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PL-\XT PHYSIOLOGY .IXD ECOLOGY 



upon the habitat through absorption, transpiration, decompo- 

 sition, etc. 



331. Vegetation zones. Zonation is the most striking fea- 

 ture of the vegetation of continents, a fact well illustrated by the 

 vegetative covering of North America. It is produced by a 

 gradual variation in both water content and temperature, though 

 the former is much the more important. In accordance with the 

 decrease of water content and temperature northward, three 



Fig. 125. Vertical zonation, or layering, in a spruce forest. 



primary belts of vegetation stretch across the continent from 

 east to west. These are forest, grassland, and polar desert. The 

 first is further divided into the secondary zones of broad-leaved 

 evergreen, deciduous and needle-leaved forests. At right angles 

 to this symmetry due to temperature and water lies a second one 

 produced by water alone. In response to this, forest belts touch 

 the oceans, but give way in the interior to grasslands, and these 

 to deserts. The interference of these two series of zones has 



