1908] HARVEY— PRAIRIE-GRASS FORMATION 297 



ductive organs possessed by such a large percentage of its com- 

 ponents. 



As to pollination, 20 per cent, of the forms are wind-pollinated, 

 while 80 per cent, have their pollen transferred by insects, the 

 sedges and grasses comprising the former group. 



Conclusions 



1. The formation is a part of the Niobrara Prairie Region of 

 Clements. In composition it is transitional. More truly a part 

 of the prairie to the west, yet it contains several pioneer forms from 

 the more mesophytic prairies to the south and east. 



2. These two groups of elements during post-glacial migration 

 have entered along two distinct lines of advance. The former mi- 

 grated northwestward from a southwestern center of dispersal, while 

 the latter followed a northwestern track up the Mississippi and 

 Missouri valleys. 



3. The prairie is pre-glacial in origin and is descended from the 

 climatic prairie of Tertiary times, which arose in response to reduced 

 precipitation caused by the upheaval of the Rocky Mountains at the 

 close of the Cretaceous. 



4. The cHmate is typically a prairie cHmate. k relatively dry 

 resting season from October to March, in which only t6 per cent. 

 (10.4*=™) of the total precipitation falls, and a moist growing season 

 from March to September, in which 83 per cent. (49.31^™) 

 of the precipitation is distributed over sixty days, with 25 per 

 cent, concentrated in April and May, insures a prairie formation. 

 On the other hand, the annual low relative humidity, the dry and high 

 winter winds accompanying high temperature, low winter rainfall, 

 absence of a snow blanket, and the hot, dry summer of low precipi- 

 tation are inimical to tree growth. 



5. The absence of trees upon the prairie is primarily to be explained 

 upon historical lines. The prairie was climatically determined and 

 successfully and successively maintains itself against tree invasion 

 from the edaphically determined arboreal fringes along flood plains 

 and in ravines. 



6. The northern slopes are the last to recover from winter, but 

 are most mesophytic. It is up these slopes that the Poa sod and the 



