CLIMATIC FACTORS 



147 



It is a significant sociological fact that the regularity in the direction 

 of the wind at seeding time in many places causes a distinct tendency 

 toward distribution and migration of species in the direction of the 



Fio. 82. — Wind form of Cedrus atlantica at the Lalla Kheditacha (2,200 m.) Algeria. 



(Photo by K. Milller.) 



prevailing wind. Christ called attention to this in speaking of the 

 immigration of rare alpine species into the Santis Alps. In southern 



Prevoif/'ng w)nd 



Shearing zone 



^j:0yv/^fer snow 



Fig. 83. — The flag form of pine at the wind timber line in the Alps. 



France the natural flight of seeds of isolated Aleppo pines is chiefly in 

 the direction of the prevailing wind (mistral), and in this direction the 

 seeds travel farthest from the mother trees. 



