METEOROLOGY. 



15 



plants rapid as compared to that which ensues after the spring and summer 

 rains. During the cool and moist period of the winter season the conditions 

 approximate closely those of a northern cool, moist, though not wet spring, 

 when the ground is wet but the air cool, a high relative humidity, with dews 

 and with an abundance of sunshine. The leaves of the plants which grow 

 at this time are in no danger of wilting, and indeed never show it, and, from 

 the teleological point of view, there is no reason why the stomata should not 

 be wide open. I have, however, not found them open more than six-tenths 

 their possible extent of opening during the warm period of the day, though 

 exceptional stomata show a somewhat wider opening. 



Table 7. — Rainfall, Tumamoc Hill, Tucson, Arizona, for 1906, in inches. 



[See fig. 6]. 



The approach of the summer is marked by cessation of rain, increase of 

 temperature, and lowering of the relative humidity.* At the same time, 



*No accurate data for relative humidity for a sustained period are available, so that gen- 

 eral statements must suffice. With reference to the possible high humidities in a given 

 area in the absence of rain see Cannon, 1906. The problem of the desert in this regard is 

 one which commends itself to study. Many areas have a distinctly desert character, but 

 are favored by high atmospheric humidities, and these are contiguous in many places to 

 the desert areas having the opposite condition, e. g., Southern California and Arizona 

 (McGee, 1906). 



