284 BOTANICAL GAZETTE [ocrosER 
lines become obscured in the floodplain societies, and still more so in 
the bluff societies, but in each of them the relation of distribution to 
soil water, as a controlling factor, is plainly marked. The definite 
relation of certain societies to soil types, shown to exist here, appears 
to be due primarily to the capacity of these various soils for water. 
4. Of other factors to which the plant societies are evidently 
Telated, the influence of light is conspicuously manifest, as for exam- 
ple in the place taken by light-requiring species in the bluff vegeta- 
tion. Quite as manifest, though far more complicated, is the 
coincident operation of biotic factors, which are so numerous and 
varied in their manifold interrelations as still to demand much 
special study. 
5. The high per cent. of northern species in early blooming 
societies, the occurrence of various southern forms along the river 
near the northern limit of their range, the occupation of favorable 
places by societies of distinctively southern cast, and of unfavorable 
ones by those of pronounced northern composition, are all indicative 
of the close relation of the members of these societies to slowly chang: 
ing climatic conditions. A discussion of the migrations of these 
plants in connection with geographic and climatic changes is deferred 
until a greater accumulation of data has been made. 
UNIVERSITY OF MICHIGAN, 
Ann Arbor. 
