CLIMATIC FACTORS 



135 



greatly even when relative humidity remains constant, for it rises with 

 the temperature (Fig. 71). 



For the measurement of saturation deficit the plant sociologists of 

 Crakow use the commercial Assman psychrometer. The saturation 

 deficit is calculated from the difference between the readings of the wet- 

 and dry-bulb thermometers by means of tables of equivalents. 



Ecologically, saturation deficit means more than humidity. 

 Regions with high relative humidity may have the character of desert 

 steppe (southwest Africa). The relative humidity of the coastal belt 



Fig. 72.- 



-Euphorbia beaumierana, a cactoid euphorbia at Cap Ghir, Agadir. 

 by Girardet.) 



{Photo 



of southern Morocco between Agadir and Cape Juby continuously 

 shows an enormously high value (90 per cent in summer). Neverthe- 

 less the xeric cactoid euphorbia scrub extends to the very shore (Fig. 

 72), Regardless of the high relative humidity, the saturation deficit, 

 and therefore the evaporation, is large. 



Measurement of saturation deficit should be made in the daytime, 

 since stomatal transpiration of higher plants is greatly reduced at night. 

 Also the saturation deficit shows a pronounced daily maximum 

 between 8 a. m. and 6 p. m. 



