VEGETATION OF THE STRAND. 595 



higher dunes landward. There is also a corresponding grada- 

 tion in the size of the vegetation comprising the formations; 

 grasses and lower shrubs with a very open formation in the 

 region of younger dunes; larger shrubs and often dense thickets 

 with a few trees in the middle region of the dunes intermediate 



Fig. 520. 



Compact, radiate clumps of willow on sand-dunes on the outskirts of the city 

 of Southport, England. (Photograph by the author.) 



in size; while the outer landward dune region is often occupied 

 by forests, which then are frequently continuous with the forested, 

 sandy plane beyond. 



1089. Active, or wandering, dunes. This name has been 

 applied to dunes which are moving, and is characteristic of 

 many younger and growing dunes. Then also stationary dunes 

 may change to active ones. One of the reasons why a station- 

 ary dune may become an active one is that when it increases in 

 size plant life is limited, owing to the fact that the plants on 

 the surface are farther from the soil moisture, since the sand 

 does not retain water readily.. In the second place, some of 

 the trees on dunes in certain regions, the cottonwood tree for 

 example, is of comparatively short life, and the same is true of 

 the sand-cherry. When the plants die, therefore, the top of 

 the dune on the windward side is blown off, because there is 

 no vegetation to protect and hold the sand. This is the begin- 



