48 VEGETATION OF A DESERT MOUNTAIN RANGE. 



At the higher altitudes the shortness of the growing season and the 

 coldness of its nights are inimical to the activity of the herbaceous 

 perennials. These circumstances make very difficult the introduction 

 into the Forest region of plants which would seem calculated to nourish 

 in a region of similar moisture conditions. 



After the close of the humid mid-summer the desert is subjected 

 to a variable period of 6 to 10 weeks of arid conditions, a season known 

 as the "arid after-summer." Although the temperature, humidity, 

 soil moisture, and evaporation may reach as extreme values in the 

 arid after-summer as in the arid fore-summer, nevertheless the total 

 duration of such extremes is not as great in the former season. A 

 general cessation of vegetative activity occurs in September and Octo- 

 ber at the higher elevations and in October and November at the lower 

 ones. On the desert it sometimes happens that occasional rains during 

 the arid after-summer prolong the activity of the shrubs and even of 

 the summer ephemerals to such a late date that they may be seen in 

 flower side by side with root-perennials which are characteristic of the 

 winter season. 



RAINFALL. 



The figures for the monthly average rainfall at Tucson, as determined 

 from the 38-year record (1876 to 1913), show that the year falls natur- 

 rally into two humid and two arid seasons (see fig. 4). Without regard 

 to the average dates upon which the heavy rains of the humid seasons 

 commence or terminate, the humid winter may be seen to fall within 

 December, January, February, and March, and the humid mid-summer 

 within July, August, and September. Making this artificial division 

 by months between the rainfall seasons, the percentages of the total 

 annual precipitation which fall in the four seasons are as follows: 

 humid winter 31.1 per cent, arid fore-summer 5.9 per cent, humid mid- 

 summer 50.6 per cent, arid after-summer 12.4 per cent. The two rainy 

 seasons yield 81.7 per cent of the total annual rainfall, and the light 

 rains of the two arid seasons (which form the remaining 18.3 per cent) 

 are of very slight influence upon vegetation. The rains of November 

 may bring forth some of the winter herbaceous perennials, without any 

 effect on the large perennials other than the inducing of leaves on Fou- 

 quieria and Parkinsonia. The rains of the arid fore-summer are usually 

 too light and too widely separated to bring into activity either the 

 summer ephemerals or the perennial plants. 



SEASONAL DISTRIBUTION OF RAINFALL. 



On the Pacific Coast the monthly distribution of rainfall brings over 

 75 per cent of the annual total within the whiter months. On passing 

 eastward through Arizona this predominance of winter rain is gradually 

 lost until it becomes less than 20 per cent of the annual total at the 

 Rio Grande River in New Mexico. Conversely, the precipitation of 



