Vegetation of Northwest Mexico [ 23 
bean, A triplex, and Typha along- the distributaries and 
sloug-hs. The mud flats are covered with salt grass (Dis- 
tichlis . sp.), arrow weed (Pluchea sp.), wild rice, wild hemp, 
and quelite, with scattered clumps of mesquite and other 
shrubs. The tangle of vegetation is quite bewildering and 
nearly impassible over large areas. 
2. Creosote-palo verde-cacti: The northwestern por¬ 
tion of Sonora is probably Mexico’s region of least precipi¬ 
tation, many of the annual values averaging under 50 mm. 
This district extends from a sand dune and salt flat seacoast 
up to about 2,500 feet, but most of it is under 1,500 feet 
elevation. Creosote bush, palo verde (Cercidium torreyana , 
and Parkinsonia microphylla ), Opuntias , mesquite, Acacias , 
Franserias, palo fierro (Olneya tesota), and the Cereus 
gigantea dominate the landscape of this truly arid district. 
Galleta grass (Hilaria sp.) is distributed sparsely through¬ 
out. The vegetation is typically Mexican, although it ex¬ 
tends northward into Arizona. There has been influence 
across the Gulf from Baja California, and in the south from 
the tropical association. 
On the coarse gravelly mesas the characteristic plants 
are creosote bush, palo verde, palo fierro, saguaro (Cereus= 
Carnegia gigantea) cholla (Opuntia sp.), ocotillo, and 
Acacias. The creosote bush extends to the coast where it is 
associated with chamiso and galleta grass on the plains 
back of the beaches, and with Ephedra in the dune country. 
Cacti seldom attain to the coast except in the southern por¬ 
tion where saguaro, pitahaya (Lemaireocereus thurheri) 
and other columnar cacti are present. The two cacti named 
are commonly associated together and with ocotillo and palo 
verde on southern slopes and outwash fans. On many of 
the desert plains there are nearly pure stands of cholla cac¬ 
tus, especially northwest of Altar towards the Pinacate 
country. The mountains and ridges of the arid district carry 
a scanty vegetation of ocotillo, cholla, and columnar cacti, 
and some Yucca , Nolina, and Dasylirions in the northeast¬ 
ern portion. 
