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 emoryt, 
Franseria deltoidea, Simmondsia californica, Jatropha cardiophylla, and 
Crossosoma bigelovit. Among the desert species which are common to 
the Arizona-Sonora region and to the Texas-Chihuahua desert are: 
Fouqueria splendens, Keberlinia 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 ia the 
Desert flora also possess representatives in the deserts of Argentine 
and Chile, as: Covillea, 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, Dodonea, Erythrina, and Gymnolomia, or have 
a world-wide representation, as Tragia and Stemodia. 
