488 BOTANICAL GAZETTE [JUNE 
directly. The diagram shows also that those associations which 
stand high genetically have a lower evaporation rate than those 
genetically lower in the successional series. 
This relation between succession and evaporation is best shown 
in the sand dune habitats, where four sets of atmometers were 
maintained. The original vegetation there is the bunch-grass 
Fic. 5.—Interior of the mixed el association, showing atmometer no. IT, 
July 9, 1910; photograph by F. 
association, but under certain conditions portions of the association 
may be destroyed by wind action, resulting in open areas of bare 
sand, the blowouts. The present blowouts are all of secondary 
origin, but their environment and vegetation probably represent 
the primitive condition of the region, shortly after its emergence 
from the glacial Illinois River. In these blowouts the highest 
evaporation occurs. The blowout is eventually revegetated by 
bunch-grass, during which process two or more consocies of the 
bunch-grass association appear. The Leptoloma cognaium consocies 
