the 9tli, 10th, and 11th of August ; when the wind was from the 

 south or southeast and banked the vapors around this station, 

 evaporation was much reduced. Station 6 was so completely- 

 sheltered from both sun and wind that evaporation was reduced 

 to a negligible quantity. 



The foregoing facts emphasize the importance of atmospheric 

 factors in influencing the distribution of the prairie and forest 

 floras. It must not be assumed that soil conditions have no in- 

 fluence, for the temperature and fineness of the soil, the amount 

 of free soil water, and perhaps other soil qualities, undoubtedly 

 produce some effect, but the determining causes of relative prai- 

 rie and forest distribution evidently lie in the atmosphere rather 

 than in the soil. 



I — THE PRAIRIE FLORA 



The writer first entered upon this work at a time when a large 

 part of the region west of the lakes was an unbroken prairie, and 

 at intervals he has watched the latter in all its moods and phases 

 to the present day. Excepting for the disappearance of the na- 

 tive flora in the cultivated areas there has been little change in 

 the character of the prairie, and where its surface has remained 

 unbroken its flora has suffered but little by elimination or by the 

 addition of introduced forms, and locally suggests even today the 

 splendor of the great prairie flower-garden which once covered 

 the greater part of our state in the mid-summer season. 



The prairie flora of this region does not differ materially from 

 that of other corresponding parts of the state. As has been re- 

 peatedly pointed out it is distinctly xerophytic. The plants pre- 

 sent the usual narrow, reduced, rigid leaves (or if the leaves are 

 broader they are leathery, or are covered with hairs or scales), 

 the large root system, the varied devices for protection, such as 

 thick cutin, protected stomata, compact leaf tissue, water storage 

 tissues, and resinous or milky secretions, all of which characterize 

 xerophytes. The compact tissues are well illustrated in figs. 2, 

 3, and 4^ on Plate lY ; ^ the cutinized epidermis in figs. 1 and 2 ; 

 trichomes in figs. 2, 3, and 4; enlarged epidermal or bulliform 

 cells, and companion cells surrounding the vascular bundles, 

 in fig. 1 (both water reservoirs) ; or with additional connecting 



8 These and the following figures are copied from the still unpublished thesis of 

 Miss Ella Shimek, on the "Ecological Histology of Prairie Plants." The drawings 

 were made from sections of leaves collected near the Lakeside Laboratory. 



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