MESOPHYl^ES. 313 



the form of plants. This they do indirectl}' by the shifting 

 of sands in sandy regions, and by their effect upon the pre- 

 cipitation and upon the moisture of the atmosphere. Winds 

 increase evaporation from the soil and from the surface of 

 plants, and thus directly influence form. Trees growing in 

 wind-swept regions are always low, bushy-branched, with 

 the trunk and limbs inclined to leeward. The twigs on the 

 windward side are often dead. Forests in wind-swept regions 

 often thin out to windward, the trees becoming smaller and 

 smaller, finally being replaced by bushes which become 

 sparser until no woody vegetation is present. The leaves 

 upon such plants are small and often peculiarly spotted. 

 These effects upon the form have been ascribed to the me- 

 chanical action of the air, to the presence of salts when in the 

 neighborhood of the ocean or salt lakes, and to the reduced 

 temperature ; but probably none of these causes is to be 

 looked upon as so efficient as the drying brought about by 

 the prevalent wind. 



425. 2. Light. — Light affects plants directly through its 

 influence upon their nutrition and upon the evaporation of 

 water from their surfaces. In this way it affects (i) the rate 

 of development. For example, the blossoming of flowers 

 and the production of leaves occur earlier upon the sunward 

 side of a tree or shrub than upon the other side. In the 

 same culti\'ated crops of the north and south there will often 

 be several days' difference in the total number between sow- 

 ing and maturing. Thus barley at northern Norway, in 68° N. 

 lat., matures in 89 days, while at Schonen, in 56° N. lat., it 

 matures in 100 days. Since the total hours of illumination 

 must be about equal, the longer days of the north enable the 

 plants to produce more food, and so to mature more rapidly. 

 The forcing of vegetables under glass by the aid of electric light 

 during the night depends upon the same principle. (2) The 

 form of plant parts is directly influenced by light. Plants accus- 



