y found in the tracheae of plants, and near surface
wounds, which it stanches. It is secreted by the cells surrounding the
injured part. It responds to the reactions of other gums and to some of
those of woody fiber. Its exact composition is not known, but probably lies
between that of the true gums and that of cellulose.

These gums are generally considered to be decomposition products of
celluloses, resulting from the action of some hydrolytic ferment, usually
stimulated by some unfavorable condition of growth, some injury, or some
morbid condition.

The =pentosans=, araban and xylan, occur normally in the stems and outer
seed coats of many common plants. They constitute a considerable proportion
of these tissues, as indicated by the following results of typical
analyses: Wheat bran, 22 to 25 per cent; clover hay, 8 to 10 per cent; oat
straw, 16 to 20 per cent; wheat straw, 26 to 27 per cent; corn bran, 38 to
43 per cent; jute fiber, 13 to 15 per cent; various wood gums, 60 to 92 per
cent.

They are white, fluffy solids, which are difficultly soluble in cold water,
more readily in hot water. They are very difficult to hydrolyze, and
indigestible by animals. When finally hydrolyzed, they yield arabinose and
xylose, respectively. The pith of dry corn stalks is a good illustration of
their general character.

MUCILAGES

These are characterized by forming slimy masses when moistened with water.
They are secreted by hairs on the skin of many plants, so that the external
walls of the leaves, fruit, and seeds are often mucilaginous when damp.
This is particularly true of aquatic plants. The chemical composition of
the mucilages is unknown. When hydrolyzed, they yield arabinose and a
hexose; the latter is sometimes galactose and sometimes mannose.

When present on the surface of plant tissues, the mucilages probably serve
to prevent the too rapid diffusion of materials through the skin, in the
case of the aquatic plants, and too rapid transpiration, in the case of
young vegetative tissues or in other plants when growing under extremely
dry conditions. When found in tubers, or other storage organs, it has been
supposed that they may serve as reserve food materials, but it seems that
such difficultly hydrolyzable compounds as these can hardly function as
normal reserve foods.

PECTINS

Many fruits, such as currants, gooseberries, apples, pears, etc.,