296 Part III. Chapter 3. 
The Edwards Plateau region in central Texas an area roughened by erosion, 
exposing the limestone strata to the reception of rainfall is a meeting ground 
for species from the Atlantic forest, the southern Rocky Mountains and from 
the north Mexican highlands. Such Mexican plants are Fuglans rupestris, 
Morus celtidifolia, Arbutus xalapensis, Sophora secundiflora, Bumelia lanugınosa. 
The trans-Pecos territory is characterized by a covering of strongly xerophytic 
chaparral which embraces the northern Mexican plateau, the arid plateaus of 
New Mexico, Arizona and southern California. It is floristically different from 
the Rio Grande chaparral, which represents a northern tongue of the great 
coastal scrub extending southward, for the trans-Pecos chaparral represents the 
northern extremity of the Mexican plateau flora. As a consequence of this, 
we find the Mimoseae and Caesalpineae represent less than ro per cent of 
the species, individuals occur less abundantly, and excepting Prosopis juliflora 
are different species from those of the Rio Grande plain. A more striking 
array of ecologic types is found in the species of this formation, as illustrated 
in the Zphedra-type, the felt-covered Zurotia- and Croton-types, the resin con- 
taining Zarrea and Flourensia, and the thorny-stemmed Fouguiera. About 
thirty species are more or less prominent constituents of the chaparral of the 
trans-Pecos area. Larrea mexicana, the Mexican greasewood, is characteristic 
of high gravelly mesas and of bolson deserts extending even to the alkali soils 
of such basins. All of these facts presented above demonstrate the predominant 
Mexican character of the plant formations described above '). 
Cactus-Vegetation. The Cactaceae afford an interesting proof of the fact 
that the Mexican tableland forms a center of origin of many peculiar plant 
forms. The headquarters of the family is found in Mexico and one is forced 
to believe that the family had its origin here”). The territory richest in species 
is in the neighborhood of Pachuca, Tula, Queretaro, Guanajuato, San Luis Potosi 
and northward to Saltillo, Monterey, Montclova and Laredo on the Rio Grande 
River. Less rich, but by no means poor in species, are the northern states 
of the Mexican Republic, Chihuahua, Coahuila and Sonora, the last is connected 
with the richest territory of the United States in cacti, viz., that covered by 
the states of New Mexico, Arizona and California, especially in their southern 
portions and Texas. Lower California is also rich in forms. The distributional 
headquarters is especially rich in species of the genera Mamillaria and Echino- 
cactus. In the moist valleys of southern Mexico, Pilocereus is regnant, only 
a single species, P. Schottii, is present in northern Mexico. Such small genera 
as Ariocarpus, Anhalonium, Pelecyphora, Leuchtenbergia are found at head- 
quarters. The genus Cereus has its development north of the center, for 
example, C. giganteus, on the Rio Gila, and such species, as Cereus ‚pecten- 
aboriginum (Fig. 9) show the prominence which plants of this character have 
in the landscape. The Opuntias are numerous both at the center and in the 
ı) Bray, W. L.: The ecological Relation of the vegetation of western Texas. Botanical 
Gazette XXXI: 275 (1901). 
2) SCHUMANN, K.: Gesamtbeschreibung der Kakteen 1899: 21. 
