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AIR 



seeds and fruits will be taken up later in connection with the subject 

 of plant succession. 



The physiological effect of wind consists almost entirely in increas- 

 ing transpiration, and in this capacity it is very important. As a 

 rule, the velocity of the wind increases with the height above the 

 ground. Therefore, the tallest plants, such as trees, suffer most 

 from the drying effects of the winds, while low plants, such as many 

 grasses, get along very well in windy regions. Furthermore, the 

 height to which many plants can grow is limited by their ability to 

 transport water upward fast enough to counteract the loss through 

 transpiration. For this reason, in the most windy places, such as 



^^r^ 



'->w.<^, :*'. 





Fig. 63.— Dwarf vegetation at the top of Specimen Mountain, Colorado. Silene 



acaulis in the center. 



exposed mountain ridges and flat windy sea-coasts, tall plants are 

 entirely absent. 



Because of this drying effect of the wind the trees are smaller on 

 the windward side of an exposed grove than on the opposite side. 

 A single tree growing in the open is almost invariably one-sided; 

 that is, a larger part of its crown is on the side opposite that from 

 which the prevailing winds blow. So true is this that one can 

 determine the direction of the prevailing winds of a region by 

 examining the trees. 



At timber line on mountains the same species of woody plants, 

 which a few hundred feet further down are upright forest trees, grow 

 as gnarled and sprawling, much-branched shrubs, known as elfin- 



