298 Plant Physiology 



is more commonly brought about by the growth of various 

 species of yeast in liquids or moist substrata containing 

 certain sugars. Some organisms show a marked specific 

 election with respect to the sugar fermented ; but in 

 general, the hexose forms are most important, while trioses 

 and nonoses are sometimes used. When abundant O 2 

 is supplied, the yeasts may grow rapidly, utilizing the 

 sugars as foods, and effecting relatively little fermenta- 

 tion. In the absence of sufficient O 2 for rapid growth 

 the organisms grow slowly, but fermentation proceeds 

 vigorously. The decomposition of glucose yields ethyl 

 alcohol and carbon dioxid as chief products, expressed 

 conveniently by the following equation : - 



CH 12 C = 2(C 2 H 6 0) + CO, 



glucose ethyl alcohol 



On account of this capacity to produce alcohol, yeasts 

 are the important organisms utilized in the production of 

 alcoholic beverages, and the proper regulation of growth 

 and fermentation is the essential factor in economic pro- 

 duction. Starch, cane-sugar, and other carbohydrates 

 transformable into the fermentable types are, of course, 

 ultimately used, if first hydrolyzed, as in the malting 

 process. 



Yeasts are unusually resistant to the alcohol produced, 

 but the fermentative activity declines rapidly at 12 per 

 cent, and at 14 per cent there is usually complete inhibi- 

 tion. This concentration, however, is far greater than 

 that which the more resistant molds may endure, com- 

 monly from 4 to 5 per cent. It is well known that the pro- 

 duction of CO 2 bubbling through and held by the cohesive 



