556 CORRELATION OF DISTRIBUTIONAL FEATURES. 



ticularly favorable subject for fuller investigation. Its ability to with- 

 stand a considerable range of temperature conditions between Missis- 

 sippi and South Dakota and its ability to range through dissimilar 

 moisture conditions from Ohio to Texas should be investigated in terms 

 of the habitat requirements of the plant in the various portions of its 

 range. From such information as is available, this plant seems to be 

 confined to the most arid situations on the eastern edge of its area 

 and to relatively moist situations or seasons on the western edge, so 

 that it would be particularly valuable to have parallel series of data 

 for the local conditions met by the most widely separated colonies. 



Bouteloua oligostachya (fig. 63). — This abundant and characteristic 

 grass of the Great Plains is found throughout the Grassland, in the 

 western part of the Grassland Deciduous-Forest Transition, and in the 

 southeastern part of the Desert. It is most abundant in the Grassland 

 and the Desert-Grassland Transition, becoming infrequent at the 

 southeastern and southwestern corners of its area. 



The temperature conditions encountered by Bouteloua are those of 

 the Grassland, but with slightly higher maxima in each case (except 

 number of cold days) . The amplitude of the precipitation conditions 

 is much greater for Bouteloua than for the Grassland, the maxima 

 being higher and the minima lower. With respect to the length of the 

 longest normal dry period particularly, Bouteloua exhibits its ability 

 to range from the conditions of the Grassland far into those of the 

 Desert, enduring 283 days in the vicinity of Phoenix, Arizona. Like 

 many other perennial grasses, it is able to withstand prolonged and 

 severe conditions of drought in its resting condition and to take advan- 

 tage of moist periods of relative infrequency. 



TCMPCRATUNC 



0*v« IN Normal FnosTLEsa S»«on T. S.) L 

 Hot Days. F. S. ■ 



Colo Days. F. S. ■ 



Physiological Summation, F, S. Gfl 



Normal Daily Mcan. colocst 14 oays of Ycar !■ 

 Normal Daily Mean. Ycam ■■ 



Precipitation 

 Normal Daily Mean, F. S. I ■ 



Days in longest Normal Rainy Period, F. S. ^H 

 Days in longest Normal Dry Period. F. S. I I 

 Mean Total, Year I 



Evaporation 

 •aily Mean, 1667-8, F. S. I 



Moisture Ratios 

 Normal P/e, F. S. 

 Normal it/e, F. S. 

 Normal P/E, Year 



Humidity 

 Normal Mean, F. S. C 



Sunshine 

 Normal Daily Duration, F. S. I 



Moisture-Temperature Indices 



Normal P/E x T, F, S., Physiological Method Di 



Fig. 63. Climatic extremes for Bouteloua oligostachya. 



