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 climate is typically a prairie climate. A relatively dry 
testing season from October to March, in which only 16 per cent. 
(10. 4°") of the total precipitation falls, and a moist growing season 
ftom 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- 
‘ation are inimical to tree growth. 
5- The absence of trees upon the prairie is primarily to be explained 
‘pon historical lines. The prairie was climatically determined and 
successfully and successively maintains itself against tree invasion 
m 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 
