108 BOTANICAL GAZETTE [FEBRUARY 
leeward side, but white to the windward because the colored 
outer layers of the bark have been wholly worn away. On the 
windward side of basswood limbs the softer portions are carved | 
away while the tougher fibers remain as a reticulated network. 
On the leeward side of these same limbs, the outer bark is intact 
and even covered more or less with lichens. & 
The indirect action of the wind produces effects that are con- 
siderably more far-reaching than any other factor, for it is the 
wind which is primarily responsible for sand dunes and hence | 
for their floras. But more directly than this, the wind playsa | 
prominent part in modifying the plant societies of the dunes 
The wind is the chief destroyer of plant societies. Its methods | 
of destruction are twofold. Single trees or entire groups of plaats 
frequently have the soil blown away from under them, leavilf | 
the roots exposed high above the surface ; as will be shown later : 
this process is sometimes continued until entire forests are under 
mined, the débris being strewn about in great abundance. Ag 
swamps, forests, and even low hills may be buried by the om | 
advance of a dune impelled by the winds; in place of a divers: 
fied landscape there results from this an all but barren waste of 
sand. 
SOIL. 
The soil of the dunes is chiefly quartz sand, since quarta a 
resistent to the processes of disintegration. The quattz ed ] 
ticles are commonly so light colored that the sand asaW -t 
appears whitish; closer examination reveals many grains ie 1 
are not white, especially those that are colored by iron oxide ] 
With the quartz there are conspicuous grains of black : 4 
largely hornblende and magnetite. These black grains - : 
accumulate in streaks, persistent for considerable depths ak 
apparently sifted by the wind; large quartz grains are ™ 7 
with these grains of magnetite and hornblende = 2 q 
would seem as if grains of higher specific gravity are sift s 
together with those of greater absolute weight. The sand 
dunes is remarkably uniform in the size of the particles as 
pared with beach sand; this feature is due to the selective 
30 
