300 



MYCORRH1ZAE AND MYCOTROPHY 



duction. The second type he noted to occur under the same con- 

 ditions as the first, but the fusion of mantles merged clusters of 

 forked roots and thereby produced knots or tuber-like growths. 

 The third type is constituted of long, thin, unbranched structures, 

 which occur upon the roots of heath-inhabiting species and are 

 believed to be conditioned by decreased "virulence" of the fun- 



Fig. 47. Diagram of root in cross-sections, one-half being normal, the other 

 mycorrhizal. The cells of the cortex are hypertrophic, and all are en- 

 veloped by fungus filaments, forming a mvcorrhiza of the ectotrophic 



type. (After Hatch and Doak.) 



gus. Decreased virulence, in turn, is the result of growth in the 

 more acid soils whose mineral content is relatively unavailable. 

 Melin's observations, involving mycorrhizae on pine, spruce, and 

 larch, led him to conclude that mycorrhizal types represent phases 

 or stages in development. The endotrophic condition is transi- 

 tional to the ectendotrophic, which finally becomes transformed 

 into a typical ectotrophic type. During this transition the invad- 

 ing hyphae are gradually digested and eliminated by the host 

 cells. 



No doubt a great deal of the confusion in understanding the 

 structure and function of mycorrhizal associations arises from 



