144 Postelsia 
torta is common in certain peculiar situations. 
These are the trees which can flourish with the 
lowest summer temperatures provided that the 
season of growth is sufficiently long. It is not- 
able that the three principal species of this for- 
mation all extend far north along the Alaska 
coast, and the spruce and hemlock are the last 
trees to disappear in that direction. The final 
limit of the forest in the vicinity of Kadiak island 
seems to be due to the lack of a sufficiently long 
season of growth, rather than to the winter cold 
which is not severe. 
The Douglas fir, on the other hand ismes- 
sentially a tree which needs considerable summer 
heat, though it will withstand more severe win- 
ters than the spruce. It is not found anywhere 
in Alaska, and although it is very abundant in 
the interior of Vancouver Island and even on the 
mountains overlooking the west coast, it does not 
flourish on the lowlands close to the ocean and 
the occasional specimens which occur there are 
small, and offer a marked contrast to those which 
have developed in the more favorable climate of 
the interior. 
