24] 
The University of New Mexico 
On the flood plains and alluvial areas, there are large 
trees of Parkinsonia, Olneya, Prosopis, and Acacia , inter¬ 
mingled with columnar cacti, smaller cacti, chamiso, Fran- 
seria , and galleta grass. In the arroyos and washes a vege¬ 
tation of mesquite, catsclaw (Acacia sp.), palo verde, palo 
fierro, creosote bush, hackberry, Ephedra , Condalia , and 
ocotillo prevails. Along the southern coast there are heavy 
stands of Pachycereus , and cirio (Idria columnaris) . Along 
the southern foothills are numerous palo bianco trees 
(Ipomea arborescens) , intruders from the tropical area. 
3. Sonoran mesquite-grasslands: The northern and 
eastern basins and plains of Sonora carry a steppe shrub 
vegetation quite similar to that of the Chihuahuan mesquite- 
grassland. The distinctive features of the Sonoran district 
are dominance of grama grasses, numerous cacti, relative 
unimportance of Yuccas , creosote bush, Flourensia, Ephedra, 
Condalia, and Koeberlinia, lesser importance of mesquite as 
a dominant, and the presence of Parkinsonia and Franseria. ' 
The arboreal constituents of the gallery forests are nearly 
the same in both districts, but there is a stronger develop¬ 
ment of oaks and Acacias in the Sonoran district. Palms 
are common in many of the western canyons, the chief 
genera represented being Washingtonia, Erythea, and 
Inodes . 
4. Subtropical mimosaceae-cacti: Covering the arid 
coastal and river valley portion of southern Sonora and 
northern Sinaloa is a spiny shrub and cacti formation domi¬ 
nated by Mimosaceae and columnar cacti. Creosote bush, 
saguaro, and Fouquiera splendens of the northern districts 
are virtually lacking, their places being taken by numerous 
Caesalpinias , Cassias, Mimosas, and palo santo (Guaiacum 
coulteri). This vegetation is not distributed uniformly, but 
is arranged in clumps, gathering into a nearly continuous 
cover in wetter spots. Various Mimosaceae and columnar 
cacti especially form large stands which extend for miles, as 
in the deltas of the Yaqui, Mayo, and Fuerte rivers. In the 
