THE DUNES AND THEIR PLANTS 143 



of the older dunes may expose enough of the loose sandy soil to 

 the wind to initiate a blow-out. 



It is very evident, then, that any given area in the dune 

 region must be interpreted in the light of the complex of forces 

 operative upon it. Frequently one will get into a region that 

 is easily recognized as a cottonwood dune, a black oak associa- 

 tion, .or an interdunal swamp area, but not infrequently a par- 

 ticular spot will be in transitional conditions, a swamp being 

 invaded by a moving dune perhaps, where within a rod one may 

 pass from abundant water to bone-dry sand, from sphagnum 

 and pitcher plants and a crowded vegetation to a barren sand 

 slope. Old dunes are invaded by new ones, oaks superseded by 

 cottonwoods. In fact, the zones as presented above, while often 

 regularly placed, are at -times mixed in a thorough jumble as 

 blow-outs occur, moving dunes blow rapidly inland, or as the 

 succession is* here or there retarded or accelerated by other local 

 conditions. 



The following table presents a summary of the plant associa- 

 tions in the Dunes. It shows in which association each plant 

 listed is most likely to occur. It would be well for the student 

 to make a similar summary for each of the succeeding chapters, 

 viii-xiii. 



