78 



THE FACTORS 



[Part I 



not attributable to the mechanical action of the wind. On the contrary, 

 as Focke was the first to state, the wind must have a power that is directly 

 destructive of life, as is shown by ' the numerous short stiff projecting 

 branches which are to be seen on the tops of shrubs and in the crowns of 

 trees, chiefly on their windward side, in exposed places, and which do not 

 exhibit any trace of external injury ' ' (Fig. 51). The directly fatal 

 influence is, as Kihlman pointed out, the excessive increase of transpiration 

 caused by the wind. When the wind blows during frosty weather — a time 

 when the loss of water cannot be made up by^ a supply from the soil and 

 from the stem — its action can easily become destructive. Injuries by frost, 



Fig. 51. Influence of wind on tree-growth: group of trees in a wood on Sylt Island showing 



branches dried up. After Borggreve. j 



as has already been shown-, are not ordinarily attributable to the tem- 

 perature in itself, but are due to desiccation during the frost. 



Evaporation is however universally weaker at low than at high tempera- 

 tures, }-et dry, cold winds possess an extraordinarily strong drying power. ^ 

 A striking example is mentioned by Middendorfif, who, on a windy and 

 frosty day, hung outside his tent a leather glove which after being washed 

 had frozen stiff — in an hour's time it was completely dr}'. The drying 

 action of the wind and its effects are, as will be shown in a later chapter ', 

 of great geographical importance, since they determine the limits of tree- 

 growth both towards the Poles and in a vertical direction. 



' Kihlmau, p. 68. 



"■ Page 39. 



Part II, Chap. I, The Formations. 



