98 CHEMISTRY OF PLANT LIFE 



various tropical trees). This latter is a true tannin, which is 

 much used in dyeing and other technical processes. 



" Quercitannic acid," obtained from oak bark, etc., is likewise 

 a catechol tannin. It yields no glucose on hydrolysis. 



A great many other tannins are known, and their possibilities 

 for technical use in tanning, dyeing, etc., have generally been 

 investigated; but so little has been learned about their composi- 

 tion and relation to the plant's own needs, that it seems unneces- 

 sary to discuss them in detail here. 



PHYSIOLOGICAL USES OF TANNINS 



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 func- 

 tion 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 glu- 

 cose 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 forma- 

 tion 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 formal- 

 dehyde, or other photosynthetic products, under such conditions. 

 It seems certain that in all such cases tannins are the result of, 



