594 RELATION TO ENVIRONMENT. 



for this purpose are: ist, perennial habit, since the death of 

 an annual would cease to afford shelter for the accumulated 

 sand; 2d, rhizome propagation; by the subterranean stems 

 the plant is enabled to spread and increase the mat and thus 

 afford an opportunity for enlargement of the dune; 3d, ability 

 of the stems to grow out of the sand when buried; 4th, highly 

 developed xerophytic structures to endure the severe conditions 

 in the sand; $th, ability to stand at least partial root exposure 

 where the roots on the windward side may be uncovered. 



The best dune-formers as found in the Lake Michigan region, 

 according to Cowles, are as follows: 



1. Grasses with rhizome formation. 



Ammophila arundinacea. Most abundant. 

 Agropyron dasystachyum (northward). 



2. Grasses forming clumps. 



Elymus canadensis. 

 Calamagrostis longifolia. 



3. Shrubs. 



Salix adenophylla. Most abundant. 



Salix glaucophylla. 



Prunus pumila. 



Cornus stolonifera (or C. baileyi). 



4. Trees. 



Populus monolifera. 



Populus balsamifera. 



1088. Character of formations on dunes of different size. 

 Very often embryonic dunes are formed on the middle or upper 

 beach, but these usually are temporary; sometimes those on the 

 upper beach may migrate landward and grow to become larger 

 dunes. In general, in age and size the dunes bear a definite rela- 

 tion to the shore-line, the younger and smaller ones lying toward 

 the beach, while the older and larger ones are usually landward. 

 There is, however, great variation in size, so that the land pre- 

 sents often a hilly or wavy, billowy appearance. The forma- 

 tions also show the same tendency from young and open ones 

 at the beach to the older and often closed onse on the older, 



