FLORA OF THE SANTA CATALINA MOUNTAINS. 37 



between the Sierra Madre Occidental and the Gulf of California, in the 

 States of Sonora and Sinaloa, the other lying at higher elevations in 

 the States of Chihuahua and Zacatecas. There are strong diversities 

 of flora between these two Mexican deserts, although they do not fail 

 to have many species in common. The Sierra Madre forms an effective 

 barrier between them in Mexico, but north of the International Bound- 

 ary the continental divide is formed by scattered mountain ranges and 

 broad valleys rather than by a continuous elevated range, and these 

 valleys, lying between 4,000 and 5,000 feet, have permitted the inter- 

 mingling of species from the two desert floras, at the same time that 

 they have constituted a barrier to many species presumably unable to 

 withstand the winter temperature conditions of the elevated valleys. 

 The deserts which border the lower course of the Colorado River in 

 Arizona and California, the Mojave Desert, and other desert regions 

 in southern California and Nevada lying below 4,000 feet, possess a 

 very small number of distinctive species as contrasted with the two 

 Mexican desert regions, and have contributed almost no species to the 

 flora of the Santa Cruz valley, although many species of wide Mexican 

 occurrence are represented in both localities. The deserts of the Great 

 Basin have likewise contributed no distinctive elements to the flora 

 of the Santa Cruz Valley and the Desert region of the Santa Catalinas. 



Among the many species characteristic of the Arizona-Sonora Desert 

 which do not cross the continental divide are: Carnegiea gigantea, 

 Parkinsonia microphylla, Encelia farinosa, Olneya tesota, Hyptis emoryi, 

 Franseria deltoidea, Simmondsia californica, Jatropha cardiophylla, and 

 Crossosoma bigelovii. Among the desert species which are common to 

 the Arizona-Sonora region and to the Texas-Chihuahua desert are: 

 Fouquieria splendens, Kceberlinia spinosa, Chilopsis saligna, Momisia 

 pallida, Coldenia canescens, Opuntia leptocaulis, Ephedra trifurca, 

 Hilaria mutica, and Baileya multiradiata. 



It would be possible to place perhaps 90 per cent of the desert flora 

 of southern Arizona in one or the other of the categories just mentioned. 

 There are a few local and endemic species, but very few species exhibit 

 ranges extending chiefly to the west, north, or east. Among the two 

 Mexican elements many species range far south of Mexico, as witness 

 the following, which are found in the deserts of Chile: Calandrinia 

 menziesii, Bowlesia lobata, Daucus pusillus, Parietaria debilis, and 

 Hydrocotyle ranunculoides. A large number of the genera found in the 

 Desert flora also possess representatives in the deserts of Argentine 

 and Chile, as: Covilka, Franseria, Encelia, Actinella, Krameria, Gutier- 

 rezia, Viguiera, Chorizanthe, Coldenia, Perezia, Menodora, Nama, 

 Amsinckia and many others. Other genera found in the Santa Cruz 

 Valley have many representatives in tropical South America or in the 

 West Indies, as Hyptis, Dodoncea, Erythrina, and Gymnolomia, or have 

 a world-wide representation, as Tragia and Stemodia. 



