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 formaldehyde with various tannic acids. Second, by various treatments of cork, decomposition compounds showing tannin-like properties may be obtained.
Some investigators have held that not only cork tissue but also other lignose, or cell-wall material, may be developed from tannins. Certain observations with Spirogyra seem to indicate that tannin may play an important part in the formation of new cell walls during conjugation, as cells which are ready to conjugate are rich in tannin, which gradually diminishes in quantity until it is practically absent at the time of spore-formation. There seems to be no evidence that tannins perform any such function as this in higher plants, however.
Again, tannins may play a very important part in pigment-formation. They are very similar in structure to the anthocyanin pigments, both being made up of practically identical decomposition units, the phenolic bodies. The disappearance of tannins during the process of ripening of fruits may be connected, in part at least, with the development of the brilliant red, blue, and yellow pigments which give such rich colors to the thoroughly ripe fruits.
Finally, certain of the tannins undoubtedly serve as protective agents to prevent the growth of parasitic fungi in fruits, etc. Recent investigations show that at least some of the varieties of fruits which are resistant to the attacks of certain parasitic diseases utilize tannins for this purpose. This protective effect may be accomplished in two different ways. Either the tannin actually serves as an antiseptic to prevent the growth of the parasitic fungus within the tissues of the host plant, or it assists in the development of a corky layer which "walls-off" the infected area and so prevents further spread of the disease through the tissue. Examples of both types of protective action have recently been reported.
It is obvious that the different forms of tannins may play different rôles in plant life, and the same tannin substance may possibly serve different purposes under different conditions.
BIOLOGICAL SIGNIFICANCE OF TANNINS IN FRUITS
The presence of tannins in fruits and the changes which they undergo during the ripening process cannot fail to attract attention to their biological significance in serving to protect the fruit from premature consumption as food by animals.