CHAP, viii MOVEMENTS OF THE AIR 39 



The transverse section of tree-trunks is also influenced by the wind, 

 since it becomes excentric the diameter in the direction of the wind 

 being longer than that at right angles to it. 



Plants vary according to their species in power of resistance 

 to the wind. 1 Of the trees common in Denmark the hardiest in this 

 respect are the following : Pinus montana, P. austriaca, Picea alba, as 

 well as some species of willow and poplar, and these are consequently also 

 the species of greatest value in afforesting dunes and heaths. 



The importance of protection against wind has thus been made clear. 

 Such protection is provided by elevations of the land, as well as by other 

 natural or artificial protective barriers : careful study will often show 

 that vegetation differing widely as regards density, stature, structure, 

 development, and admixture of species, can arise respectively on the 

 windward and lee sides of such a barrier, even when this is only an 

 insignificant rock, stone, or shrub. The hills of Central Jutland appear 

 when viewed from the east to be clothed with forest, but when viewed 

 from the west to be clad with heath. In beech forests the vegetation 

 clothing the soil of places where light and wind can penetrate is quite 

 different from that where these are excluded. In this case the wind has 

 inter alia an indirectly injurious action, in that it removes the carpet of 

 dead leaves which protects and variously affects 2 the nature of the soil, 

 and in that it leads to the conversion of mild humus into acid humus, or 

 prevents the production of humus. Arctic and alpine vegetation, as was 

 shown by Kihlman 3 , receive very material protection from snow, and 

 where this remains lying, particularly in calm and sheltered depressions, 

 the vegetation is consequently of a stamp different from that on more 

 elevated, windy spots. 4 



The defences against wind that have hitherto been mentioned are 

 topographical, but many plants have by adaptation acquired special 

 structural features, both morphological and anatomical, by which they 

 are protected. To this category belong bud-scales, covering hairs, 

 remnants of leaves and stems that are long persistent, and other features 

 which will be dealt with later in this work. 5 



Distribution of vegetation. It may be added that though the absence 

 of trees from many places on the Earth is mainly due to wind, yet it is 

 also due to cold and other conditions unfavourable to growth. Wind 

 is partially responsible for the delimitation of the boundaries of forest 

 in Polar lands, and of forest and bushland up high mountains. On 

 mountains, forest ceases where the mountain commences to divide into 

 separate peaks. Above this limit forest can still occur where there is 

 local shelter from the wind, for instance, within the crater-valleys of 

 Java. 6 Also, it is in valleys sheltered from the wind that forest extends 

 farthest north in arctic lands ; for instance, along the Lena and Mackenzie 

 rivers. Middendorff 7 was the first to recognize the significance of the 

 wind in assigning limits to the extension of forest. 8 



Illustrations are given in L. Klein, 1905. 2 See Chapter XVIII. 



Kihlman, 1890. 



For further particulars see C. Schroter, 1904-8, and Chapter XVIII. 

 Also see Chapter VI, p. 24; and Chapter XXX. 

 Schimper, 1893. ' Middendorff, 1867. 



The importance of wind has been treated in an attractive and detailed manner 

 by Kihlman, 1890; and more recently by Bernatzky, 1901; Buchenau, 1903; 



