plants.

It is apparent that this important group of compounds, with its exceedingly
varied and complex constituent groups, may play a variety of significant
roles in plant growth.

REFERENCES.

ARMSTRONG, E. F.--"The Simple Carbohydrates and Glucosides," 239 pages,
_Monographs_ on Biochemistry, London, 1919 (3d ed.).

VAN RIJN, J. J. L.--"Die Glykoside," 511 pages, Berlin, 1900.

CHAPTER VII

TANNINS

Using the term in its general application to a group of substances having
similar chemical and physical properties, rather than in its limited
application to a single definite chemical compound known commercially as
"tannin," the _tannins_ are a special group of plant substances, mostly
glucosides, which have the following characteristic properties. First, they
are non-crystalline[4] substances, which form colloidal solutions with
water, which have an acid reaction and a sharp astringent taste. Second,
they form insoluble compounds with gelatine-containing tissues, as shown by
the conversion of hide into leather. Third, they form soluble, dark-blue or
greenish-black compounds with ferric salts, the common inks. Fourth, they
are precipitated from their solutions by many metallic salts, such as lead
acetate, stannous chloride, potassium bichromate, etc. Fifth, they
precipitate out of solution albumins, alkaloids, and basic organic coloring
matters. Finally, most tannins, in alkaline solutions, absorb oxygen from
the air and become dark brown or black in color.

FOOTNOTES:

[4] The needle-like forms, in which commercial "tannin" comes on the
market, are not true crystals, but are broken fragments of the threads into
which the colloidal tannin is "spun-out" from the syrupy extracts of
nutgalls, etc.

OCCURRENCE

Tannins occur widely distributed in plants. Practically every group of
plants, from the fungi up to the flowering plants, contains many species of
plants which show tannin in some of their tissues. Among the higher plants,
tannins occur in a great variety of organs. Thus, they are found in the
roots of several species of tropical plants; in the sterns, both bark and
wood, of oaks, pines, hemlock, etc.; in the leaves of sumac, rhododendron,
etc.; in many fruits, especially in the green, or immature, stages; and in
the seeds of several species, either before or after