1901 | VEGETATION OF WESTERN TEXAS 103 
Jatropha macrorhiza, J. multifida, Janusia gracilis, Aspicarpa 
hyssopifolia, A. longipes, Galphimia angustifolia, Malpighia glabra, 
Amyris parvifolia, Helietta parvifolia, Schaefferia cuneifolia, Ser- 
jania incisa, S. brachycarpa, Urvillaea Mexicana, Wissadula 
mucronulata, Hermannia Texana, Melochia tomentosa. 
SEMITROPICAL.— The tropical elements, which are but faintly 
indicated even in the southernmost part of Texas, quickly merge 
into a semitropical zone, which is visible and persistent even to 
the cafions on the border of the plains region, and especially up 
the Rio Grande into the Great Bend and its side cafions, and in 
the lower Pecos cajion, and this in spite of the great preponder- 
ance of Sonoran species of the Lower Austral. The range of 
Acacia Farnestana and Parkinsonia aculeata would represent fairly 
the area of this zone outside the Rio Grande valley, A number 
of the species included in genera cited for the tropical portion 
extend also over this area. It is further indicated by such gen- 
era as Guajacum, Mortonia, Pistacia, Peganum, Ascyrium, Per- 
sea, Myrica, Balodendron (listed by V. Bailey from Corpus 
Christi), Cardiospermum, Castela, Choisya, Hermannia, Ayenia, 
Corchorus, and Malachra. 
Lower Sonoran.—The Lower Sonoran elements so pervade 
the regions containing tropical and subtropical species as to be 
by far the dominant flora, and to constitute the characteristic 
formations. If the Lower Sonoran division of the Lower Austral 
zone be counted from the 98th meridian in Texas, it exists in 
several grades of purity or intensity over the following provin- 
ces, varying with differences in temperature, moisture, and soil : 
(1) the Rio Grande plain, especially interior to the tropical and 
subtropical divisions; (2) the erosion mountainous portion of 
the Edwards plateau; (3) the central plains of northern Texas 
(including the cafion of the Canadian river across the Staked 
plains); (4) the Staked plains below 3500 feet and the adjoin- 
ing plateau prairie to the south; (5) the Toyah basin; (6) the 
bolson plains and mesas of trans-Pecos Texas; (7) the Stock- 
ton plateau and slopes of the Guadalupe, Davis, and other 
mountains at the north up to about 4000 feet, and the slopes of 
