384 PHARMACEUTICAL BOTANY 
effect on the medium in which they live. Yeast causes a whole- 
sale destruction of sugar in the surrounding fluid. One of the 
decomposition products of sugar is alcohol. The alcohol of 
commerce is produced by the activity of this plant. . 
The protoplasm of the yeast plant manufactures two kinds of 
enzymes, viz., an extracellular enzyme called invertase which 
passes out of the yeast cell and hydrolyzes cane sugar to dextrose 
(invert sugar) and an endocellular enzyme called zymase which 
Fic. 284.—Saccharomvees cerevisie. The form or variety known as brewers’ top 
yeast. (Oberhefe.) 
remains within the yeast cell and breaks down dextrose into 
alcohol and carbon dioxide. 
Saccharomyces has its times of danger and stress when the cells 
perish in great numbers from cold, starvation, poisons, etc. If 
not too suddenly exposed, however, they are able to meet 
adverse conditions by eliminating most of their water, suspend- 
ing physiological processes, and becoming dormant. Some- 
times they enter the resting condition after a process of division, 
when each cell divides into four parts, each of which becomes 
nearly dry and is surrounded by a thick wall. Such cells are 
