108 VEGETATION OF A DESERT MOUNTAIN RANGE. 
species of the Desert (for elevations see page 37). On a high ridge 
tributary to Bear Cafion have been found the highest individuals of 
Opuntia sp., the highest species of that genus on the mountain, and 
Mamillaria grahami, the highest-ranging plant of the Desert. The 
individuals which most nearly approach these highest stations for 
Opuntia and Mamillaria have been found on south exposures about 
600 feet lower, in the Bear Cajion drainage. 
On an exposed ridge, with a considerable inclination to the south, 
at 7,800 feet are found the highest individuals of Binus cembroides, 
Juniperus pachyphlea (with one known exception), Yucca schottii, 
Echinocereus polyacanthus, and Arctostaphylos pungens. In this station 
the influences of slope exposure and of topographic relief are combined, 
thereby bringing about the pronounced conditions that are expressed 
in the highest occurrence of 5 species of the Upper Encinal. On the 
ridges above Marshall Gulch are found the highest occurrences of 
Quercus hypoleuca and Quercus reticulata, both of which forms extend 
further down the south faces of these east-and-west ridges than they 
do down the north faces. ; 
When Desert plants are found on the ridges of the Encinal region 
they fail to appear on the south-facing slopes just below these ridges. 
When the plants of the Encinal are found at their highest locations 
on ridges of the Forest Region they are also absent on the south-facing 
slopes just below the ridges. This does not appear to be the case with 
respect to the highest occurrences of plants which are believed to have 
their true climatic limit just below the summit of Mount Lemmon, 
such as Quercus hypoleuca and Quercus reticulata. 
The extent by which the highest individuals on ridges exceed the 
highest individuals on south slopes is never more than 500 to 600 feet, 
except in the case of Pinus cembroides, in which it is about 700 feet. 
Opuntia sp. and Mamillaria grahami, which have their upper limit in 
the vicinity of 7,000 feet, agree in this respect with Opuntia versicolor, 
Echinocactus wislizeni, and Fouquieria splendens, which have their 
limit in the vicinity of 5,500 feet. 
Perhaps the most common explanation of the highest occurrence of 
species on ridges is that the soil is driest in such situations and therefore 
offers to plants from lower elevations a habitat more like that in which 
they are abundant. The principal objection to such an explanation 
is the unquestionable fact that a somewhat more moist soil is not 
inimical to the plants of the Desert nor to the plants of the Encinal. 
Neither is there a sufficient difference between the soil moisture at 
the bottom of a slope and on the ridge at the top of the slope, in the 
arid seasons, to cause a differentiation of the vegetation. 
The explanation of the phenomenon may be sought partly in the 
existence of cold-air drainage, which is at least responsible for the 
absence of Desert and Encinal plants from the bottoms of cajions at 
