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 bur 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 



