200 or more. The
fact that these compounds are insoluble in most of the solvents which can
be used for molecular weight determinations makes it difficult to determine
their actual molecular constitution.

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,
Naegeli 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. T