524 INFLUENCE OF HEAT ON CONFIGURATION AND DISTKIUUTION OF PLANTS. 



rock gullcys, aud growing out from them overspreads like ivy the vertical rock- 

 faces. 



In all these cases it is certain that the weight of snow cannot liave any deter- 

 mining influence upon the form of the plants, and some other explanation must be 

 sought. May it not be perhaps that strong winds render it impossible for woody 



r.;,y-4g' a^l^-ia.\ 



Fig. 131. — Alpine Willows with stems and branches clinging to the ground in the Tyrol 



plants with erect stems to grow in liigh Alpine regions? Observing the mist and 

 volumes of clouds rushing across the tops of the mountains, one gets some idea of 

 the strength of the air currents which operate there, and whoever has experienced 

 the eflects of a storm on a high mountain ridge can estimate the force of the power- 

 ful gusts of wind. And yet it would be erroneous to suppose that the force of the 

 storms on lofty mountain heights is greater than in mere hill regions. In the case 

 of many winds it is even certain that they increase in violence as they rush down 



