MECHANICAL EFFECTS OF WIND ON STRAND PLANTS. 383 



ronment of the strand vegetation were discussed in a very general 

 way. Of these factors, the most important in their effect upon plant 

 growth are probably to be referred to two categories. To the first 

 belong the movements of the atmosphere, so far as their mechanical 

 effects upon the vegetation are concerned, whether direct, or indirect, 

 i.e., through their action upon the substratum. The second com- 

 prises all such factors as tend to bring about excessive transpiration, 

 and consequently necessitate protective modifications. 



MODIFICATIONS DUE TO THE MECHANICAL ACTION OF THE WIND. 



Exposure to frequent strong winds laden with tine particles of sand 

 is the more or less probable cause of certain modifications in the 

 vegetation, especially that of the open dunes. Three principal results 

 of such exposure may be mentioned: 



Direct effect on ike external form of the plant. — This is evident 

 in individual trees and large shrubs of the open dunes, which are 

 marked by their trunks usually leaning in a direction opposite to 

 that of the prevailing winds, the branches on the wind-exposed side 

 being often entirely denuded, leaving the crown of foliage to leeward 

 of the trunk; by their gnarled and twisted stems and branches, the 

 latter often rigid and comparatively short; and frequently by their 

 torn and ragged-looking foliage (especially in such inland species 

 as Platanus occidentalis and Taxodiuni distichum, when straying 

 into this area). 



Indirect or histological effect. — This often appears in the develop- 

 ment of much strengthening tissue (stereome, wood), and in a consid- 

 erable thickening of the cuticle and epidermis cell walls of the leaves, 

 giving the surface a hard, resistant, polished texture. The latter is 

 especially notable in the evergreen leaves of Quercus virginiana, and 

 in the whole surface of the larger grass-like plants, Uniola panieidata, 

 Ammophila arenaria, Panicum amarum, etc. It is a modification 

 chiefly important to the plant as a protection against excess of light 

 and of transpiration, but is perhaps also useful as a protection against 

 the impact of strong, sand-laden winds. 1 



Effect through redistribution of soil. — The wind blowing upon the 

 incoherent surface soil keeps it in almost constant motion, and often 

 shifts great masses of it in a short time. Consequently some plants 

 are being uprooted, while others are quickly buried beneath the 

 sand. This action is, of course, most violent in winter. Large dunes 

 that have been cut open by the wind exhibit dead roots and root- 

 stocks (of Smilax, Ammophila, etc.) far below the surface. To the 

 necessity of adapting themselves to the mobile soil in cooperation 



1 How strongly the wind may act upon plants not protected in some such way 

 is evident in the tattered older leaves of the banana, which usually become seg- 

 mented by the splitting through to the midribs of the tissues between the parallel 

 lateral nerves. 



