THE METABOLISM OF PLANTS. 237 



Mixed ethers : anethol or oil of anise, C 10 H 12 O = CH 3 , C 6 H 4 (C 3 H 5 ), O. 



Compound ethers : benzyl benzoate, C M Hi2O 2 , and benzyl cinnamate, 

 C 16 H H O 2 , in various balsams; methyl salicylate, C 8 H 8 O 2 =CH 3 , 

 C 7 H 6 O 3 , in the Winter-green (Gaultheria procumbens). 



b. Ethereal oils not containing oxygen : 



These oils are hydrocarbons of the formula Ci H 16 ; but though they 

 have the same ultimate chemical composition they are not all iden- 

 tical, but some are isomers and polymers; they are commonly termed 

 terpenes. 



Terebenthene is the terpene which is present in turpentine ; a ter- 

 pene is also present in the essential oils of plants, such as oil of Neroli 

 obtained from Orange-flowers, oil of Lemons, oil of Peppermint, etc. 



In connexion with the terpenes we may consider two hydrocarbons, 

 Caoutchouc and Gutta Percha which have the formula (C 5 H 8 ) x. Caout- 

 chouc is obtained from the latex of the Urticaceas, Euphorbiaceae, and 

 Apocynacese: Gutta Percha from the latex of various species of Isonan- 

 dra (Dichopsis) and other Sapotaceae. 



With regard to the function and fate of these aromatic 

 substances, it appears that they are of no use in the con- 

 structive processes; they are to be regarded as waste-products, 

 destined, for the most part, to be thrown off. Even when they 

 actually remain in the plant, in resin-ducts or in laticiferous 

 vessels, they are outside the sphere of the metabolism of 

 the plant. Hartig was led, by his observations on the oak, 

 to the conclusion that tannin (tannic acid ?) is used in the 

 constructive processes, but Sachs and others do not find 

 that this is the case. According to Schell, tannin can only 

 serve as a plastic substance when the plant possesses in- 

 sufficient stores of carbohydrates and of fats: this amounts 

 to saying that, under ordinary circumstances, tannin is merely 

 a waste-product. Again, it has been found (Wagner), that 

 when hippuric acid is presented to the roots of one of the 

 higher plants, it is decomposed into glycocoll and benzoic 

 acid, and that only the former of these bodies is absorbed. 

 On the other hand, we must not forget that tyrosin is 

 absorbed when presented to the roots, but we cannot con- 

 clude from this that the aromatic radical which it contains 

 is actually used in the constructive processes. Nor must we 

 overlook the fact that Naegeli has succeeded in cultivating 



