278 BOTANICAL GAZETTE [APRIL 
series of gradations. First there are hills with bull pine and a few 
Douglas spruce on the north and east slopes and few or none on the 
exposed slopes; then there are hills where a considerable number of 
bull pines find favorable conditions for development on the exposed 
sides of the hill (fig. 18), and with these there is some Douglas spruce. 
On the protected slopes the Douglas spruce becomes more abundant 
and may even destroy new growths of bull pine by their shade. 
Another set of hills nearer the forest formations show the south slopes 
fairly well covered with bull pine and Douglas spruce, while the bull 
pine has become less abundant on the protected slopes where the west- 
ern larch element has been introduced. Thus there are all gradations 
from hills with few or no trees to those that have mesophytic elements 
on the north slopes. Of course there are many variations; for 
instance, hills a little to the north of west of the town of Big Fork 
have their southern slopes almost destitute of trees, while the pro- 
tected slopes have a stand of Douglas spruce, western larch, and 
some bull pines. The series may be carried still further. It has 
been shown that the west slope of Mission Mountains has a forest 
of Douglas spruce and bull pine, while on the protected slope western 
larch is the most prevalent, and with it trees like lowland fir, silver 
pine, and giant arborvitae, which require still more moisture, are found. 
If the rainfall be sufficient, a mesophytic forest may be found on the 
exposed as well as on the protected slopes. 
It may be well to analyze the conditions that make the so-called 
protected slopes more desirable for trees. Of course the great 
factor that prevents tree growth is the lack of moisture. If an annual 
rainfall of 4oo™™ falls upon a hill similar to those just described, 
and in gentle showers so that there will be little or no run-off, the page 
that leaves the soil directly must do so by evaporation, the remainder 
soaking in and becoming available for absorption. It is obvious 
that the slopes receiving the strongest insolation and exposed to dry- 
ing winds will lose the most water by evaporation, and these are the 
south and west slopes; hence the north and east slopes will have 
more moisture in the soil. Again, the plants growing on these pro- 
tected slopes do not receive so much heat and are not exposed ae 
drying winds, hence do not lose so much water by transpiration; 
they not only get more moisture but do not need so much to supply 
