558 CORRELATION OF DISTRIBUTIONAL FEATURES. 



The failure of Agropyron to extend farther into the northern part of 

 the Great Basin Desert is apparently due to the moisture conditions 

 of that region. A more precise correlation of its distribution with that 

 of the moisture ratio (ir/E) would probably show that it does not 

 occur where the values of this ratio are lower than 0.20, the same value 

 that limits it in Arizona and New Mexico. The actual moisture-ratio 

 conditions of the areas occupied by Agropyron in Utah and northern 

 Nevada are poorly exhibited by our data, which are from stations 

 located in the valleys. 



TCMFIKATVflC 



Oats in Normal Fiio*TLta> Suaon (F. SJ I 

 Hot Oats. F. S. ■ 



Cold Oati, F. S. ■ 



Pmysiolooical Summation, F. S. C 



Normal Dailt Mian, coldest 14 dats or Vcar I 

 ,NoRMAL Dailt Mian, Ycau I 



Priciritation 

 Normal Dailt Mian, F. S. C 



Days in longest Normal Raint Piriod, F. S. ■ 

 Oats in lonscst Normal Drt Period, F. S. C 

 Mian Total, Yiar C 



Evaporation 

 Dailt Mean. 1S87-e, F. S. C 



MoiSTURi Ratios 

 Normal p/e, F. S. 

 Normal ir/E, F. S. 

 Normal P/E, Yiar 



humioitt 

 Normal Mean, F. S. C 



sunshini 

 Normal Daily Duration. F. S. I 



' MOISTURI-TlMPERATURI INDICES 



NoRtML P/t X T, F, S., Pntsiolosical Mctmoo cm 



Fig. 64. Climatic extremes for Agropyron spicatum. 



Hilariajamesii (fig. 65). — This is a characteristic grass of the Desert- 

 Grassland Transition region, ranging from western Texas to southern 

 Nevada and northward to extreme southwestern Wyoming. All of 

 the climatic conditions exhibit narrower ampUtudes for Hilaria than 

 for the Grassland, with the exception of the number of days in the 

 longest normal dry period. The narrowest amplitude among the 

 temperature conditions is that of the normal daily mean for the coldest 

 14 days of the year, which ranges from 27° to 45°. At few points does 

 the limit of Hilaria extend north of the isoclimatic line for 25° as the 

 daily mean of the coldest fortnight, and the correspondence of these 

 fines would indicate that this condition is an important one in limiting 

 the northward distribution of the grass. 



The eastward extension of Hilaria in Texas is such that it nowhere 

 encounters moisture ratios (t/E) higher than 0.40, nor rainy periods 

 of more than 25 days. These are the same conditions that appear to 

 limit the eastward range of other grasses, but different intensities are 

 involved in the case of Hilaria from those mentioned in connection 

 with Bouteloua. 



