196 BOOKS AND CURKENT LITERATURE 



concludes, however, for the following reasons that ectotrophic mycorhi- 

 zas are not to be regarded as symbiotic associations but as instances of 

 parasitism by fungi on the roots of trees. (1) Healthy trees may grow 

 entirel}^ free from mycorhizas even in humus. (2) Mycorhizas are 

 developed in late summer at a time when the tree is no longer active. 

 (3) It is at this season too, that more food stored in the roots is avail- 

 able to the fungus than would be the case in spring. The parasitism 

 is comparatively harmless, however, because only a few roots are at- 

 tacked and only those near the surface. In the case of endotrophic 

 mycorhizas the situation is more complex. Sufficient benefit may 

 be obtained by the digestion of fungous hyphae to justify its being 

 regarded as a symbiotic association. — Frederick A. Wolf. 



