YEASTS. 
39 
only to that she bears for some of the bacteria. So 
many more of the bacteria are friends rather than 
foes that it may be more just to place mold as her 
chief enemy. 
YEAST 
The third variety of plants found in dust is yeast. 
These are not usually so numerous as either the bac¬ 
teria or ^he molds, although about apple trees in the 
country wild yeasts are common. Like a bacterium, 
the yeast plant is a single microscopic cell of pro- 
tpplasm enclosed by the cell wall. It is round or oval 
in shape and often one two-thousandth of an inch in 
diameter. Fig. 22. It is therefore quite a giant com¬ 
pared with the smallest bacterium. 
If a drop of tepid water in which bread yeast has 
been dissolved be carefully watched under the micro¬ 
scope, the changes shown in Figs. 23 and 24 may be 
seen. One cell will be &een to swell a little at one 
part. This bud or daughter cell will bulge out more 
and more from the parent and may even produce one 
or more generations from itself before it breaks away. 
This ‘‘budding’' is the method of reproduction common 
to yeast plants of which there are many varieties. 
Some species, however, reproduce by spores very 
much like the molds. Such yeast cells will be seen 
to divide within the cell wall into two or four rounded 
bodies which in growing soon rupture the parent 
cell and escape. Fig. 25. Each of these liberated 
spores forms a new plant which may produce buds. 
Size and 
Structure 
of Yeast 
Spores 
