Abrasive Action of Medium 



55 



growing buds. These effects, augmented by excessive evaporation on 

 the windward side, often produce extremely asymmetrical growth. 

 "Flag-form" trees with the upper branches restricted to one side of 

 the trunk are commonly found on exposed mountain ridges. Groves 

 of trees in windy lowlands are often similarly wind-trained, the more 

 protected individuals growing many times higher than those on the 

 exposed side. The outline of such a grove presents a smoothly rising 

 contour (Fig. 2.14). Along the sea coast the harmful action of salt 



Fig. 2.14. Live oaks on wind-swept shore at Morehead City, N. C. Wind action 

 has stunted the trees on the e.xposed side of the grove at the left. 



spray is added to the other effects of the wind in progressively re- 

 stricting growth toward the exposed beach and in causing a zonation 

 of the species present (Costing and Billings, 1942). At high eleva- 

 tions in the mountains low temperatures act with the severe winds to 

 produce the familiar dwarfing and gnarling of the vegetation. The 

 combined effect of these ecological factors often limits the growth of 

 trees to a sprawling mat only a few centimeters high near the summit, 

 whereas in the protected valleys trees of the same species grow to 

 heights of over 15 m. 



In the water medium abrasive action is an even more serious in- 

 fluence with which the organisms must contend. Only strongly at- 



