50 LOUIS PASTEUR 



may be seen to put out buds which slowly increase 



in size and finally constrict off as new yeast cells. 



^ ^r^^ Sometimes the substance of the yeast 

 HP m^ 



q q cells breaks up within the cell wall 



into (usually four) rounded bodies 



called spores, and these bodies, which 



Fig. 4 Cells of are more resistant to drought and 



Yeast ° 



Showing bud- other destructive influences than the 



onefcas 11 f *r y east pl ants themselves, may give rise 

 spores. to new individuals. There are many 



kinds of yeast plants, each producing its own peculiar 

 kind of fermentation. The importance of yeasts 

 in making bread, alcoholic beverages, and other 

 products of industry has greatly stimulated the 

 study of their different varieties and modes of life. 

 The most important of all one-celled forms of 

 plant life are unquestionably the bacteria. These 

 are also the simplest and most primitive of all 

 living organisms. Their discovery dates back to 

 our industrious old Dutch observer, Leeuwenhoek, 

 who in 1683 in one of his notes to the Royal So- 

 ciety describes and figures several very small crea- 

 tures that undoubtedly belong to this group. Al- 

 though countless in the number of their species, 

 the bacteria present little variety of external form 

 as compared with the Protozoa. A very common 



