THE WESTERN VALLEYS 131 



Negatively the species demands abundance of moisture in 

 soil and air, its temperature limits are relatively narrow, lying be- 

 tween about -35 degrees and 95 degrees in this region. It is easily 

 damaged by fire and is moderately susceptible to disease. 



The positive qualities of this species are practically confined 

 to two things of particular importance, viz: the seeding habits 

 and tolerance. The hemlock is especially fortunate in both ; in 

 the former as providing a great number of individuals in the 

 competition for space, in the latter as successfully opposing the 

 suppressing influence of other species. These are important ele- 

 ments within the suitable areas, but so important in the economy 

 of this tree is the constant presence of a considerable amount of 

 soil moisture, that this factor alone is of sufficient weight to 

 over-balance all other considerations in local distribution. No 

 more definite mark of the nature of the tree in this regard can 

 be found than the habits of the root system, which spreads in 

 all its complex ramifications at the very surface of the soil. 

 Often the roots extend for fifteen feet or more on all sides of 

 the tree just under the layers of forest moss and mould and are 

 more than half exposed when a chance ground fire consumes the 

 combustible material. In such cases, of course, the tree is killed 

 and this constitutes another of the limiting influences which 

 affect the prosperity of the species. Thousands of seedlings of 

 the/ hemlock establish themselves upon fallen logs, boulders, 

 stumps, etc., wherever mosses and fallen leaves furnish some 

 check to evaporation, and under such conditions they may flour- 

 ish to maturity. It is not unusual in hemlock woods to find old 

 trees with roots astride a log and reaching down to the ground 

 on either side. Sometimes under such conditions the young 

 seedlings form a veritable carpet. The behavior of the hemlock 

 in these respects, however, is possible only under the most favor- 

 able conditions of atmospheric and soil moisture. 



The western hemlock is not especially susceptible to dis- 

 ease except in the case of Echinodontium tinctorium, by which 

 it is often seriously affected or destroyed. 



One other tree of importance remains to be discussed as a 

 member of the mesophytic forest. The arbor vitae (Thuja 

 plicate) is frequently conspicuous, and is somewhat more widely 

 distributed than the two preceding species, though sharing to a 



