Chap. IV] 



THE AIR 



77 



the greater mobility of their segments, the leaves come into contact with 

 larger quantities of air and their interchange of gases is correspondingly 

 increased. 



Considerable mechanical damage by exceptionally strong storms is 

 commoner in countries where the weather is usually calm, than in regions 

 that are normally windy, partly because in the latter the form of growth 

 assumed by stem and twigs conveys protection, and partly because, as 



Fig. 50. Primus spinosa to the left, Crataegus Oxyacantha to the right, on the north coast of 

 Zealand in Denmark. Both trees bent and unilaterally branched through the influence of the 

 sea-breeze. From a photograph by Warming. 



Hegler proved, a continuous tensile stress occasions an increase of strength 

 and of the mechanical tissue. 



The mechanical effects of the wind are by no means those most 

 important to the oecology of tall and tree-like plants. They are for the 

 most part markedly exhibited only where very strong continuous air- 

 currents prevail, and then chiefly cause harmless deviations from the normal 

 shape. The destructive influence that in certain areas completely 

 prevents l the growth of trees and is chiefly exerted during the winter, is 



1 Borggreve, for example, states : ' One frequently hears, for example, the assertion 

 made, that afforestation of the west coasts and islands of Schleswig is impossible ' 

 (p. 251). 



