When completely hydrolyzed, the polysaccharides yield only hexoses. They are, therefore, technically known as "hexosans." Each individual polysaccharide which has been studied thus far yields only a single hexose, although the particular hexose obtained varies in different cases. In fact, the polysaccharides are often classified according to the hexoses which they yield on hydrolysis, into the following groups: the dextrosans, which yield glucose, and include starch, dextrin, glycogen, lichenin, etc.; the levulosans, which yield fructose, and include inulin, graminin, triticin, etc.; the mannans; and the galactans. The more common representatives of each of these groups are discussed below. (A) THE DEXTROSANS These are by far the most common type of polysaccharides to be found in plants. =Starch.=--It is probable that no other single organic compound is so widely distributed in plants as is ordinary starch. It is produced in large quantities in green leaves as the temporary storage form of photosynthetic products. As a permanent reserve food material, it occurs in seeds, in fruits, in tubers, in the pith, medullary rays and cortex of the stems of perennials, etc. It constitutes from 50 to 65 per cent of the dry weight of seeds of cereals, and as high as 80 per cent of the dry matter of potato tubers. Starch occurs in plant tissues in the form of microscopic granules, composed of concentric layers, there being apparently alternate layers of two types of carbohydrate material, which have been distinguished from each other by several different pairs of names used by different authors: thus, Nägeli uses the terms "granulose" and "amylocellulose"; Meyer, "[alpha] and [beta] amylose"; Wolff, "amylo-cellulose" and "amylo-pectin"; while Kramer asserts that the layers are alternate lamella of crystalline and colloidal starch. Many theories as to the nature of these concentric layers and their mode of deposition have been advanced, but it would not be profitable to discuss them in detail here. For purposes of study, starch may be prepared from the ground meal of cereals, potatoes, etc., by kneading the meal in a bag or sieve of fine-meshed muslin or silk, under a slow stream of water. The starch granules, being microscopic in size, readily pass through the cloth with the water, and may be caught in any suitable container. The starch is then allowed to settle to the bottom, the water poured off and the starch collected and dried. Starch is insoluble in water; but if boiled in water, the granules burst and a slimy opalescent mass, known as "starch paste," is obtained. This is undoubtedly a colloidal suspension of the starch in water. By various processes, such as boiling with very dilute acids, treatment with acetone, etc., starch is converted into "soluble starch" which dissolves in water to a clear solution. Soluble starch is precipitated out of solution by alcohol, or by lead subacetate solution.