position and constitution, but which
serve entirely different purposes in the plant. As a class, they are
condensation products of pentoses, known as pentosans and having the
formula (C_{5}H_{8}O_{4})_{n}, or hexosans having the formula
(C_{6}H_{10}O_{5})_{n}, or combined pentosan-hexosans.

In general, these compounds make up the skeleton, or structural framework
material, of the plant, in contrast with the protoplasmic materials or food
substances for which most of the other types of organic compounds
(discussed in other chapters of this book) serve. They are the principal
constituents of "woody fiber," of cell-walls, and of the "middle lamella"
which fills up the spaces between the plant cells. They are, therefore,
found in largest proportions in the stems of woody plants; but they are
also present in every other organ of plants, as the cell-wall or other
structural material.

For purposes of study, these compounds may conveniently be divided into
three groups; namely, the natural gums and pentosans, the pectins and
mucilages, and the celluloses. The segregation into these three groups is
not sharply defined. The distinction between the groups is based upon the
solubility of the compounds in water. The gums and pentosans readily
dissolve in water; the pectins form colloidal solutions which are easily
converted into "jellies"; the mucilages do not dissolve but form slimy
masses; while the celluloses are insoluble in and unaltered by water. Some
authors add a fourth group, known as "humins"; but as these are the
products of decay (usually in the soil) of these structural compounds,
rather than of growth and development, they need not be taken into
consideration in a study of the chemistry of plant growth.

THE NATURAL GUMS AND PENTOSANS

The natural gums, when hydrolyzed, yield large proportions of sugars, but
most of them also contain a complex organic acid nucleus, by means of which
they form salts with calcium, magnesium, etc. Some of them, such as cherry
gum and those which are found in the woody stems of plants (wood gum, and
those found in corn stalks, the straw of cereals, etc.) yield practically
pure pentoses. These are known as pentosans. They bear the same relation to
the pentose sugars as do the dextrosans to glucose, etc. The wound gums,
for example, yield arabinose, and the wood gums yield xylose. But most of
the natural gums yield a mixture of galactose, some pentose, and some
c