PHYSIOLOGICAL USES OF TANNINS

Tannins are probably not direct products of photosynthesis. They are,
however, elaborated in the green leaves of plants and translocated from
there to the stems, roots, etc. Their close association with the
photosynthetic carbohydrates has led many investigators to seek to
establish for them some significant function as food materials, or as
plastic substances in cell metabolism. Many conflicting views have been
advanced, but a careful review of these leads inevitably to the conclusion
that tannins probably do not serve in any significant way as food material.
The glucose which is generally present in the tannin molecule may, of
course, serve as reserve food material, but it seems probable that it
functions as a constituent of the tannins only to assist in making them
more soluble and hence more easily translocated through the plant tissues.

Some fungi, and perhaps other plants as well, can actually utilize tannins
as food material under suitable conditions and in the absence of a proper
supply of carbohydrates. But this does not prove that tannins can normally
replace carbohydrates as food material for these species of plants.

There seems to be ample evidence that tannins are elaborated where intense
metabolism is in progress, such as occurs in green leaves during the early
growing season; in the rapid tissue formation which takes place after the
stings of certain insects, producing galls, etc.; during germination, and
as a result of any other unusual stimulation of metabolism. It may be,
therefore, that tannins serve as safety accumulations of excessive
condensations of formaldehyde, or other photosynthetic products, under such
conditions. It seems certain that in all such cases tannins are the result
of, and not (as some investigators have supposed) the causative agents
for, the abnormally rapid metabolism.

It seems to be fairly well demonstrated that tannins are intermediate
products for the formation of cork tissue. This may account for their
common occurrence in the wood and bark of trees. Indeed, it has been shown
that gallic and tannic acids are present in considerable proportions in
those parts of the plant where cork is being formed. Further, that they
bear direct relation to cork-formation has been demonstrated in two
different ways. First, cork-like substances have been artificially produced
by passing a stream of carbon dioxide through mixtures of fo