D 



28 The Ohio Naturalist. [Vol. XI, No. 6, 



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 occu]3ying dr}^ 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 

 mesophytes 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 

 gymnosperais by the later gymnospenns 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 arc profoundly and rapidly modified 

 by 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 fomiations, 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 Calltma, 

 Empetrum, several species of pines (Pinas 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 b}' 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. arbutif olia) , the blueberries (Vaccinium 

 corym.bosum, V. canadense), the black huckleberry (Gaylussaccia 

 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 

 (Vaccinium sp.), creeping snowberry (Chiogenes hispidula), and 



