12 



FORREST SHREVE 



TABLE 1 



Rainy days grouped according to amount of rainfall, together with the total rainfall 



yield of each group 



are approximately the same as the total for the lighter rains, up 

 to 1 in. (2.5 cm.). The rains of more than 1 in. are still less fre- 

 quent, but all of them together form a considerable proportion 

 of the annual total. In fact, the 32 days of 1907 to 1912 on 

 which there were rains of 0.75 in. (1.9 cm.) or more yielded 46 % 

 of the total rainfall of the six years, although these 32 days were 

 only 8.6 % of the total number of rainy days. 



The existence of two seasons of precipitation, separated b}^ 

 weeks or months of pronounced aridity, gives to the vegetation 

 a marked periodicity of activity. The desert perennials of the 

 upland situations do no more than maintain their existence dur- 

 ing the arid seasons, while the herbaceous flora confines its 

 activities to the two humid seasons. ^ 



1 For descriptions of the seasonal behavior of the vegetation at Tucson, see 

 MacDougal, D. T., The Course of the Vegetative Seasons in Southern Arizona, 

 The Plant World 11: 189-201, 217-231, 237-249, 261-270, 1908; and Spalding, V. M., 

 Distribution and Movements of Desert Plants, Publ. 113, Carnegie Institution, 

 1909. 



