germination. Tannins
are also found in certain special structures, such as gland cells, cells of
the pulvini, laticiferous tissues, etc. Further, they are especially
abundant in the pathological growths known as galls, which often contain
from 40 to 75 per cent of tannin and constitute the most important
commercial source for these materials.

The principal commercial sources of tannin, which is used in the
manufacture of inks, in the tanning of leather, in certain dyeing
operations, etc., are oak-galls, the bark and wood of oak, hemlock, acacia,
and eucalyptus, the bark of the mangrove, the roots of canaigre, and the
leaves of several species of sumac.

CHEMICAL CONSTITUTION

Tannins are either free phenol-acids or, more commonly, glucosides of these
acids. Common "tannin," when hydrolyzed, yields from 7 to 8 per cent of
glucose, which indicates that it is a penta-acid ester of glucose, i.e.,
each glucose molecule has five acid groups attached to it. The formula for
such a tannin is, therefore, as follows,

------O-------
| |
CH_{2}OR.CHOR.CH.CHOR.CHOR.CHOR

in which the R represents a complex phenol-acid like tannic acid, or
digallic acid. These acids are derivatives of the common phenols, whose
constitution will be brought to mind by the following series of formulas:

Ordinary phenol Pyrocatechol Resorcinol Hydroquinone
/\ /\ /\ /\
/ \ / \ / \ / \
| |OH | |OH | |OH | |OH
| | | |OH | | HO| |
\ / \ / \ / \ /
\/ \/ \/ \/
OH

Pyrogallol Oxyhydroquinone Phloroglucinol
OH OH OH
/\ /\ /\
/ \ / \ / \
| |OH | |OH | |
| |OH | | HO| |OH
\ / \ / \ /
\/ \/ \/
OH

These phenols themselves do not occur as constituents of tannins, although
they are often found in other glucosides