5 8o 



RELATION TO ENVIRONMENT. 



not condense. 6th. The lessened precipitation, so that the snow 

 is not so deep except in colder regions where it does not all melt 

 during the summer, but forms great glaciers or permanent snow- 

 fields, which, of course, prohibit plant life except a few low 

 organisms on the surface of the snow. In addition the high 



L 



Fig. 515. 

 Willows from Greenland, one third natural size. 



winds often drive the snow from extensive tracts and leave them 

 bare during the long winter night. 



1068. Responsive type of vegetation. The vegetation responds 

 to these conditions of environment by the development of certain 

 types of plants, as the radiate dwarf-cushion type, the rosette 

 type, the succulents, etc. The succulents are plants with thick 

 and fleshy stems and leaves for water-storage. These general 

 characters are very similar to those of many desert plants. In 

 plants with these habits there is less radiation of heat and mois- 

 ture, and they are also protected better from the drying effect 

 of wind. The leaves are either leathery, stiff, and hard, cr 

 small scale-like leaves with a thick cuticle, as in Cassiope, or 



