TANNINS 95 



the seeds of several species, either before or after germination. 

 Tannins are also found in certain special structures, such as gland 

 cells, cells of the pulvini, laticiferous tissues, etc. Further, they 

 are especially abundant in the pathological growths known as 

 galls, which often contain from 40 to 75 per cent of tannin and con- 

 stitute the most important commercial source for these materials. 

 The principal commercial sources of tannin, which is used in 

 the manufacture of inks, in the tanning of leather, in certain dyeing 

 operations, etc., are oak-galls, the bark and wood of oak, hemlock, 

 acacia, and eucalyptus, the bark of the mangrove, the roots of 

 canaigre, and the leaves of several species of sumac. 



CHEMICAL CONSTITUTION 



Tannins are either free phenol-acids or, more commonly, glu- 

 cosides of these acids. Common " tannin," when hydrolyzed, 

 yields from 7 to 8 per cent of glucose, which indicates that it is a 

 penta-acid ester of glucose, i.e., each glucose molecule has five 

 acid groups attached to it. The formula for such a tannin is, 

 therefore, as follows, 



-0 , 



CH 2 OR - CHOR CH - CHOR - CHOR - CHOR 



in which the R represents a complex phenol-acid like tannic acid, 

 or digallic acid. These acids are derivatives of the common 

 phenols, whose constitution will be brought to mind by the follow- 

 ing series of formulas : 



Ordinary phenol Pyrocatechol Resorcinol Hydroqu'none 



| 



OH 



Pyrogallol Oxyhydroquinone Phloroglucinol 



OH OH OH 



H 



i i 



H 



HOIJOJ 



