76 BOTANICAL GAZETTE [JULY 
transition portion Prosopis and other shrubs, together with several grasses, 
become more abundant, and such forms as Agave Palmert and Dasylirion 
Wheeleri appear. In the encinal the grass cover largely persists, although 
small evergreen trees and shrubs are the distinctive feature of the vegetation. 
Among the most abundant of these are several aa of Quercus, Juniperus 
pachyphloea, Pinus cembroides, Arctostaphylos pungens, Mimosa biuncifera, 
us trilobata, Garrya Wrightii, and Vauquelinia lie Herbaceous 
perennials are also abundant. 
The forest begins at about 6000 feet, with an open stand of Pinus arizonica, 
in which the evergreen oaks of the encinal become gradually more scattered. 
At slightly higher elevations Arbutus arizonica, Pseudotsuga mucronata, and 
Pinus strobiformis begin to be found, while the shrubby and herbaceous flora 
gradually changes by the appearance of less xerophytic forms. This forest 
reaches its highest expression by the appearance of Abies concolor above 7000 
feet, where the individual trees are larger and the stand becomes close and 
heavy. Such a mesophytic conifer forest in the midst of a desert calls for a 
close analysis of the climatic factors involved. The location of the Santa 
Catalina Mountains in a continental desert gives their lower slopes the climate 
characteristic of this desert, its main features being a low rainfall unequally 
distributed, a short winter with severe frost, and a long summer with high 
temperatures and low humidity. The major portion of the rainfall is confined 
to a short but well marked rainy season in July and August, within which a 
little over half of the annual precipitation usually occurs. At Tucson this has 
averaged 6.6 inches during the years 1907-1912, while in the mountains, 
although there have been very few observations, the average for the years 1907- 
1914 inclusive, at stations at 7600 feet, for the humid midsummer, was 17.45 
inches, or a probable annual average of 35 inches. The frostless season de- 
creases from a range extending from March to December at Tucson, to one 
from the middle of May to the beginning of October at 8000 feet, but perhaps 
the change at higher altitudes most important in its effect upon vegetation is 
the prolongation of spring shortening the arid fore-summer from 15 weeks on 
the desert, to 11 weeks at 6000 feet, and 6 weeks at 8000 feet. The influence 
of this modification will become apparent when it is recalled that this is the 
most trying season of the year, and the one in which moisture conditions are 
critical for the survival of individuals or species. The humid midsummer, on 
the contrary, is the season when conditions upon the desert and in the moun- 
tain are most alike, and during which there is the greatest vegetative activity 
in both habitats. A close analysis of these effects of the variations in the 
amount and distribution of the rainfall is given, together with a few data 
regarding the resulting conditions of soil moisture 
The evaporative power of the air was determined by the use of standard 
atmometers at six stations, situated at regular intervals of 1000 feet from 
3000 to 8000 feet. At each station a pair of instruments, one with a north 
exposure and the other with a south exposure, were used, and some data were 
