THE TRUE PRAIRIE. 121 



and Brauneria a somewhat similar result is secured by means of the basal 

 rosette. The roots of the great majority of these are deep-seated, apparently 

 for the purpose of escaping the competition of the grass roots in so far as 

 possible. Most of them place their roots at depths of 5 to 12 feet, and some 

 penetrate as deeply as 15 to 20 feet. 



Developmental relations. — From what has been said of the range of subclimax 

 dominants, it follows that the several associations are closely related in suc- 

 cessional development. The consocies and socies belong chiefly to the same 

 genera, and a large number of species, especially those in water, saline areas, 

 and Bad Lands, occur throughout. Phylogenetically, the formation shows 

 evidence of having derived its dominants originally from two distinct vege- 

 tations. Stipa, Agropyrum, and Koeleria appear to have come from an 

 original northern climax, which was forced southward during glacial times into 

 the steppes of Eurasia and the prairies and plains of North America. Bou- 

 teloua, Bulbilis, Aristida, and Andropogon are genera of southern origin, which 

 had probably pushed into the prairies and plains during the Miocene. It 

 seems likely that the four most vigorous species, Bouteloua gracilis, Aristida 

 purpurea, Bulbilis dactyloides, and Andropogon scoparius pushed still farther 

 northward after the Pleistocene, and came to be at home with the tall-grasses 

 of the northern prairies of the Dakotas, Montana, and Saskatchewan. The 

 ecological unity of this particular association is emphasized by Carex filifolia 

 and C. stenophylla, which resemble the short-grasses in life-form, but are 

 holarctic in origin. To the east of this central matrix was differentiated the 

 Stipa-Koeleria and to the west the Agropyrum-Stipa association, the one in 

 response to a moderate rainfall of the summer type, the other to winter pre- 

 cipitation. Within these there was a further tendency to separate into a 

 northern Agropyrum area and a southern Stipa one. This was well-marked 

 in the Pacific region, but it has completely stopped as a consequence of settle- 

 ment. In the south, a similar differentiation resulted in the Aristida-Bou- 

 teloua association, which still finds its best expression in Mexico, and the 

 Andropogon subclimax of the Mississippi Basin. 



THE TRUE PRAIRIE. 



STIPA-KOELERIA ASSOCIATION. 



Extent. — The true prairies occupy a distinct belt between the subclimax 

 and mixed prairies, reaching from Manitoba to Oklahoma. This position as 

 well as their relationship is shown by the presence of Andropogon scoparius 

 derived from one and Stipa comata from the other. The ecological relation 

 is well illustrated in northeastern Nebraska, where Andropogon furcatus and 

 A. scoparius occupy the meadows and moister slopes, and Stipa comata and 

 Bouteloua gracilis the drier upper slopes and crests of the Stipa-Koeleria hills. 

 To the southeast, increasing rainfall enables first Andropogon scoparius and 

 next A. furcatus to extend over the rolling hills, while to the west and north- 

 west reduced rainfall causes Stipa comata to dominate and then replace S. 

 spartea, and permits Bouteloua and Bulbilis to become constant associates of 

 the prairie grasses (plate 21). 



Cultivation has perhaps destroyed this association to a larger extent than 

 any other community of the grassland, and its limits are accordingly difficult 



