20 ] The University of New Mexico 
grassland district, as mesquite is still dominant, and most of 
the associated plants are typical. A somewhat similar 
extension of mesquite-grassland is found along- the lower 
Nazas into the Coahuilan sink. 
3. Succulent desert: Southward from the Conchos 
and the Big Bend of the Rio Grande there is a marked in¬ 
crease in the number of species and individuals of succulent 
and semi-succulent plants. Cacti, Agaves, Nolinas, Dasy- 
lirions, and Yuccas are dominant members of the vegetation, 
along with various Leguminoseae, creosote bush, and oco- 
tillo. Along the washes grow mesquite, Acacias, Mimosas, 
Paroselas, hackberries, Lyciums, and Condalias. Creosote 
bush dominates most of the district, but yields to the succu¬ 
lents on the slopes and ridges. The grass vegetation is 
poorer than in the creosote-yucca-mesquite association. Both 
capsular and baccate yuccas are common. Cacti increase in 
size and numbers southward, and include numerous Echino- 
cacti, Mamillarias (Neolloydia), members of the Cevens 
group, etc., in addition to the Ojmntias found to the north. 
Agaves, especially lechuguilla, Nolinas (bear grass), and 
Dasylirions (sotol) have such a development that this dis¬ 
trict is sometimes referred to as the sotol, or lechuguilla 
country. Two species of ocotillo are important items in the 
landscape. The richness of this district in succulents prob¬ 
ably expresses a close relationship to the cradle of such arid 
climate plants more than any special suitability of climatic 
or edaphic factors. 
Sierra Madre Occidental: The Sierra Madre Occidental 
area comprises the western highland margins of the Mex¬ 
ican central plateau or tableland, with elevations between 
5,000 and 10,000 feet. The precipitation values run from 
500 mm. to over 1,200 mm., with mean annual temperatures 
under 18°C. The annual rainfall, although concentrated in 
the summer months, is sufficient to maintain a mixed deci¬ 
duous-coniferous woodland at the lower elevations and a 
fairly pure, open coniferous forest higher up. In general, 
