60 ORGANIC ACIDS, TANNINS 



cases their application is unnecessary, since crystals of other 

 inorganic compounds are very rare. 



Many by-products occur in solution in the cell-sap, and of 

 these the commonest are diverse organic acids and bodies known 

 as tannins. The former are responsible for the frequently acid 

 reaction of the sap, and are especially abundant in unripe fruits. 

 As examples mention may be made of malic acid (in Apples), 

 citric acid (in Lemons), tartaric acid (in Grapes), etc. The name 

 of tannins is given to diverse organic substances, whose chemical 

 constitution is not fully established, but all of which possess 

 an astringent taste and are characterised by the following 

 reactions : They reduce Fehling's solution, are precipitated by 

 the salts of many metals (e.g. basic lead acetate), and take on a 

 blue-black or greenish colour with ferric chloride. Dilute iodine 

 solution, together with a little 10 per cent, ammonia, gives a 

 brilliant red colour even with small quantities of tannins, whilst 

 they are readily precipitated by dilute solutions of caffein 

 (cf. p. 13). 



Tannins are particularly common in the bark of trees (e.g. 

 Oak, Alder), in unripe fruits (e.g. Pear, Acorn), in leaves (e.g. 

 Bracken), and occur abundantly in certain abnormal growths 

 (e.g. Oak-galls). They are often accompanied by a yellow or 

 reddish pigment which facilitates recognition of the cells in 

 which they occur. The bark of certain trees (e.g. Oak), owing 

 to the large quantity of tannin present, has long been employed 

 in the conversion of hide into leather, which process depends on 

 the coagulation, by the tannin, of the albuminous substances 

 contained in the animal skin. The reactions of tannins with 

 ferric salts have been extensively used in the preparation 

 of ink. 



Another group of by-products, encountered particularly 

 in certain families of Flowering Plants (e.g. Ranunculacese, 

 Scrophulariaceae, Solanaceae, Umbellif erae) , are the alkaloids. 

 These are complex basic organic compounds containing nitrogen, 

 which are either dissolved in the cell-sap or present in the solid 

 state ; in the plant they are often combined with organic acids. 

 The alkaloids are of such importance, owing to their poisonous 

 and medicinal properties, that the following list of some of the 

 more familiar is given : 



