306 -STRUCTURE AND LIFE HISTORIES 
YEASTS 
We are all familiar with "yeast," but many persons do 
not realize that yeast is a plant, and that there are vari- 
ous species. The baker's yeast is different from the 
brewer's, and there are more than one kind of the latter 
one for example causing "top fermentation," another 
"bottom fermentation." There are also various "wild" 
yeasts, present in the air. The nature of fermentation 
has been discussed in Chapter VIII. It is interesting to 
reflect that this group of microscopic plants, in its relation 
to bread making (from one kind of grain) adds to the 
wealth and happiness of the world, and ministers to one 
of the most fundamental needs of our physical being, 
while in its relation to brewing (the formation of alcohol 
from another kind of grain) it contributes to one of the 
greatest sources of poverty, misery, and crime. 
The use of yeast in bread making dates from prehistoric 
ages. It is mentioned in old testament history as early 
as the patriarchal age. It is of interest also to reflect 
that in the various great migrations of the human race, 
this little plant must have been preserved and transported 
with as much care as domestic animals. In such a 
movement as the colonization of a new continent, as, 
for example, North and South America, and Australia, 
provision must have been made for maintaining the 
supply of yeast for the making of bread. 
BACTERIA 
289. Extent of Their Influence. The study of bacteria, 
the smallest of all known plants, has become so extensive 
as to form a separate science, bacteriology; and it will 
not be possible here to more than suggest in outline the 
