VEGETATION OF THE SANTA CATALINA MOUNTAINS. 23 



yos on the bajada, is found here growing with Parkinsonia microphylla, 

 which it eventually exceeds in vertical distribution by nearly 500 feet 

 (153 m.). Prosopis is even more abundant at 4,000 feet (1,220 m.) 

 than it is on the lowest slopes, and attains a trunk circumference of 

 6 feet (2 m.) at 4,200 feet elevation (1,280 m.), within 600 vertical 

 feet (183 m.) of its upper limit. Such common shrubs of the bajada 

 as Lycium, Zizyphus, Krameria, Jatropha, and Momisia are now very 

 sporadic in their occurrence, and the compact, hemispherical and 

 vividly green Chrysoma laricifolia has become very frequent and con- 

 spicuous, together with the white-tomentose Artemisia ludoviciana and 

 the less conspicuous Eriogonum wrightii. 



On northerly slopes, just below 4,000 feet (1,220 m.), are encountered 

 the first individuals of the rosaceous tree Vauquelinia californica and 

 of Agave palmeri and Dasylirion wheeleri (sotol). Along the arroyos 

 the most conspicuous forms are Erythrinaflabelliformis, the large leaves 

 and brilliant scarlet flowers of which recall its tropical congeners, 

 Manihot carthaginensis, with leaves of striking form, and Ingenhousia 

 triloba (wild cotton), with tripartite leaves and large white flowers 

 which strikingly resemble those of the cotton plant. 



THE UPPER DESERT SLOPES. 



The slopes lying between 4,000 and 4,500 feet (1,220 and 1,372 m.) 

 constitute the upper edge of the desert. On these slopes all the char- 

 acteristic species of the bajada are confined to southerly slopes, and 

 all but half a dozen of them find their uppermost limits. On the Upper 

 Desert slopes Vauquelinia becomes common, although confined to 

 ledges of rock, and Juniperus pachyphlcea, Quercus oblongifolia, and 

 Quercus arizonica occur for the first tune away from canons. On the 

 northerly slopes, where these trees form the lowest attenuated edge of 

 the Encinal region, Dasylirion occurs in abundance together with the 

 lowest individuals of Nolina microcarpa (bear grass), Arctostaphylos 

 pungens (manzanita), Agave schottii, and Yucca macrocarpa. 



The physiognomy of the Upper Desert slopes is made distinctive 

 from that of the Lower Desert slopes not only by the entrance of these 

 plants of striking form, and the exit of the desert species, but also by 

 the abundance of perennial grasses, root-perennials, and small shrubs, 

 which combine with the ephemeral plants, or their dead remains, to give 

 a much more complete ground cover than is to be found in any part of 

 the bajadas. The compact and extended patches of Agave schottii are an 

 important element in this low cover, and so are the scattered plants of 

 Boutelouarothrockiiand the bunches of Boutelouacurtipendula, Bouteloua 

 oligostachya, Muhlenbergia dumosa, Andropogon scoparium, Eragrostis 

 lugens, and Heteropogon contortus (see plates 8 and 9). 



Commonest among the low shrubs and other perennials of the Upper 

 Desert are: Chrysoma laricifolia, Acacia suffrutescens, Eriogonum 



