OCCURRENCE 267 



the tannin ; the colour is hable to be destroyed by mineral 

 acid and to be turned green by organic acids, 



4. They are precipitated from solution by many metallic 

 salts such as copper or lead acetates or stannous chloride, etc. 



5. They are precipitated from solution by a strong aqueous 

 solution of potassium bichromate or by a i per cent solution 

 of chromic acid. 



6. They precipitate from solution both alkaloids and sub- 

 stances of a basic nature, such as basic organic colouring 

 matters, including methylene blue. 



7. In alkaline solution the tannins, and many of their 

 derivatives, readily absorb oxygen, becoming dark in colour. 



8. With a solution of potassium ferricyanide and ammonia 

 they give a deep red colour. 



It must be borne in mind, however, that none of these re- 

 actions, taken separately, are specific for tannins ; they may be 

 given by many other substances as well, but all true tannins 

 answer them as a whole. 



OCCURRENCE. 



Tannin, using the word as a generic term, is generally 

 looked upon as an aplastic substance, and is very widely dis- 

 tributed in the vegetable kingdom. 



In certain Algae, e.g. Spirogyra, Mesocarpus, and Zygnema, 

 it occurs in the cells in the form of numerous small vesicles ; 

 in the Fungi, tannin is stated to be more abundant in parasites 

 than in saprophytes, thus hardly any occurs in the Agaricineae 

 whilst in the Polyporeae it is present in much larger amounts. 



In the higher plants it occurs more or less generally through- 

 out a tissue, for example in bark, or it may be restricted, in 

 the more mature parts, to special cells which may be isolated 

 or superposed one above the other in the form of chains. 



Amongst the higher plants there is no great phylum in 

 which tannin is not found ; it occurs in the ferns, e.g. Angio- 

 pteris and Aspidium ; in Gymnosperms, e.g. Pinus ; and also 

 in innumerable Angiosperms, in all parts. 



Thus it obtains in the roots of Trianea, Desmanihus, and 

 Pistia ; in the stems, where it may be accumulated, especially 



