ADAPTATION OF ORGANISMS 



333 



in partnership, they are superior to vicissitudes with which 

 many other plants cannot cope, and thus sonic forms become 

 the vanguard of vegetation in repopulating rocky, devas- 

 tated areas. 



From the practical standpoint of agriculture the symbiotic 

 nitrogen-fixing Bacteria are of first importance. It will be 



FIG. 180. Rose Aphids visited by Ants. (After Kellogg.) 



recalled that these Bacteria form small tubercles on the 

 rootlets of higher plants and make atmospheric nitrogen 

 available to the latter. Thus in return for an abode and cer- 

 tain food elements, such nitrogen-fixing Bacteria render their 

 symbiotic associate largely independent of soil nitrogen. 

 Again in the higher animals, including Man, evidence is 

 accumulating which indicates that certain kinds of Bacteria 



