78 PRACTICAL FORESTRY 
ducers which would cover the earth but for certain 
hindrances or barriers which prevent their spread. 
2. Barriers To Forest ExTension 
The spread of seeds is often hindered by high 
mountains and seas. In many instances plants have 
been thus confined or imprisoned in small areas. The 
greatest of all barriers to tree-growth is cold. It is 
this which determines the northern limit of all trees, 
and the timber-line on mountain sides. Below about 
43° Fahr., vegetative activity is not possible. The 
temperature for at least six weeks of the year must 
exceed that figure. Many claim that the plants of 
temperate regions need the rest of winter. If so, 
the arctic willow has mostly rest, and is in conse- 
quence only a stunted tree form. Although north- 
ern species have, in the course of time, become 
adapted to this condition of affairs, long inactivity can 
be of no advantage whatever to any plant or animal. 
In many cases in our north it is a case of mere ex- 
istence, and not growth. Conifers cover vast areas 
of our north mainly because they are able to work 
at a lower temperature than deciduous trees. They 
are, on the whole, less frost-tender, although there 
are pines, such as the famous stone-pine of Italy and 
the Cuban pine, which are very sensitive to cold. 
When pines or spruce are frozen throughout, as is 
