82 PRACTICAL FORESTRY 
aspen, on the other hand, with its light seeds and 
ability to grow almost anywhere, extends from Labra- 
dor, Hudson Bay, and the Yukon River in Alaska, 
southward to Mexico, and from the Atlantic to the 
Pacific. 
The second most important factor in determining 
the distribution of species is moisture. Without 
some moisture no tree can live. The conifers, in tem- 
perate regions, use per year, according to some au- 
thorities, as much, if not more, water than broad- 
leaved species. .While the broad-leaved species are 
in leaf, however, they use much more water than do 
the conifers. The drier the climate and the larger 
the leaf surface, the greater the amount of evapora- 
tion. Therefore, plants of extremely dry regions have 
developed a small leaf surface, or the leaves are cov- 
ered with wax or mucilage in such a way that evapora- 
tion is hindered. As a rule, the conifers, or cone- 
bearers, owing to their endurance, inhabit the cold 
zones of the earth. They grow also on the upper zone 
of mountains, and in dry places in the tropics. They 
would grow in other parts of the tropics were they 
not crowded out by the more vigorous and more 
highly developed broad-leafed species. They are truly 
pioneers paving the way for the broad-leafed kinds 
which are constantly creeping in, and because of their 
vigorous coppice-growth are holding their own. In 
