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. 



The third variety of plants found in dust is yeast, size and 



Structure 



These are not usually so numerous as either the bac- of Yeaat 

 teria or the 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- 

 toplasm 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 seen 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 Spores 

 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. 



