38 A. DaohnowsM—JProblem of \Xeromorphy. 



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. The extensive change in 

 floral types which is particularly evident through the subordi- 

 nation of the ferns to grasses and heath plants, and the elimi- 

 nation and replacement of the primitive gymnosperins by the 

 later gymnosperins and angiosperms is largely one of range 

 and variability of protoplasmic forces. In some types the 

 characteristics often bear no apparent relation to the environ- 

 ment and are retained under the most varied conditions ; yet 

 many others are profoundly and rapidly modified by changes 

 in climate, physiograpby, and soil processes. 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-invasions, undoubtedly 

 accentuated the ecological evolution of this type of vegetation. 

 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 the cycads, Bennet- 

 tites and conifers now T inhabit desert areas. Not less interest- 

 ing is the fact that many species of heather- plants of Europe 

 such as Calluna, Empetrnm, several species of pines (Pinus 

 sylvestris, P. montana), juniper (Juniperus communis) 

 birches (Betula pubescens, B. nana), Labrador tea (Ledum 

 palustre), bladderwort ( Utricularia comuta) and others, can 

 grow both on extremely dry, warm soil and on extremely cold 

 or wet soils. The observation 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 tamar- 

 ack (Larix laricina) the chokeberries (Aronia nigra, A. arbuti- 

 folia), the blueberries ( Vaccinium corymbosum, V. canadense), 

 the black huckleberry (Gaylussaccia bacata), the shrubby 

 cinquefoil (P otentilla fruticosd), sweet gale (Myrica gale), the 

 steeple bush (Spiroza tomentosa) and several other xerophytes 

 of the peat bogs of Ohio. The cranberries ( Vaccinium sp.), 

 creeping snowberry (Chiogenes hispidula), and wild rosemary 

 (Andromeda polif olio) occur in moist ravines and rich woods, 

 while leather leaf (Chamwdaphne calyculata), the buck bean 



* Bower, F. 0., The origin of a land flora. London, 1908. 



