268 VEGETABLE PHYSIOLOGY 



bark of Betula lento, , act on various glucosides, after the 

 manner of emulsin. 



The digestion of the glucosides, we may notice, is 

 always accompanied by the appearance of sugar, which is 

 one of the products of their decomposition. The fate of 

 the other bodies into which they split is not well ascertained, 

 though there is some evidence that cyanogen compounds, 

 even such as hydrocyanic or prussic acid, are used for 

 nutritive purposes by certain plants. 



The digestion of fat or oil has not been very fully 

 investigated, though certain facts are known concerning 

 its fate in germinating seeds. The digestion is generally 

 accompanied by the appearance of starch grains in cells 

 near the seat of digestion, and it was formerly considered 

 that the starch arose directly from the oil. It appears 

 now that the oil is split up by an enzyme, lipase, the result 

 being the formation of a free fatty acid and glycerine. The 

 fatty acid undergoes further decomposition, being oxidised 

 into simpler acid bodies, which are crystalline instead of 

 being viscid like the fatty acid first liberated. These pass 

 into the general body of the seedling. The glycerine in 

 its turn seems to be converted into some variety of sugar, 

 from which the plastids of the seedling construct the starch 

 which has been referred to, its formation indicating, as in 

 other cases, a temporary surplus of carbohydrate supplies. 



Within the last two years it has been ascertained that 

 the production of alcohol from sugar is brought about by 

 another soluble enzyme which has been prepared from 

 yeast. Though the existence of this body has been long 

 suspected, it is only quite recently that it has been demon- 

 strated. Like the decomposition which is brought about 

 by myrosin, the splitting up of the sugar is apparently not 

 a process of hydrolysis. It may be expressed by the follow- 

 ing equation : 



C 6 H 12 6 = 2C0 2 f 2CH 3 CH 2 OH 



In the reaction the sugar is decomposed, alcohol is formed 

 and carbon dioxide given off. 



