starch is formed. There is usually a variety of sugars present in the leaf; in only relatively few cases have these been carefully investigated as to their structure. It has been difficult to account for the presence in plants of sugars, such as the pentoses, rhamnose, I-galactose, whether they are direct products of photosynthesis or are produced in subsequent metabolic reactions in the plant cells. In leaves the starch is present chiefly in the chloroplasts, and its normal appearance has therefore been associated with the photosynthetic activity of these organs. The starch granules of the chloroplasts do not have the characteristic structure of ordinary starch grains. The isolation of starch from the chloro-