602 



RELATION TO ENVIRONMENT. 



bluets (Houstonia purpurea), alum-root (Heuchera americana), 

 Arenaria stricta; grasses like Agrostis hiemalis, etc. Shrubs like 

 Cornus, Spiraea, some of the cedars, etc., while trees like the 

 white pine (Pinus strobus), also Pinus divaricatus, oaks (Quer- 



Fig. 524. 

 Rock lichen (umbilicaria) from Greenland. 



cus macrocarpa), ash (Fraxnus americana),, and trembling pop- 

 lar (Populus tremuloides) occur. 



1096. Transition of open formations into close ones. This 

 vegetation together with greater accumulations of fine rock ingre- 

 dients provides soil in the crevices where plants with greater 

 stem development may gain a foothold in the crevices. These 



