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 0.1 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. 
48 
