TANNINS 99 



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 inter- 

 mediate 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 rela- 

 tion 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 for- 

 maldehyde with various tannic acids. Second, by various treat- 

 ments 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 decomposi- 

 tion 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. 



