110 BOTANICAL GAZETTE [ FEBRUARY 
situations which are most exposed to cold winds, one finds the 
best illustrations of the arctic type, while the deserf or xerophil- 
ous type is shown in its purest expression on protected inland 
sandy hills. The discussion of the various arctic and desert 
structures and their relations to each other will be deferred to. 
the second part of this paper. 
OTHER FACTORS. 
Certain other factors are of minor importance in determining 
the character of the dune flora. Forest fires occur occasionally, 
and, as will be shown later, they may considerably shorten the 
lifetime of a coniferous plant society. 
Near cities the vegetation is unfavorably influenced by smoke 
and other products issuing from chimneys. In the neigh 
borhood of the oil refineries at Whiting, Ind., the pine trees 
especially have been injured or destroyed. A careful study 
would probably show many plant species that have suffered? 
similar fate. : 
rectly affects the soil, since it is mainly in the depressions thi 
ence of man is seen, although such influences are slight ¥ ‘. : 
the sand is removed bodily for railroad grading and other a 
poses. ; 
The influence of plants, which so often becomes the 
nant factor, is relatively inconspicuous on the dunes. The 
