282 THE NORTH AMERICAN GRASSLAND 



formation. AVith an extent of nearly 12 degrees in latitude and 4,000 

 feet in altitude, the average precipitation ranges from 18 inches or 

 more in the east and at the upper elevations to 6 inches or there- 

 abouts along the margin of the desert. 



Like the mixed prairie, the eastern third of the area falls within 

 the rainfall belt characterized by receiving 70 per cent of its precipita- 

 tion between April first and September thirtieth. The percentage drops 

 to the west rather rapidly, falling to about 45 at the eastern line of 

 the desert. Over much of the region, in consequence, there are two 

 rainy seasons, winter-early spring, and late summer, separated by two 

 almost rainless intervals, and two corresponding growing seasons. 



Dominants. The dominants of the desert plains are as follows: 



Bouteloiia eriopoda Eragrostis lugcns 



rothrocki Andropogon cirrntus 

 radicosa hirtiflorus 



chondrosioides Boutdoun gracilis 

 Aristida californica Jdrsula 



arizonica curtipendula 



temipes trifida 



Hilaria mutica Aristida dii'aricata 

 Muhlenhergia arenicola purpurea 



monticola Hilaria cenchroides 



porteri Triodia pilosa 

 emersleyi mutica 



Sporoholus crypt andrus-flexuosus pulchella 



c-contractus Epicanipcs rigens 



c-giganteus Panicum obtusum 



Scleropogon brevifolius Setaria macrostachya 



Panicum halli Sporoholus cryptandrus 

 Trichachne californica airoides 



Leptochloa dubia Andropogon scoparius 



The desert plains possess a larger number of dominants than any 

 o'her unit of the grassland, as well as much the largest group of eu- 

 dominants, amounting to half or more of the total, as shown by the 

 left-hand column. This is in accord with the assumption that deserts 

 and adjacent arid regions are centers of active evolution. This asso- 

 ciation may well be regarded as the true short-grass community, since 

 half of the species belong to this life form which characterizes three- 

 fourths or more of the area. As would be expected, the bunch form is 

 overwhelmingly the rule, the ratio to the sod-formers, viz., Bouteloua 

 eriopoda, Hilaria cenchroides, and Panicum ohtusuin, being more than 

 12 to 1. Of the total number of dominants, 34 are found in Mexico, 

 9 extend to Central or South America, and only 10 northward of the 

 ecotone with the mixed prairie. 



