Studies in Prairie and Woodland 23 



more xerophilous during the summer of 1917 than in 1916. But 

 nevertheless they also indicate a close correlation between the 

 evaporating power of the air and the stage in the succession. 

 This relation was even more emphatically demonstrated in 191 7 

 than in 1 91 6. 



Our data for 191 7 also corroborate in an interesting and con- 

 clusive degree the really quite close approach of the conditions in 

 the Rhus thicket to the mesophilous conditions of the woodland, 

 a fact that appeared during our 1916 studies. This is particularly 

 significant when we remember that the Rhus thicket is but a very 

 small area on a southwest slope and completely surrounded by 

 typical prairie, the xerophilous conditions of which have been 

 already pointed out. 



The course of evaporation upon the prairie during the season 

 of 1917 doubtless indicates some of the peculiarly trying condi- 

 tions experienced by the propagules of shrubs and trees, and in 

 fact of whatever plants may have found their way there, in be- 

 coming established in such a habitat which, at best, is a real test 

 of the amplitude of physiological response possessed by the plants 

 in question. The relations of these extreme conditions of the 

 prairie to the possibility of tree establishment have been investi- 

 gated by a series of studies conducted in 191 7, the results of 

 which will appear in another paper. 



The monthly recurrence of relatively high evaporation maxima 

 recorded for the season of 191 7, especially during the physiolog- 

 ically important fore-summer, are doubtless of great significance 

 in the maintenance of the typical xerophily of the prairies. If 

 such seasons occur in fairly close succession we have much added 

 light upon the reasons for the very slow invasion of prairie by 

 woodland species. Such would also go far toward showing why 

 trees probably never become directly established upon the prairies. 

 Because of these and other important relations we ought to secure 

 data to show the march of evaporation for a long period of years 

 in such stations as we have established. 



29 



