The Relation to Nitrogen 



243 



136. Mycorhizal fungi. - Ternetz has isolated and 

 tested a fungus from the roots of certain heaths. The 

 association of root and fungus is known as mycorhiza. 

 This fungus proved to be a species of Phoma, and several 

 strains of it were found to show a high capacity for nitro- 

 gen fixation. In fact, while the total amount of nitrogen 

 fixed in a given period of time is relatively small, the fixa- 



FIG. 64. Mycorhiza of the orchid Corallorhiza, also cells showing the 



hyphse. [After M. B. Thomas.] 



tion per gram of dextrose used by the fungus indicates that 

 in pure cultures fixation is more economic than with any 

 other organisms yet investigated. The following table 

 affords a comparison of the efficiency of several strains of 

 this fungus as compared with certain soil bacteria : - 



The Phoma discussed above develops an endophytic 

 mycorhiza, penetrating the cells. This endophytic form 



