260 VEGETABLE PHYSIOLOGY 



bark of Betula lenta, 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 in- 

 vestigated, 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 

 subsequent decompositions are very complex, among the 

 products being lecithin, a peculiar fatty substance containing 

 phosphorus, as well as several 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 appears to contribute to the forma- 

 tion of the lecithin. The decomposition is accompanied by 

 the appearance of sugars and starch, which are probably 

 formed by the protoplasm of the cells. 



The production of alcohol from sugar is brought about by 

 another soluble enzyme, which has been prepared from 

 yeast. 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 following 

 equation : 



C 6 H 12 6 = 2C0 2 4- 2CH 3 CH 2 OH. 



In the reaction the sugar is decomposed, alcohol is formed 

 and carbon dioxide given off. 



This enzyme, which has been called zymase, has been 



