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 
447 448 449 
Fias. 447-449. — Forms of common yeast (Saccharomyces cerevisie): 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: What 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 
