u6 



BOTANICAL GAZETTE 



[FEBRUARY 



an encroaching dune, are bent leeward {fig. i). The cause of this 

 to be found in the continuous pressure, or in sudden gusts of wind 

 which bend the trees while the onrushing sand prevents return to the 

 original position. As a rule the sand on the lee side of a dune i> 

 moister, and the slope is consequently steeper. Often slides of sand 

 take place, and they also bend or even break the trunks of the trees. 



Dunes which have been made stable by a cover of plants are 

 sometimes again broken up by the wind (fig. 2). Such formation 

 are often met with on the Baltic coasts. On these broken-up dunes 

 the usual series of development of vegetation begins anew and thus 



~ 





j t ■ 1 * 



^^m^^^^mm 



Fig. 3.— Embryonic dunes inland from littoral dune, south of Cliff House, San 

 Francisco, Cal. (Photograph by author.) 



they have a peculiar character, remnants of the old communities 



A new life-history of the 



-~~e ""^" vvuj.1 me new immigrants. A new ine-nibiuij « 



plant community is started, and during the course of development it 

 may take a direction entirely different from the former series. 



Vegetation covering on the ground will greatly slacken the speed 

 of the air current which comes into immediate contact with the 



are first struck 



i or other obstructions are in the path 

 formed behind them (fig. 3). The planl 

 the whole force of the wind are mos 



tlv 



injured, not only by its mechanical action, but to a greate 



r extent 



