WIND 



141 



have been worked out for nearly all possible combinations of air 

 temperatures and wet bulb depressions showing relative humidities 

 and dew points, the dew point being the temperature at which con- 

 densation would take place with the existing water vapor, that is, 

 the temperature at which the air would become saturated without 

 any increase in the absolute humidity. The tables most used in 

 America are the 'Tsychrometric Tables" of the United States 

 Weather Bureau, contained in its Bulletin 235. From these tables 

 the relative humidity may be obtained directly and the saturation 

 deficit is obtained by taking the difference between the saturation 

 pressure at the current dry bulb temperature and the saturation 



Fig. 62.— "Elfin timber." Engleman spruce at timber line in the Arapaho 



National forest, Colorado. 



pressure at the current dew point. Such data, when extended over a 

 long period of time, are very useful in determining whether the 

 environment is suited to plants that are well protected from excessive 

 transpiration or to those that are relatively unprotected. 



90. Wind.— Wind is moving air and is more important from an 

 ecological viewpoint than is still air. Its effect on plants is both 

 physical and physiological. The breaking-over and uprooting of 

 plants and the breaking of branches are familiar examples of the 

 physical effect of wind. Wind is also important as an agent of 

 pollination and of dispersal. It has already been discussed as an 

 agent of pollination in Chapter VI and its action in the dispersal of 



