PYROCATECHOL TANNINS 217 



CATECHIN. 



This substance, which is obtained from Acacia catechu, 

 Ouroparia catechu, mahogany wood, Mimosa, and pyrocatechol 

 tannins in general, is not in itself a tannin since it does not 

 precipitate gelatine, but it is converted into a tannin, namely 

 catechu tannic acid, by loss of water, a change which may be 

 rapidly brought about by heating to 1 20 or above. 



Catechin may be prepared by extracting powdered catechu 

 with ether ; the crude material obtained on evaporating off the 

 ether may be purified by crystallization from water. 



Catechin forms colourless glistening needles, which, when 

 dry, melt at 175-177. It is readily soluble in alcohol and 

 ethyl acetate, not so readily soluble in ether, and only slightly 

 soluble in cold water. 



With ferric chloride alone it gives a green colour, but with 

 ferric chloride and sodium acetate a dark violet. 



It gives the phloroglucin reaction with pine wood shaving 

 and hydrochloric acid. 



Potash fusion gives protocatechuic acid and phloroglucinol. 



QUERCITANNIC ACID. 



Quercitannic acid is the name given to the tannin of oak 

 bark, which is not identical with the tannin of oak galls. 



Pure quercitannic acid yields no glucose on hydrolysis, 

 though levulose is nearly always present in oak bark. 



Although much work has been done on the oak bark 

 tannins by various workers, notably Etti, Bottinger and Lowe, 

 nothing definite is known as yet regarding their constitution. 



Procter summarizes the present state of our knowledge by 

 saying that, on the whole, it seems probable that the principal 

 tannin of oak bark is a purely catechol tannin, and that the 

 gallic and ellagic acids which have been detected in it are due 

 to an admixture of the gallotannic and ellagitannic acids 

 present in oak wood. 



A great many more tannins are known, but too little is 

 known about their composition to justify their inclusion here. 



PHYSIOLOGICAL SIGNIFICANCE OF TANNINS. 



It is manifestly a difficult matter to ascertain the signifi- 

 cance of tannins in the life of the plant, more especially as 



