200 TANNIN ESSENTIAL OILS 



Gallo-tannic acid, for example, is said to have the constitution 

 C 6 H 2 (OH) 3 .CO.O.C 6 H 2 (OH) 2 .COOH. 



It is hydrolysed by dilute acids or by ferments, with the produc- 

 tion of gallic acid, C 6 H 2 (OH) 3 .COOH. 



The natural tannins, however, are usually associated or combined 

 with glucose, and, on hydrolysis, yield glucose in addition to gallic 

 acid. Fischer and Freudenberg x now regard tannin, not as a glucoside, 

 but as a pentacid derivative of glucose, resulting from the combination 

 of 1 molecule of glucose with 5 molecules of digallic acid (C 76 H 52 46 ). 



Tannin is present in almost all plants, occurring in the bark, 

 leaves, root, or fruit. Its origin and function in the plant are much- 

 debated questions. By some investigators it is thought to be formed 

 by chlorophyll from carbon dioxide and water, and, in many ways, to 

 resemble starch in its relationship to the plant. The tannins possess 

 a strongly astringent taste, give intense blue- or green-black colours 

 with ferric chloride solution, and possess the power of converting 

 gelatin into an insoluble substance (leather). They tend to absorb 

 oxygen, especially in presence of alkalies, giving a dark-brown 

 colouration. 



The nature of the acids present in the root hairs of plants does 

 not appear to have been much investigated. Probably a number of 

 acids are present in most cases, the predominating one varying with 

 the species of plant. Dyer ' 2 has investigated the amount of acidity of 

 the liquid contained in the root hairs of a large number of plants. 

 He expresses his results in terms of hydrogen and in the equivalent 

 of citric acid. He found, as is to be expected, considerable variations, 

 but the average of about 100 different plants gave 



As hydrogen 0-013 per 100 of water. 



As citric acid 0-910 



IV. ESSENTIAL OILS AND RESINS'. 



These are excretions or secretions of plants, sometimes normal, 

 sometimes as the result of injury or disease. To them the character- 

 istic odours of certain plants are due. 



The essential oils and resins often occur associated in the plant, 

 and the latter may be regarded as products of oxidation of the former. 



They are present in various parts of the plant, sometimes being 

 distributed over all portions, as in many coniferous trees, while in 

 many cases they are confined to particular organs, e.g., the petals of 

 the flowers, the rind of the fruit, the bark, or the roots. Their 

 function is difficult to understand. When present in the flower, they 

 may serve as an attraction to insects and thus favour cross fertilisa- 

 tion. When present in the leaves or twigs they may serve to render 

 the tissues unpalatable to animals and thus protect the plant from 

 injuries by grazing animals or parasitic insects. 



The oils can be extracted from the vegetable tissues in several 

 ways, all of which may be classed as belonging to three principal 

 types : 1. Expression. 2. Distillation. 3. Extraction with a solvent. 



1 Ber. 1912, 45, 915 ; Jour. Chem. Soc., 1912, Abstracts, i. 471. 



2 Jour. Chem. Soc., 1894, Trans., 115. 



