CORRELATION OF DISTRIBUTIONAL FEATURES. 557 



The minimum conditions of evaporation and the maximum condi- 

 tions of humidity are very similar for Bouteloua and the Grassland 

 region, indicating that conditions which determine the eastern limit 

 of the Grassland also limit one of its most characteristic plants, and 

 this is also true of Bulhilis dactyloides and Bouteloua hirsuta. The 

 maximum conditions of evaporation and the minimum conditions of 

 humidity, however, are respectively higher and lower for Bouteloua 

 than for the Grassland. The moisture ratios for the frostless season 

 are very similar for this plant and for the Grassland as a whole, although 

 the minimum values for Bouteloua are lower. 



The eastern limit of Bouteloua, like that of many of the grasses 

 associated with it, is apparently set by some one of the moisture con- 

 ditions, or by a combined operation of several of them. Neither the 

 area of Grassland nor that of Bouteloua extends very far into the 

 region with more than a daily mean precipitation of 0.100 inch with 

 more than 75 days in the longest rainy period, or with a mean annual 

 precipitation of more than 25 inches. The position of the western 

 boundary of Bouteloua indicates that it is there again limited by mois- 

 ture conditions. Although we have presented no data bearing directly 

 on the seasonal distribution of precipitation, it is apparent that this 

 grass is unable to penetrate far into the portion of the Desert, in which 

 the sunomer rainfall is light. The abihty to withstand dry periods of 

 as much as 283 days has enabled it to range as far as the area of uncer- 

 tain summer rains in the lower Colorado Valley. It is not able, how- 

 ever, to extend its area into the region in which there is frequently 

 no summer rain for many successive years. 



Agropyron spicatum (fig. 64). — This grass is found throughout the 

 Grassland north of Oklahoma and New Mexico and in the outlying 

 portions of that vegetation which fringe the northern edge of the Great 

 Basin Microphyll Desert. Although withstanding the entire amplitude 

 of cold days this grass encounters lower and narrower amplitudes of 

 the other temperature conditions. It has a relatively low maximum 

 (125) for the number of hot days and a low maximum (11,600) for the 

 physiological temperature summation. With respect to precipitation 

 it shows wider amplitudes than those of the Grassland, and this is 

 true of evaporation and the moisture ratios for the frostless season. 



Agropyron appears to have its eastern limit set by the same con- 

 stellation of moisture conditions that controls the Grassland and other 

 grasses, but it does not follow the conditions favoring Grassland as far 

 as the southern Hmit of that vegetation. Nowhere does it encounter 

 more than 100 to 120 hot days, nor does it range into regions with a 

 physiological summation of temperature greater than 12,500. These 

 and associated temperature conditions appear to be responsible for its 

 southern limitation. 



