THE PRAIEIES 207 



However a limit must be reached, and moreover where hot, dry 

 winds blow almost constantly, as they do in summer during the 

 daytime in our territory, the loss of water from the unprotected 

 younger structures and the interference with the stomatal ap- 

 paratus which to some extent controls transpiration, must ulti- 

 mately result in the elimination of all plants which cannot well 

 resist these conditions. 



This limits the flora of these exposed areas largely to xero- 

 phytes, and it is a fact especially worthy of note, and one which 

 will be set out in detail in the near future, that the flora of the 

 prairies, especially that of the usually dry mid-summer period, is 

 decidedly xerophytic. This is further emphasized by the similar- 

 ity of the floras of the prairie and the xerophytic sand-dune areas, 

 the plants of the latter being with very few exceptions identical 

 with those of the dry prairie. That they are xerophytic is re- 

 vealed in the tufted habit, as illustrated in plate XI, figure 1, the 

 development of large root-systems and the stunting of the exposed 

 tops; the development of thick cutin, deep-seated stomata and 

 strong protective tissues ; the production of hairs and scales on the 

 exposed surfaces ; and various other adaptations which are recog- 

 nized as adding to the protection of plants growing on dry sur- 

 faces. Hence the structurally protected xerophytes of the prairie 

 persist in exposed places while the mesophytes of the forest fail, 

 and any cause or combination of causes tending to bring about 

 xerophytic conditions will eliminate mesophytes and give the 

 field to the xerophytes of the prairie. 



Rainfall should receive attention in connection with evapora- 

 tion, but it is here given but little prominence as a factor in de- 

 termining the treelessness of the prairies for the reasons that it is 

 entirely sufficient for forest growth within our territory, as shown 

 by ;i\Ir. Stern's table, and indeed it is sufficient throughout the 

 prairie sections of the Mississippi valley, if only properly dis- 

 tributed, and that it cannot be a determining cause because fre- 

 quently, as illustrated by several of the figures, prairie and for- 

 est are in close proximity, upon opposite sides of the same ridge, 

 where they evidently receive the same amount of rain. 



The conclusion is therefore inevitable that the question is one 

 of conservation rather than precipitation of moisture, and the 



