48 A Study of the Vegetation of 



ences are pointed out here since it will be shown later that cer- 

 tain plants are quite confined to moist north and northeast 

 hillsides. 



Studies of the water content of these soils have been carried 

 on since the spring of 1912. It will be unnecessary to burden the 

 reader with all of the data and graphs obtained, and only enough 

 will be given to make plain the seasonal march of soil water. In 

 Fig. 12 are graphs giving the march of soil water from April 25 

 to September 25, 1913, on a typical northeast and southwest slope, 

 respectively. The ordinates represent percentages of soil mois- 

 ture in the first ten inches of soil. The rainfall between the in- 

 tervals of readings is also shown in inches, each ordinate repre- 

 senting o.i inch. The horizontal solid and broken lines show 

 the wilting coefficients of the soils on the northeast and southwest 

 slopes respectively. The greater amount of moisture on the 

 northeast slope (in some cases'being twice that of the southwest 

 slope) may be noted at a glance, while the fact that the soil on 

 the exposed slope reached its wilting coefficient about July 15, 

 and more than five weeks before similar conditions obtained on 

 the sheltered slope is significant. Records for the fall of 1913 

 were discontinued when the rains of late September replenished 

 the parched soil. In 1914 these ten-inch soil moisture determina- 

 tions were made only at longer intervals and with the object of 

 determining the time at which the wilting coefficient was reached. 

 The water contents on the dates of these determinations are indi- 

 cated by the light lines, the solid line representing soil moisture 

 on the northeast slope. The rainfall for June, 1914, being ap- 

 proximately normal (and not 1.6 in excess of the mean, as in 

 1913) the wilting coefficient of these soils was reached much 

 earlier than in the preceding year. An examination of these 

 determinations, together with the rainfall records at Pullman, 

 shows that at no time after June 8 and until September 14, was 

 there water available for plant growth in the first ten inches of 

 soil on the southwest slope. The autumn and winter rains 

 replenish the soil of the water lost during the long period of 

 drought and in the following spring it again shows a maximum 

 water content. 



