42 OUTLINES OF FORESTRY. 



IV. CONDITIONS NECESSARY FOR THE 

 GROWTH OF TREES. 



IF a soil exists in any locality, and certain con- 

 ditions of light, heat, and moisture are present, 

 the character of the vegetation that naturally 

 grows in such a region will depend more on the 

 peculiarities of the distribution of the heat, light, 

 and moisture, than on the character of the soil 

 itself. 



If moisture be entirely absent, or if it exists in 

 such a form as ice or snow, in which it cannot be 

 readily appropriated by plants, then that region 

 must become a desert. 



Deserts occur either in dry, arid regions, or in 

 the regions of perpetual snow of the polar zones, 

 or on the higher mountain slopes. 



If the rainfall is absent during certain seasons of 

 the year, but occurs during the rest of the year, 

 that is, if one part of the year is dry and the rest 

 is wet, the vegetable forms, which die or disap- 

 pear at the beginning of the dry season, reappear 

 at the beginning of the wet season. Areas of the 



