7 6 



THE FACTORS 



[Part I 



by an abnormal tree-growth, if this be present at all, whilst the humbler 

 vegetation exhibits the effects of the wind to a very small extent, or not at 

 all. This difference between tree-growth and low shrubby and herbaceous 

 growth in relation to the action of the zviud, is due to the increasing movement 

 of the air as the distance from the ground increases. 



Some of the results of observations made by Stevenson 1 of Edinburgh 

 upon the increase in velocity of the wind with the height above the 

 ground are shown in the following table : — 



From the above tabular statement it is quite clear that plants 

 that are only slightly raised above the ground experience the effects of 

 the wind much less than tall plants, and especially trees. The action of the 

 wind on plants is partly direct, by tensile stresses and by pressure, and 

 partly indirect, by increasing transpiration : both these actions are the 

 more energetic, the taller the plants, or the higher the spots they 

 occupy. 



The direct effect of the wind on the growth of plants is for the most 

 part strongly exhibited only in places where the wind blows continuously 

 and with considerable strength. It is a common feature in such localities 

 that stems and branches are bent away by the prevailing wind from their 

 normal direction of growth and follow the direction of the wind (Fig. 50). 

 It is also obvious that such trees also suffer direct damage, by breakage of 

 branches, rending of foliage, and so forth. The action of the wind in 

 tearing the leaves of arborescent or lofty plants and tall herbs may, however, 

 prove to be quite a normal and useful feature, as in the case of the banana 

 (Fig. 49) and some other plants, the huge leaves of which when young 

 are quite entire, and remain so in stations that are sheltered from the wind, 

 but are always torn in more exposed ones. The functional activity of the 

 leaves is not thereby in any way prejudiced ; on the contrary, owing to 



1 Stevenson in Journ. Scot. Meteorol. Soc, New Series, Vol. V, 1880, p. 348. 



