THE GRAVEL-SLIDE COMMUNITY. 91 



tion. Perhaps 95 per cent of the rocky soil surface is bare except for 

 Parmelia saxatilis and a few other crustose lichens (plate 23). This 

 habitat is somewhat similar to that of the sandhills in respect to 

 the dry surface mulch and the supply of moisture in the upper layer. 

 The root habit is clearly a response to the environment. Undoubtedly 

 the large number of roots which have such a strong tendency to run 

 up the slope serve in part for anchorage. The distribution of soil 

 nutrients may also have some share in this. The bisect in figure 37 

 shows that root competition on the gravel-slide is far from severe. 



