96 BOTANICAL GAZETTE [ FEBRUARY 
response to varying conditions. Plant formations should be 
found which are rapidly passing into other types by reason of a 
changing environment. 
These requirements are met par excellence in a region of sand 
dunes. Perhaps no topographic form is more unstable than a_ 
dune. Because of this instability plant societies, plant organs, 
and plant tissues are obliged to adapt themselves to a new mode — 
of life within years rather than centuries, the penalty for lack of 
adaptation being certain death. The sand dunes furnish a favor- | 
able region for the pursuit of ecological investigations because 
of the comparative absence of the perplexing problems arising 
from previous vegetation. Any plant society is the joint product 
of present and past environmental conditions, and perhaps the 
latter are much more potent than most ecologists have thought. | | 
As will be shown in another place, even the sand dune floras are | 
often highly modified by preexisting conditions, but on the 
of a region, and with their death there pass away the influences 
which contributed so largely to their making. In place of the 
rich soil which had been accumulating by centuries of plant and 
animal decay, and in place of the complex reciprocal relations - : 
between the plants, as worked out by a struggle of centuries, . ce 
advance of a dune makes all things new. By burying the past, 
the dune offers to plant life a world for conquest, subject alm 
to 
=> 
‘ence of opinion as to the influence of salty soils and atmo 
upon the vegetation. It would seem that a comparison of d 
