34 



BACTERIA AND SOIL FERTILITY 



tached to the mother cell may begin to bud. This gives rise to 

 a small colony of cells. 



Yeasts produce spores which will tide them over adverse con- 

 ditions, but they differ from bacteria in that a single yeast cell 



Fig. 1 8. — Yeast cells (after Van Hest). 



may give rise to a number of spores; hence, this becomes a process 

 of multiplication. The yeasts ferment sugars with the forma- 

 tion of alcohol and carbon dioxid and are often classified accord- 

 ing to the specific sugar which they ferment. Few of the yeasts 

 produce disease in man or the lower animals. However, they 



Fig. 19. — Different types of yeast cells. 



have been used to combat in man certain bacterial infections as 

 boils and carbuncles. Their action is probably due to the vitamin 

 which they contain as well as to the fact that they increase the 

 activity of the defensive substances, white corpuscles, of the body 



