Vegetation of Northwest Mexico [ 19 
where their expanse may be broken only by popotillo 
(Ephedra sp.), a few yuccas, an occasional opuntia or oco- 
tillo, and some mesquite and hojasen. Tobosa grass grows 
mainly in areas of fine textured soils, and the bunch grasses 
are found over areas normally subject to annual flooding. 
Various grasses vegetate the borders of the barren playas, 
the dominant species being salt grasses (Distichlis spicata, 
and Eragrostis obtusiflora) , and alkali sacaton (Sporobo- 
lus airoides). The edges of the flooded areas usually mark 
the inner limits of the grassy zone, although in some places 
clumps of sacaton extend well out into the flats. 
The banks of the larger streams often carry a consid¬ 
erable tree vegetation consisting of cottonwood, willow, 
sycamore, walnut, hackberry, ash, and alder. The Fremont 
cottonwood (Popidus fremontii=ivislizeni) is the outstand¬ 
ing member of the gallery forest. Walnuts (Juglans rupes- 
tris, and J . major), desert willow (ChUopsis linearis), and 
hackberry (Celtis reticulata) also occur singly and in small 
groves and clumps along the arroyos. Occasional oaks and 
junipers fringe the upper margins of the western basins, 
and mark the change to the Sierra Madre Occidental area or 
to the higher mountain ranges of the district. These desert 
and steppe ranges have a vertical succession from mesquite- 
grass-creosote, through juniper, agave, and oak, to pines 
—in the case of the highest ranges. Oaks are dominant, 
Quercus emoryi prevailing on the lower slopes and Quercus 
grisea on the upper slopes. In the steppe lands of the Son¬ 
oran Desert area, there is an association quite similar in 
general to that of the Chihuahua mesquite-grassland, but 
there are sufficient floristic differences to justify a distinc¬ 
tion. 
Along the Rio Grande and lower Conchos valley in Chi¬ 
huahua, the vegetation resembles that of the larger river 
valleys in the steppe country. There is, however, an increase 
in the Acacias and Mimosas so typical of the lower Rio 
Grande. This gallery vegetation is more scrubby than ar¬ 
boreal. It has been mapped as an extension of the mesquite- 
