THE PEAIKIES 211 



It will be observed that the maximum annual rainfall (56.60 

 inches) occurred in 1881, and the minimum (16.63 inches) in 

 189-4, while the average was 33.05 inches. The annual rainfall 

 dropped below 22 inches but once during the 43 years, — in 

 1894. During that year some of the trees in artificial groves 

 perished, but many more survived together with most of the 

 trees in the native groves of that part of the state. During the 

 remaining years the rainfall was on the whole sufficient and trees 

 suffered, especially in the drier years, only in the exposed situa- 

 tions which are discussed in the preceding part of this paper. 



Temperature 



"While the influence of temperature upon plants is marked, 

 it has not been regarded as the direct cause of the floral peculiar- 

 ities of the prairie. Dana (1849) early attached some impor- 

 tance to it, and later (1875) considered it in general terms, to- 

 gether with topography and winds. The writer (1900) con- 

 sidered it briefly in relation to injury to trees, and Warming 

 (1909) ascribes the treelessness of the prairie to long winters 

 and hot, dry summers, and to cold nights during the earlier 

 growing season. 



Whitney (1876) regarded the factor as of little importance, 

 indeed he says (p. 581) that he is ''not aware that this has 

 ever been suggested as having anything to do with the phenom- 

 enon in question." We must not forget, however, that while the 

 direct influence of temperature is not great because of its varia- 

 tion within comparatively safe limits during the growing season, 

 the great importance attaching to it in its relation to evaporation 

 makes it one of the most potent of the causes which influence the 

 welfare of plants. 



Wind 



Wind has been discussed in various relations to this subject. 

 Nutt (1833) seems to have been the only Avriter who considered 

 the mechanical influence of tornadoes sufficient to cause treeless- 

 ness on the southern prairies. Phillips (1844) considered wind 

 in conjunction with prairie fires. Dana (1849) refers to ocean 

 winds as a factor, and again (1875) to winds in connection with 



