INTRODUCTION. 13 
the summit ridge. Live oaks grow upon the rocky sides of the ravines, and with them is 
frequently associated a species of cedar. Among the more interesting of the herbaceous plants 
found here, are Dryopetalon runcinatum, Vesicarie, and Colomie, The narrow valley of the 
stream which runs through the bottom of the Pass is wooded with ash, cotton wood, and Mexican 
sycamore. 
VEGETATION OF THE UPPER VALLEYS OF SAN BERNADINO, SAN PEDRO, AND SANTA CRUZ WITH THAT OF 
THE ADJOINING MOUNTAIN RANGES, 
The country embraced in the above limits, includes that portion of Northern Sonora, 
which divides the waters which flow north towards the Gila river from those whose course is 
south, towards the Gulf of California. It is, as we have before stated, diversified with high 
wooded mountains and upland plains, well watered valleys and dry and barren tracts. The 
arborescent growth is not esseutially different from that we have noticed in speaking of the 
other divisions of country. Live oaks, the nut pine, cedar, ash, walnut and cotton wood are 
produced either upon the mountains or in the upland valleys. Its plains are covered with an 
uniform growth of upland grama grass, or in the more arid localities by mezquit and its thorny 
associates. This region furnishes a number of singular and highly interesting genera and 
species, most of which are described by Dr. Gray, in the second part of ** Plantae Wrightiane,’’ 
in the Smithsonian Contributions. A reference to this work will give a better idea of the 
character and distribution of the flora of this district than our limits will allow us. As it 
occupies a station between several botanical divisions, so its flora partakes of that of those 
regions. The following list of some of the plants found here will be seen to embrace species 
belonging to California, Texas, Mexico, and New Mexico, viz: Lschscholtzia douglasii, 
Zauschneria Californica, Eulobus Californicus, Bowlesia tenera, Anemone Caroliniana, Draba 
caroliniana, Corydalis aurea, Androsace — Rutosma texana, Erodium texanum, 
Layia Neo-mexicana, Cowania Mexicana. 
VEGETATION OF THE LOWER SANTA CRUZ VALLEY, THE DESERT OF TUCSON, AND THE CENTRAL VALLEY 
OF THE GILA, 
The region defined as above constitutes a very distinct botanical district, many of its peculiar 
plants not being found elsewhere, As we have mentioned in a previous sketch, the valley of the 
Santa Cruz, as it leaves the mountains in its northward course towards the Gila, gradually 
looses its fertile character, and finally terminates in the desert plain which forms the table land 
of the Gila. The vegetation of this tract comprises many of the forms which are found on all 
the barren plains of the country. Here, as in similar situations elsewhere, the mezquit and 
the creosote bush are conspicuous. Cacti are abundant here, and of various types. There are 
the low arborescent Opuntias, generally bearing proliferous fruit, as well as several elliptical- 
stemmed species. The enormous Echinocactus wislizenii and Caespitose mammillarius and Cerei 
“аге common, while, either standing solitary or collected in groups, the lofty Cereus giganteus 
towers above all. А species of misletoe is common upon the mezquit trees of this region. 
The sheltered crevices of the neighboring mountains produce a species of agave, and i in these 
localities we find Franseria deltoidea, Encelia farinosa, and Perityle nuda. 
