Chihuahuan Desert Region. 645 
Agave heteracantha forms a very characteristie association of very rough stony slopes in 
which this Agave grows associated with Dasylirion texanum. — The Cactus Association 
includes several species of Mamillaria, Cereus (C, stramineus), Echinocactus (E. [Ancistrocactus] 
longihamatus) and Anhalonium. Such an association indicates extreme exposure to intense light, 
burning midday and chilling midnight temperatures, and moistureless air and soil. Mixed in with 
the above associations is Fouquiera with slender, thorny wand-like stems growing in the most 
arid spots of the desert. 
Alkali Playa- and Desert Dune Formation. Typically the Alkali playas occur on 
the Pecos flats and in the Salt Lake basin (the Howard basin). The prevalent grass according 
to HAvARD!) on the Pecos flats is Sporobolus airoides while on alkaline soils occur Suaeda suf- 
frutescens, S. depressa and Spirostachys occidentalis. The low sand hillocks in the Howard basin 
are covered with Spirostachys, Larrea mexicana and Frankenia Jamesii. — South of EI Paso, 
according to CovıLLe and MAc DouGAL2), is a long stretch of sand dunes about forty feet 
i The vegetation consists of Poliomintha incana, Yucca radiosa (= Y. elata), Sporobolus 
eryptandrus and species of Artemisia, Chrysothamnus and Andropogon. 
River Valleys and Canyons. At Presidio del Norte, 953 miles from the 
mouth of the Rio Grande and at an altitude of 2,780 feet (847 m), the sparse 
tree vegetation is confined to alluvial bottoms and ravines. The trees are 
Populus Fremonti, Celtis occidentalis, Prosopis juliflora, P. pubescens, Salıx 
longifolia and S. nigra. Shrubs and bushes are common, but in scattered 
thickets nowhere, according to HAVARD, forming dense thickets. 
In Chihuahua associated with Populus Fremontii var. Wislizeni along water courses from the 
mountains down into the arid plain are Salix irrorata, S. taxifolia, Fraxinus velutina, Sambu 
mexicana, Juglans rupestris and Celtis oceidentalis var. reticulata3). On a few bottoms Prosopis 
juliflora occurs associated with Celtis pallida (in dense clumps), Mimosa biuncifera, Acacia Greggii, 
Microrhamnus ericoides, Condalia spathulata and Koeberlinia spinosa. e herbaceous plants of 
the bottom lands are species of Hoffmanseggia, Sphaeralcea, Lepidium, Tetraclea, Philibertia, 
Argemone, Oenothera, Martynia, Tribulus and Collomia longiflora with the coarse maton grass, 
Sporobolus Wrightii. At Eagle Pass the vegetation begins to change into the chaparral forma- 
tion of the lower Rio Grande. The canyons of the Rio Gr ande below Presidio are characte- 
rized by the presence of Acacia Schottii, Boerhaavia bracteosa, Bouchea linifolia, Mimosa fragrans, 
Henryi and Nama undulatum. In general throughout these canyons, . 
tation is scant, sometimes entirely absent. — Those plants preponderating on mesas and hills 
are (Mesa Formation): Acacia Greggii, A. constricta, Mimosa borealis, M. biuncifers, Dalea 
formosa, Condalia obovata and C. spathulata, Zizyphus obtusifolius, Koeberlinia spinosa, Larrea, 
Jatropha, Celtis pallida, Lycium carolinianum, L. Berlandieri, L. puberulum, Agave heteracantha, 
Dasylirion texanum ucca angustifolia, Cereus (Peniocereus) Greggü ange: eier 
Anhalonium (Ariocarpus) fissuratum, A. (Lophophora) Williamsii and Ephedra antisyphilitica#). 
At Eagle Pass 495 miles above the mouth of the Rio Grand 
— 
Acacia filieina, Salvia 
Grande and elevated 1,460 feet 
(= 445 m) above sea-level, Dasylirion texanum has disappeared and Yucca baccata becomes 
dwarfed. Acacia Greggii and A. constrieta of the gravelly mesas at Presidio ar here ee 
Acacia amentacea, A. Berlandieri, A. Coulteri, and A. Wrightii. Prosopis ne ge 
vanished, but P. juliflora is common and attacked by a parasite, Phoradendron flavescens. FTere 
begin Parkinsonia aculeata and Acacia farnesiana, transitional to the chaparral Bew: 
together with Lippia Iycioides (= L. ligustrina), L. graveolens, Salvia ballotaelora and Rue 
angustifolia. 
1) Havaro, V.: See Bibliography p. 68. 
2) Covıız, F. V. and Mac Doucar, D. T.: See BIbHOgeapNg Be 7% 
3) PRinGLe, C. G.: Garden and Forest I: 105—ı106 (see Bibl. p. 85). 
4) Havaro, V.: L. c. in Proceedings U. S. Nat. Mus. VII. 1885: 470474 
