16 VEGETATION OF A DESERT MOUNTAIN RANGE. 
dropuntias), Opuntia toumeyi, Opuntia blakeana (procumbent plato- 
puntias), and Acacia paucispina. : 
The shorter and steeper Upper Bajadas which fringe the southern 
and southwestern edge of the Santa Catalinas are clothed with a much 
more diversified vegetation, in all respects similar to that of other 
Upper Bajadas which lie below 3,500 feet (1,067 m.) in other localities 
in southwestern Arizona. The freedom of the soil from caliche is here, 
as elsewhere, responsible for the existence of a diversified vegetation 
rather than a pure stand of Covillea. 
The Upper Bajadas, as exemplified along the south face of the Santa 
Catalinas at about 3,000 feet elevation (915 m.), bear what may be 
regarded in many respects as the most highly developed type of desert 
vegetation to be found in southern Arizona or northern Sonora. In the 
Upper Bajadas may be found a greater number of species of perennial 
plants than in any other distinctly desert situations. In them also 
the number of individual perennial plants per unit area is greater than 
in any areas outside the flood plains of such rivers as the Santa Cruz 
and Gila. The only areas that compare with the High Bajadas in 
these respects are the volcanic hills in which basaltic rock has weathered 
to a fine clay which is very retentive of soil moisture, as is well exempli- 
fied in Tumamoc Hill, the site of the Desert Laboratory. The andesitic 
and rhyolitic hills in the vicinity of Tumamoc are much poorer than 
it is in the number of individual plants per unit area, although perhaps 
nearly as rich in their flora. 
On the Upper Bajadas there often occur, in almost equal admixture, 
from 15 to 25 perennial species of plants of such size as to dominate the 
physiognomy of the vegetation. These same species may be found on 
the more nearly level Lower Bajadas, but any one of them may often 
be absent for many miles, may be sporadically represented by a few 
individuals, or may occur in dense but local colonies (particularly in 
the case of the cacti). Occasionally as many as 5 to 10 of the species 
may be within sight at the same time. 
The flora which characterizes the Upper Bajadas of the Santa 
Catalinas ranges without substantial loss down to sea-level on the gulf 
of California,* and the vegetation formed by their commingling may 
be found as a belt covering the high bajadas which encircle all of the 
mountain ranges and clothing all of the low basaltic hills. A climb of 
2 hours from the base of the Santa Catalinas will discover greater 
changes of vegetation and flora than can be encountered in the 150 
miles (242 km.) between Tucson and Adair Bay. 
The Upper Bajadas present the desert characteristics of openness 
of stand, lowness of stature, and commingling of diverse vegetation 
types. The first of these features is common to the vegetation of all 
*See Hornaday,W.T. Camp Fireson Desertand Lava. New York, Scribner, 1909. MacDougal, 
D.T. Across Papagueria. The Plant World, 11: 93-99, 123-131, 1908. 
