OCCURRENCE OF TANNINS 195 



Further, tannin is often found in more or less special 

 structures, e.g. the cells of the pulvini of Mimosa pudica 

 and Robinia pseudacacia ; in the gland cells of Sarracenia and 

 Utricularia ; in the hairs o{ Primula and Hedera ; and also in 

 laticiferous tissue. 



Finally, it may be remarked that it is especially abundant 

 in pathological growths such as galls, which may contain from 

 25 to 75 per cent of tannin. 



Kraemer* has investigated the galls formed by the agency 

 of Cynips aciciilata, a gall fly, upon Quercus coccinea. He found 

 that during the chrysalis stage gallic acid was produced, prob- 

 ably at the expense of the starch, and as the imago developed 

 the gallic acid gave place to tannic acid. 



In the cell, the tannin occurs in solution in the cell sap, 

 and since tannin forms a precipitate with albuminous matter 

 it follows that the layer of protoplasm around the tannin 

 vesicles must be impermeable to it ; if this were not so the 

 protoplasm would be tanned on the production of tannin. 



Economically tannin is of great value although it is per- 

 haps not so extensively used at the present day as in the 

 past. As is well known its principal use is in the making of 

 leather f; in this connexion salts of chromic acid have come 

 into use as a substitute, especially in the manufacture of the 

 cheaper grades of leather. Formerly tannins were almost ex- 

 clusively used in the manufacture of black ink, whereas at the 

 present time various preparations of aniline dyes are in vogue. 

 Among other uses for tannins may be mentioned their value 

 for medicinal purposes and their use as mordants in certain 

 dyeing operations. 



The chief sources of tannins are the bark or the wood of 

 various species o{ Acacia, Castanea, Eucalyptus, and Quercus; 

 the bark of the mangrove Rhizophora Mangle, the roots of 

 Rumex hymejiosepalus (Canaigre) and the leaves of Rhus 

 Coriaria (Sumach). 



Gallotannic acid is obtained from galls, especially the galls 

 which occur on Quercus lusita7iica ; but the galls on other 

 species of oak, e.g. Q. sessiliflora, Q. pedu?iculata, Q. Flex, Q. 



* Kraemer : " Bot, Gaz.," igoo, 30, 274. 



t An excellent account of leather manufacture will be found in Procter's 

 •♦ The Making of Leather," Cambridge, 1914. 



13* 



