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 flourish 
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 winter 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 
