Slime Moulds, Bacteria, and Yeasts. 261 



which are injurious instead of beneficial, such as those pro- 

 ducing consumption, diphtheria, typhoid fever, lockjaw, 

 blood poisoning, bubonic plague, etc. (Figs. 133-134). 

 These diseases are brought about 

 by poisons produced by the bac- 

 teria within the body. Great 

 advances in surgery and in the 

 treatment and prevention of dis- 

 eases have been made by an un- 

 derstanding of the life history 

 and habits of these microscopic 

 forms of life. It is now known, 

 for instance, that gangrene and 

 blood poisoning, which formerly 

 often followed in the wake of 

 surgical operations, were brought 

 about by bacteria clinging to the 

 surgeon's knife, or which were 

 in the water, bandages, etc., used 

 in dressing the wound. Now 

 every instrument or object em- 

 ployed in such operations is thor- 

 oughly sterilized, and the healing 

 of the wound goes forward with- 

 out complications. So, too, it is 



now known that the expectorations of consumptives and 

 the dejecta of typhoid patients are teeming with the bac- 

 teria causing these diseases, and that unless the bacteria 

 are destroyed by suitable poisons or heat they may spread 

 disease. 



158. The Nature of Yeasts. Yeasts are low forms of 

 plants which, like bacteria, are destitute of chlorophyll, 

 and are dependent for their food upon materials built up 



FIG. 135. 



Experiment showing the impor- 

 tance of nitrogen-fixing bacteria 

 to leguminous plants, t, peas 

 grown in a nitrogen-free soil with 

 bacteria; u, the same in all re- 

 spects, but without bacteria. Af- 

 ter FRANK. 



