NATURAL AREAS AND REGIONS 



609 



The mes quite has a wide range through- 

 out southern Texas, and extends west 

 as well as north beyond the region of 

 typical semi-desert country. The north- 

 ern extension takes the mesquite a long 

 way into the grassland country, but 

 here it is no longer associated with the 

 other dominant semi-desert species. To 

 the westward, however, the situation is 

 somewhat different. The semi-desert 

 tends to invade the succulent desert for 

 a considerable distance up the valley of 

 the Devils and Pecos rivers, and up the 

 valley of the Rio Grande a short distance 

 beyond the mouths of the Pecos and 

 Devils rivers. Plotting the distribution 

 of the trees above mentioned as asso- 

 ciates of the mesquite, together with the 

 mesquite, we find these typical semi- 

 desert forms carrying the formation well 

 into the realm of the succulent desert 

 as two arms : one short Rio Grande arm 

 around the mouth of the Pecos River, 

 and a longer arm reaching up the valley 

 of the Pecos. Beyond this they give 

 place to the true succulent desert 

 dominants. 



Among the animal forms we find here 

 several reptiles that are typical: the 

 long-tailed holbrookia (Holbrookia pro- 

 pinqua) is confined very closely to this 

 region, and is also an inhabitant of 

 Padre Island, the sand dune region 

 across the tidal flat from the mainland. 

 The horned toad (Phrynosoma cornu- 

 tum), is abundant here, though this is 

 probably an extension of range from 

 the succulent desert. The western dia- 

 mond rattler is confined to the region 

 and is typical of it. A map of its dis- 

 tribution shows that it follows out the 

 extensions of the mesquite-persimmon- 

 cresote-bush country both northward 

 and up the valley of the Pecos and Rio 

 Grande. The same may be said of the 

 Texas armadillo (Dasypus novemcinctus 

 texanus) though its range exceeds the 

 limits of the region. The Texas opos- 

 sum (Didelphis marsupialis texensis) 

 likewise is found here, following up 

 the Rio Grande as far as the mouth 

 of the Pecos, but to the north it 

 oversteps the limits of the area. Here 



also are the Texas grasshopper-mouse 

 (Onychomys leucogaster longipes) and the 

 Texas deer-mouse (Peromyscus leucopus 

 texanus). The ocelot (Felis pardalis 

 ablescens) and the Rio Grande spotted 

 skunk (Spilogale leucoparia) follow very 

 closely the distribution of the mesquite 

 in the region. 



8. Rio Grande (Eastern) Succulent 

 Desert 



This is the real desert region of Texas, 

 a great expanse made up mostly of the 

 valley of the upper Rio Grande, Pecos 

 and Devils rivers. The vegetation is 

 scanty, and occurs most typically not 

 in these valleys but on the arid plateau, 

 the bolson basins and the foothills or 

 the slopes of isolated mountain groups. 

 It is an area of loose sandy soil extending 

 to the base of the mountains, where the 

 debris of erosion is so loose and mobile 

 that anything like a close plant forma- 

 tion is impossible. The vegetation is 

 conspicuously open with no crowding 

 of species nor competition for light and 

 space. The landscape is typically a 

 gray-white color due to the soil cover of 

 eroded limestone. The white glare of 

 this soil as one traverses the region on 

 the Southern Pacific is most trying. 

 The rainfall is very scant, from 7 to 10 

 in. and often less. During an exces- 

 sively wet season the grass tends to 

 increase, but never does it approach a 

 grassland formation. The characteris- 

 tic desert species of plants may be 

 grouped into four general types : 



1. The cactus type: represented by 

 approximately 90 species. 



2. The yucca-agave type: represented 

 by about 15 species. 



3. The dwarf shrub type : represented 

 by approximately 50 species (grease- 

 woods, mimosae, etc.). 



4. Miscellaneous: such as the rock 

 ferns, resurrection plant (Selaginella 

 lepidophylla), dry plains grasses (Hech- 

 tia) and a number of desert compositae. 



s These may be grouped into two classes 

 from the point of view of adaptation: 

 1. "Those in which a marked develop- 

 ment of storage tissue results in unusual 



