CORRELATION OF VEGETATION AND CLIMATE. 107 
shown themselves incapable of withstanding the atmospheric aridity 
at Tucson even when grown under the most liberal irrigation. The 
inability of a plant to pass water on to its transpiring tissues as rapidly 
as it is withdrawn by a desert atmosphere is undoubtedly a feature 
common to very many mesophilous plants, and it is apparently the 
cause which prevents a greater number of palustrine mountain plants 
from descending the large streamways to the Desert, and it doubtless 
prevents a lower descent upon the part of many Forest species which 
reach the flood-plains of the Lower Encinal. 
It might be argued that the low occurrence of Forest along the 
streams of the Encinal and the descent of the Encinal into the Desert 
Slopes are due to the influence of cold-air drainage rather than to the 
effects of soil moisture, or that cold-air drainage is at least an important 
contributory factor. It is difficult to believe that low temperatures, 
especially those of the winter months, should be a favoring factor for 
plants which are subjected during the day to just as high temperatures as 
are the plantsof the upland. During the summer months the low noctur- 
nal temperatures might be of someslight importance, but such importance 
would reside solely in aiding the plant to recover from the excessive 
transpiration of the preceding day and to build up a reserve of water 
against the transpiration of the following day, as has been shown by 
Edith B. Shreve to occur in Parkinsonia microphylla.* The facts that 
it is the highest diurnal temperatures that are apt to be deleterious 
to low-ranging mountain plants and that their effect can be only 
indirectly and slightly offset by the lowest nocturnal temperatures 
make it appear that cold-air drainage has at least a very minor réle 
in this connection as compared with the moisture conditions. 
THE ROLE OF TOPOGRAPHIC RELIEF. 
Each of the leading types of vegetation in the Santa Catalinas 
reaches the uppermost limit of its occurrence on ridges and high south- 
facing slopes. This carries the Desert upward into the Encinal and 
carries the Encinal up into the Forest in such a manner that there is 
an interdigitation of the vegetistic regions throughout the portions of 
the mountain in which the topography is mature enough for it to be 
manifest. This appearance of interdigitationis partly brought about by 
the influence of streams (which has just been discussed) and is some- 
times merged with the influence of slope exposure. These facts do not 
in the least obscure the high range of each type of vegetation on the 
narrow ridges which point due south or north and are therefore free 
from the influence of slope exposure. 
On the ridges which lie between the tributaries of Soldier Cafion 
have been found the highest individuals of all of the characteristic 
* Shreve, Edith B. The Daily March of Transpiration in a Desert Perennial. Carnegie Inst. 
Wash., Pub. 194, 1914. 
