278 



PLANT PHYSIOLOGY AND ECOLOGY 



it is xerophytic. Both become finall}' more or less mesophytic, 

 but in one this takes place by decreasing water content and in 

 the other by increasing it. 



297. Succession in aeolian soils. Wind-borne soils are rep- 

 resented chiefly by inland and coastal dunes. These consist of 

 rounded sand particles of almost uniform size, although this 

 varies greatly in dunes of different ages. The reaction of the 

 pioneers upon dunes plays an important part in building them 

 up, by virtue of binding and holding the shifting soil. A dune 





^^i 



Fig. 113. First stage of an seolian succession, sand dunes of Cape Henry. 



succession usually starts with xerophytes and terminates in 

 mesophytic meadows or forests. Inland dunes occur in dry 

 regions, and their successions are xerophytic throughout. 



298. Secondary successions. Practically all successions on 

 denuded soils are secondary. Exceptions occur only when the 

 action of the denuding force is so intense that an entirely new 

 soil is exposed, as in landslides. The great majority of secondary 

 successions owe their origin to erosion, floods, or the activities 

 of man. They occur ordinarily upon soils of medium water con- 



