The Environment of Plants 



149 



Gravity. Gravity is an im- 

 portant environmental factor, 

 largely because of its influence 

 on the direction of growth in 

 stems, roots, and other organs 

 (page 131). Light influences 

 also the growth of the various 

 parts of the plant. Conse- 

 quently the position of the 

 aerial organs of plants is to a 

 large extent determined by the 

 combined influences of light 

 and gravity. 



Wind. Winds and air cur- 

 rents are of importance, as 

 they affect the rate of tran- 

 spiration or modify the tem- 

 perature. It may take 10 

 minutes for your wet hands to 

 dry in still air, but if you hold 



them before an electric fan they will be dry in 2 or 3 minutes. 

 The passing of an increased volume of air over a wet surface 

 increases evaporation, and wind affects the transpiration of 

 plants in the same way. In drying your hands before a fan, 

 notice also the cooling effect of the breeze. Leaves are cooled 

 by transpiration in the same way. Winds also may cause 

 important modifications in the forms of plants, and occasion- 

 ally violent winds may destroy large areas of timber and 

 crops. 



Animals. Leaf-eating insects, such as the potato beetle, 

 injure the plant by destroying the food-makmg apparatus. 



Vt'. S. Cooper 



Fig. 86. Western white pine on Long's 

 Peak, Colorado, showing the effects of 

 violent winds and of wind-driven snow. 



