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 is 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 is 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. 



