von Schrenk — A Severe Sleet- storm. 149 



the branches at such very great intervals of time that 

 any reaction or adaptation to the same cannot possibly 

 take place. 



One can ask, therefore, how it happens that so many of the 

 branches were able to withstand successfully a weight so great 

 as the one described. The only other force which might act 

 in depressing branches is snow. But its weight is exceedingly 

 small when compared with ice, and its influence on the 

 branches of deciduous- leaved trees is of the smallest. It is 

 therefore necessary to account for the strength of the 

 branches in another way. 



The wind is a factor, constantly at work, and its influence 

 in shaping the form and strength of the plant body has been 

 duly noted.* It exerts a pressure on the branches which at 

 times is very considerable. In the course of time the 

 branches of trees have acquired sufficient strength to with- 

 stand this pressure, and only when the same exceeds a certain 

 limit, as in tornadoes and hurricanes, do the branches break. 

 The pressure exerted by the ice is much like that exerted by 

 the wind, in many ways. It is a steady pressure, and in that 

 respect, perhaps, more effective than the pressure exerted 

 only now and then by the gusts of wind. But the action is 

 one affecting the strength of the branch. It therefore be- 

 comes less astonishing that so many of the trees successfully 

 bore the great weights of ice placed upon them, when one 

 reflects that these same trees resist similar strains which act 

 at frequent intervals. They have adapted themselves to 

 these, and were consequently able to withstand the unwonted 

 ice pressure. 



* Schimper, A. F. W. Pflanzengeographie. 84. 1898.— Metzger. DerWind 

 als massgebender Faktor fur das Wachstum der Baume. Mundener forst- 

 liche Hefte. 8. 1893; also 5 and 6. 1894. 



