LOSS OF WATER 251 



affect the evaporation from any moist surface. First of all is the 

 relative humidity of the air. If the atmosphere is relatively full 

 of water vapor, diffusion from the plant is retarded. In a saturated 

 atmosphere as during rain, mist, or fog, there is practically no 

 transpiration. As the humidity decreases from 100% to the 70% 

 of a moderate day in mesophytic climates or to the 50% and less 

 of desert areas, evaporation increases. As the humidity varies 

 from day to day and hour to hour, the changes in transpiration 

 run parallel. 



Temperature likewise affects transpiration largely through its 

 influence upon humidity. As the temperature increases, the rela- 

 tive humidity becomes less and transpiration increases. Increase 

 in soil temperature also increases transpiration. 



Barometric pressure is important only because of its relation to 

 the vapor pressure and the ease with which water vapor can pass 

 off into the air. The lower the pressure the more will be the 

 transpiration. Although this is of importance only when com- 

 paring plants at low and high altitudes, on mountain tops where 

 the low pressure is associated with low humidity and intense 

 light, this may be a powerful factor in the distribution of plants. 

 Whitfield (1932) found that transpiration decreased with altitude, 

 but his data seem to show merely that the difference in barometric 

 pressure was more than offset by the lower temperature and greater 

 humidity, so that the net result was a decrease in transpiration. 



Air movements affect transpiration through their influence upon 

 the humidity. Clothes hung upon the line dry more quickly 

 on a windy day since the layer of saturated air becomes removed 

 by the wind as fast as formed. In a similar manner, transpira- 

 tion varies with the amount of motion of the surrounding air. 



The chemical properties of the soil affect transpiration. This 

 effect may be due to quantity (concentration) of materials or to 

 the quality (nature) of the materials present in the soil. A small 

 amount of salts in the water has little effect upon transpiration, 

 but, as the amount increases, there is a decrease which is probably 

 due to the osmotic effect produced by the higher concentration of 

 salts. Acids may accelerate while alkalies may retard transpira- 

 tion. Specific salts may also exercise such inhibitory and acceler- 

 ating effects. 



Of the various external factors, light is one of the most im- 

 portant, partly through its action upon the stomata in connection 



