



Igor] PHYSIOGRAPHIC ECOLOGY OF CHICAGO 171 
B. The embryonic or stationary beach dunes.—Wherever plants 
occur on a beach that is swept by sand-laden winds, deposition 
of sand must take place, since the plants offer obstacles to the 
progress of the wind. If. these plants are extreme xerophytes 
and are able to endure covering or uncovering without injury, 
they may cause the formation of beach dunes. Among the 


Fic. 33.— Beach at Dune park, showing the smooth and naked lower beach, the 
ene beach with its line ot débris, the upper beach with scattered shrubs, and the 
dune 
dune-forming plants of this type are Ammophila arundinacea, 
Salix glaucophylla and S. adenophylla, Prunus pumila, and Populus 
monilifera. The shapes of these beach dunes vary with the 
characteristics of these dune-forming plants. Ammophila dunes 
are extensive but low, because of strong horizontal rhizome prop- 
agation. Prunus and Populus dunes are smaller but higher, 
because of the relative lack of horizontal propagation and the 
presence of great vertical growth capacity. Dunes are formed 
more slowly in protected places, and here the dune-forming 
species may be plants that are ill adapted to the severest beach 
conditions, such as the creeping juniper. 
