216 TANNINS 



produced by warming concentrated solutions of tannin with 

 sulphuric acid. 



Inasmuch as phlobaphenes are produced by any process 

 which tends to remove water, such as heating tannins to a high 

 temperature or prolonged boiling or heating under pressure, 

 they are regarded as anhydrides of the tannins ; besides being 

 thus produced artifically, they occur also in nature side by 

 side with the tannins from which they can be produced. 



They are red-coloured substances and are practically in- 

 soluble in water though they dissolve in solutions containing 

 tannic acid; also they dissolve in alcohol and in alkaline 

 solutions. 



The formation of phlobaphenes by treatment of a tannin 

 with acid is characteristic of pyrocatechol tannins (p. 208) in 

 just the same way as ellagic acid is produced from pyrogallol 

 tannins. 



A number of different phlobaphenes are known, such as 

 kino-red, catechu-red, oak-bark red, etc. 



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 

 these substances vary in different species, so that what may be 

 true for one is not necessarily true for all. 



It is, therefore, not surprising to find that several ideas 

 have been put forward. 



With regard to the origin of tannins practically nothing of 

 fundamental importance is known. 



According to the investigations of Kraus, tannin, although 

 not a direct photosynthetic product as is indicated by the 

 fact that the tannin does not increase in the leaves of plants 

 which are able to photosynthesize in dull light is not formed 

 unless carbon dioxide and light are available. He found that 

 etiolated leaves produced no tannin, and that the amount of 

 this substance in shaded leaves was less than that contained 

 in the leaves of the same plant fully exposed to the sun. 

 The tannin thus formed is translocated to the stem and root 



Similarly Dekker* finds that light is requisite for the 

 formation of tannin, and that the tannin content of leaves 



* Dekker: " Rec. trav. bot. nSerlandais," 1917, 14, i. 



