208 NATUKAL HISTOEY BULLETIN 



claim usually made by meteorologists that forests have no effect 

 on precipitation has no significance in connection with our prob- 

 lem, nor has it any application to the question of the influence of 

 the forest on moisture, for the forest must be considered as a 

 conservator of moisture rather than a rain-maker. 



It must be remembered that a tree or any other ordinary 

 plant is quite as much dependent for the possibility of carrying 

 on its functions on the moisture of the air as it is on the 

 moisture of the soil, and any conditions which serve to dissipate 

 the moisture of the air will be fatal to many plants, especially 

 those which are mesophytie, even though sufficient moisture falls 

 upon the soil. 



Both the moisture of the air and of the soil will be conserved 

 by protection against evaporation, and this may be accomplished 

 by topography or groves. 



The influence of these factors upon the melting of snow should 

 also be considered in this connection, for the retarding of melting 

 by either factor (see plate I, figure 2) results in more complete 

 absorption of the resulting water by the soil and the water is 

 made available for plants during a longer period. 



Previous Work 



The climatic and topographic factors here especially empha- 

 sized as causing the treelessness of the prairies, have received 

 previous attention, both individually and in combination, but 

 the references have been largely general or incidental. For the 

 most part these factors have been given a secondary place, or 

 have been treated as of minor importance, as was done by Whit- 

 ney (1858, 1876 and 1882), who argued against their effective- 

 ness, declaring (1876) in his comparison of prairie and forest 

 floras, that "by no amount of ingenuity can the peculiarities of 

 the isothermal and isohyetal lines be made to play in with the 

 marked differences of the vegetation." 



In a few cases only were these factors held to be collectively 

 the chief cause of the prairie. Dana (1875) concluded (p. 45) 

 "that prairies, forest-regions, and deserts are located by the 

 winds and temperature in connection Avith the general config- 

 uration of the land", and Todd presented a more elaborate ar- 



