208 THE CONSTITUENTS OF PLANTS. . IX. 



plant. The tannins possess a strongly astringent taste, give 

 intense blue- or green -black colours with ferric chloride solu- 

 tion, 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 coloura- 

 tion. 



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* has investi- 

 gated the amount of acidity of the water 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 ... -013 per 100 of water. 

 As citric acid ... -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 characteristic 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. 



The essential oils may be divided into two great classes : 



1. Hydrocarbons or Terpenes. 



2. Substances containing, in addition to carbon and hydro- 



gen, other elements, e.g., (i.) oxygen, (ii.) sulphur. 



1. The Terpenes are again subdivided into 

 (i.) Hemiterpenes, C S H H . Unsaturated compounds of the 

 valerylene series, isoprene, a product obtained from 

 crude india-rubber, being the best known, 

 (ii.) Terpenes proper, C 10 H 16 . The most important class, 

 (iii.) Poly-terpenes, (C 5 H 8 ) n . Colophene and caoutchouc belong 

 to this class. The sesqui-terpenes, C 15 H 24 , occur fre- 

 quently in essential oils. 



* Jour. Chem. Soc. 1894, 115. 



