386 ENZYMES 



This fermentation, however, may not take place immedi- 

 ately on the introduction of the yeast to the particular sub- 

 stance : for instance, before yeast can ferment galactose, it 

 must be educated with regard to this material by being culti- 

 vated for some time in a solution containing it. A yeast so 

 educated yields a juice which can ferment galactose, the fer- 

 menting mixture, according to Harden and Norris, reacting 

 with phosphate in a manner exactly similar to yeast-extract 

 and glucose ; further, the process is accelerated by the addition 

 of a small quantity of sodium arsenite. 



In addition to ordinary, alcoholic fermentation, yeast also 

 exhibits the power of auto-fermentation.* This is brought 

 about at the expense of the reserve food materials of the 

 plant, chiefly glycogen, two enzymes being concerned in the 

 process. Glycogenase changes the glycogen into sugar, which 

 is then converted by zymase into alcohol and carbon dioxide, 

 the rate of fermentation being dependent on the rate of sugar 

 production by the glycogenase. Harden and Paine also 

 found that the rate of auto-fermentation is greatly increased 

 by the removal of water from the cell, which means, of course, 

 a concentration of the cell-sap. This may be accomplished 

 by partial desiccation or by the use of dissolved substances 

 which plasmolyse the cells. Alcohol in solutions above 10 

 per cent also have the same effect ; on the other hand, salts 

 which do not produce plasmolysis, even in concentrated 

 solutions, such as urea, have no such accelerating effect. 



OCCURRENCE OF ALCOHOLS IN PLANTS. 



Methyl Alcohol has been found to occur in the aqueous 

 distillates and in the essential oils of a very large number of 

 different plants, amongst which might be mentioned Juniperus 

 Sabina, Zea Mais, Lolium perenne, Iris germanica, Euonymus 

 europaea, Thea sinensis, Eugenia caryophyllata^ Carum carvi y 

 Anthriscus cerefolium, etc. 



Ethyl Alcohol is not quite so widely distributed as methyl 

 alcohol, but occurs in distillates from Cananga odorata 

 (Ylang Ylang), Pyrus Malus, Mespilus germanica, Eucalyptus y 

 Anthriscus cerefolium, Pastinaca sativa, V actinium Myrtillus, 

 Betula alba^ etc. 



* Harden and Paine : " Proc. Roy. Soc., Lond.," B., 1912, 84, 44 . 



