The Ohio Naturalist. 
[Vol. XI, No. 6, 
3 28 
habit reactions. The effects of dessication in the physiologically 
arid habitats resulted in greater differentiation of organs, in pro- 
tective and resistance features (9), and in a greater range of 
dispersal. The vegetation had now developed to forms capable 
of occupying dry land-, and able to maintain themselves as bog or 
desert vegetation in localities restricting functional activity. The 
general movement finally resulted in a land flora of which the 
mesophvtes are the highest expression. The lowland basins and 
regions of coal formation were undoubtedly the regions of the 
evolution of the flora as a whole and of the several natural plant 
formations which include many diverse species in a unity of 
characteristic physiognomy and growth form. Probably the 
arctic regions were then the most favorable for the growth and 
development of xeromorphic forms. Migration from northern 
•centers of dispersal, the periods of climatic aridity, and the 
changes immediately before and after ice invasion, undoubtedly 
accentuated the ecological evolution of this type of vegetation. 
The extensive change in floral types which is particularly evi- 
dent through the subordination of the ferns to grasses and heath 
plants, and the elimination and replacement of the primitive 
gvmnosperms by the later gymnosperms and angiosperms is 
largely one of range and variability of protoplasmic forces. In 
some types the characteristics often bear no apparent relation to 
the environment and are retained under the most varied condi- 
tions, yet many other types are profoundly and rapidly modified 
bv changes in climate, physiography, and soil processes. 
The great development of form in response to the environ- 
mental stress was attended by a rapid and luxuriant expansion in 
range, in successions of vegetation formations, and in sequence of 
associations. Several forms of cycads, Bennettites and conifers 
now inhabit desert areas. Not less interesting is the fact that 
many species of heather-plants of Europe such as Calluna, 
Empetrum, several species of pines (Pinus sylvestris, P. 
montana), Juniper (Juniperus communis), birches (Betula, 
pubescens, B. nana), Labrador tea (Ledum palustre), bladder- 
wort (Utricularia cornuta), and others, can grow both on extremely 
dry, warm soil and on extremely cold or wet soils. The observa- 
tion has repeatedly been made by the writer that in the northern 
parts of Michigan several species of bog plants leave the peat 
soils entirely and are only found upon dry and poor soils. This 
is notably the case with tamarack (Larix laricina), the choke- 
berries (Aronia nigra, A. arbutifolia) , the blueberries (Vaceinium 
corymbosum, V. canadense), the black huckleberry (Gavlussaccia 
bacata), the shrubby cinquefoil (Potentilla fruticosa), sweet gale 
(Myrica gale), the steeple bush (Spiraea tomentosa) and several 
other xerophytes of the peat bogs of Ohio. The cranberries 
(Vaceinium sp.), creeping snowberrv (Chiogencs hispidula), and 
