272 TANNINS 



acid, for in the case of the former a precipitate immediately 

 comes down, whilst in the case of tannic acid, according to 

 Drabble and Nierenstein, the reaction is either very slow or 

 entirely negative. 



2. Potassium cyanide in aqueous solution gives a pink 

 coloration with gallic acid. 



3. With Nessler's solution gallic acid gives a grey-green 

 precipitate. 



With this same reagent pyrogallol immediately yields a 

 brown precipitate ; pyrocatechol forms a deep green precipitate 

 which changes to greenish brown ; and a dirty green precipi- 

 tate is given by protocatechuic acid. 



Vinson * gives a method of simultaneous fixing and staining 

 of tannins in situ, by exposure to the vapour of sweet spirits 

 of nitre. 



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 f finds that light is requisite for the 

 formation of tannin, and that the tannin content of leaves 



* Vinson : " Bot. Gaz.," 1910, 49, 222. 



t Dekker : " Rec. trav. bot. n^erlandais." 1917, I4» i- 



