THE POOTHIIiL VEGETATION 107 



bution. South and southwest slopes are usually grassy, or 

 covered with an open forest of yellow pine and Douglas spruce, 

 the former the more abundant. The northern and eastern slopes 

 usually bear heavy forests in which Douglas spruce prevails, or 

 in which larch is locally strong sometimes over considerable 

 areas. 



The difference between opposite slopes is a common and 

 striking phenomenon over wide ranges of landscape and con- 

 cerns mainly the yellow pine and Douglas spruce. That this 

 is a reaction to soil moisture woidd seem to be evident. Experi- 

 mental operations, however, do not wholly support this view. 

 Soil samples taken from the depth of a foot at five different 

 stations on each of two slojjes for comparison, the one facing 

 north, the other south, were placed in glass stoppered bottles 

 and removed to the laboratory. This was at the close of a rain- 

 less period of three months. By the removal of the stoppers 

 they were allowed to become air dry. After a lapse of several 

 weeks the samples from the slope facing the north had lost an 

 average of 3.0% of their original weights; those from the oppo- 

 site slope had lost 2.5% of their original weights. The samples 

 were then kept for three weeks in an oven at a temperature of 

 120 degrees C. At the conclusion of this period the samples of 

 the former lot had lost an additional .9% and of the latter .8%. 

 The total loss therefore, in terms of the original weight, was 

 3.9% from soil of the northern and 3.3% from soil of the south- 

 ern exposure. There was a difference therefore of only .6% 

 in moisture between the two series. The southern exposure 

 presented the usual open yellow pine forest with almost bare 

 soil; the other bore a dense cover of young Douglas spruce and 

 larch with ground cover oP mosses and other small plants. Com- 

 parisons were also made on a series of soil samples from the 

 level gravelly plain and an open treeless slope, covered with 

 grasses and other plants, according to the same methods and 

 subject to the same conditions. The former showed a total 

 water content of 1.9%, the latter 4.3%. 



"While the evidence from soil examinations is not so con- 

 vincing as one might expect, the data are too meager to justify 

 conclusions. It may be expected that the longer the period of 

 drouth the more closely -will the different sites approach uni- 



