CRYPTOGAMS 



315 



brands. This kind of yeast is not known to exist except in 

 a state of cultivation, and probably owes its survival and 

 present condition of development to a symbiosis with man, 

 on account of its usefulness in bread making, and still more, 

 perhaps, to its part in the gratification of his bibulous pro- 

 pensities, for among savage tribes the manufacture of fer- 

 mented liquors is practiced long before the wholesome art of 

 bread making. 



There are other yeasts existing in a state of nature, such as 

 those on the surface of fruits, which cause the latter, under 



448 



449 



FIGS. 447-449. Forms of common yeast (Saccharomyces cerevisice) : 447, 

 brewers' yeast ; 448, household yeast (the large grains are starch) ; 449, yeast from 

 beer sediment, showing budding. (Figs. 447, 448 X 250 ; Fig. 449 X 1270.) 



certain circumstances, to ferment and decay. For this reason 

 artificial ferments are not needed in making wine and 

 other alcoholic liquors from fruits. Fermentation is also 

 caused by certain forms of bacteria, as in the formation of 

 vinegar and the souring of milk. Such bacteria often con- 

 taminate the yeast ferments. 



357. Microscopic examination. Place a drop of the 

 cultural liquid on a slide and examine under the highest 

 power of the microscope. What do you see? These egg- 

 shaped bodies are yeast plants, unicellular organisms like 

 the pleurococcus. Do you see any chlorophyll? Are the 

 yeasts parasitic? How do you know? What do they live 

 on ? (Suggestion : WTiat food substance that has disappeared 

 was put into the culture liquid?) In getting their nourish- 

 ment from the sugar, these fungi disintegrate it into alcohol 

 and carbon dioxide, which is a process of fermentation. It 



