38 THE BOOK OF FORESTRY 
produce fertile seed plenty of light, as well as food and 
moisture, is absolutely necessary. 
The influence of heat upon tree growth is very much 
the same as light, for each species to assimilate its food 
properly, to grow and to reproduce must have a cer- 
tain amount of heat. Tropical plants, of course, find the 
climate of the north temperate zone inhospitably cold, 
and trees from the north cannot long endure in the 
moist tropics on account of the continuous warmth. 
Plant life practically ceases growth at freezing temper- 
ature although in the polar region certain arctic 
plants are found growing through the snow in the 
springtime. 
Moisture, aside from its influence in controlling 
temperature of the trees and the forest by evaporation 
and transpiration, is necessary to dissolve the salts in 
the ground so that they can be drawn in through the 
cell walls of the root hairs. It 1s also necessary for 
assimilation and growth, for in addition to being a com- 
ponent of plant food, a large amount of moisture is 
needed by the dividing cells in the growing parts of 
the tree. The amount of moisture consumed by a 
mature forest 1s extremely large. A mature beech 
forest consumes no less than 350,000 gallons of water 
per acre during the growing season. 
Satisfactory soil conditions are indispensable to forest 
growth and in spite of the fact that a relatively large 
part of the tree is made from water and air, unless 
the soil be deep and permeable the tree cannot thrive 
although it may drag out a poor existence for years. 
On thin, stony or barren soils the form of a tree is 
generally crooked and stunted, just as a child who has 
been compelled to live on scanty fare never becomes 
a straight robust man. 
