Minnesota Plant Life. 73 



are sometimes impure or "bad" as the saying is. This is owing 

 to the mixture with the genuine yeast of other fungal organ- 

 isms, which liberate other and often undesirable substances. 

 For example, certain acids are produced through the activity of 

 yeast-like plants, and if such acid-forming yeasts are present in 

 sufficient quantity, the bread or the beer becomes sour. Hence 

 the cultivation of pure yeast is a prime necessity and when the 

 housewife or the brewer finds by experience that veasts in use 

 are no longer pure, a fresh supply must be obtained from a 

 neighbor or in the market. 



Yeast is a widely distributed plant and will often appear 

 as if spontaneously, just as black mould does when a substance 

 suitable for its development is exposed to the atmosphere for a 

 sufficient length of time. The yeast-plant itself consists of thin- 

 walled, egg-shaped cells which have the power of budding. 

 Somewhere the wall bulges and the bulge enlarges until it is the 

 size of the parent cell. Thus a branching body can be built up, 

 but the branches and buds readily separate from each other 

 and in a favorable condition of temperature and food supply 

 the growth of the plant is extremely rapid. When the yeast- 

 plant forms spores, which it does not do very abundantly under 

 ordinary conditions of growth, the contents of one of the egg- 

 shaped cells will be seen to divide into four portions each of 

 which becomes spherical in form and secretes about itself a wall 

 of its own. Then, when the wall of the mother-cell breaks down, 

 the spores separate from each other and may be distinguished 

 from the ordinary yeast-cell by their smaller size, spherical 

 shape and thicker wall. It is on account of their thicker wall, 

 and probably, too, in consequence of some difference in the 

 structure of the living substance within that they are able to re- 

 sist the harmful influence of extreme temperatures much better 

 than the ordinary yeast-cell. If the conditions favorable to 

 rapid growth become for any reason unfavorable, the yeast plant 

 is likely to undertake the formation of spores. 



The particular substance that yeast attacks is cane-sugar. 

 This it splits up, during its life-processes, into carbonic-acid-gas 

 and alcohol. Such a process is called fcnucntatioiu or more pre- 

 cisely, alcoholic fermentation, because there. are various kinds 

 of fermentation which go on under different circumstances 



