1898] VEGETATION REGIONS OF THE PRAIRIE PROVINCE 385 
common to the entire province, while one hundred and twenty- 
nine are found throughout the central and southern portions, 
and ninety-one throughout the central and northern portions. 
The vegetation-center of the prairies is found in Nebraska, 
lowa, Kansas, and the Dakotas. From this center the typical 
plains flora slowly shades out toward both north and south; 
naturally enough over a country so little diversified with about 
equal rapidity. The effect of higher altitudes and greater 
distance from the vegetation-center accounts for the greater 
ultimate reduction in number of species to the northward. 
The fact that species of distinctly northern range are found 
only in one extreme of the prairie province, or those of southern 
origin only in the other, is of trifling importance. All vegeta- 
tion regions, and none more readily than a plains region, where 
dissemination is so easily effected, borrow floral elements from 
adjacent regions, and it is only when this invasion has resulted 
toa pronounced degree that the original floral covering changes 
aspect. As has been stated above, the facies and principal 
Species of the province are essentially the same throughout, and 
i the comparatively small number of strictly southern, or northern 
- Species are of purely secondary importance in the consideration 
of the floral covering. 
In the characterization of the Missouri prairie region, Drude 
Utes Bouteloua oligostachya and Bulbilis dactyloides as the most 
: mmon prairie grasses. Both of these grasses, in fact, are 
- “qualed in abundance and in importance by several species, such 
Andropogon scoparius, Aristida purpurea, Stipa comata, Agropy 
p Beudorepens, and Koeleria cristata, all of which are of the widest 
» “stribution, Bulbilis, on the other hand, is entirely lacking 
er the vast prairies of the Saskatchewan. It may be remarked 
| » Passing that the supposed disappearance of buffalo-grass, 
1S dactyloides, connected more or less poetically with the 
is a popular myth. The buffalo-grass 
