94 JOURNAL OF THE ARNOLD ARBORETUM [vol. n 



mines and rich cities of the interior, rumors of which had reached them 

 from earlier explorers and from native traditions, the burrows, which were 

 the pack animals of their caravans, were fed largely upon the beans of the 

 Mesquite, and in this way the tree was first established in the country. My 

 informant assured me that their route from the Rio Grande, up through the 

 Spanish Pass northwest of San Antonio, towards San Saba and the interior, 

 could formerly be traced by the growth of Mesquite along it, and that sub- 

 sequently it had spread over the country in all directions from this nucleus. 

 I cannot vouch for the authenticity of this account, but certainly the trail of 

 the old Spaniards pointed out through the region is near the centre of dis- 

 tribution of the Mesquite in Texas and it does not seem impossible that it 

 may have been, at least in part, introduced in this way. In receht years its 

 dissemination over wide areas has been accomplished largely by the domes- 

 tic animals that feed upon it. It is claimed that in the arid regions the seeds 

 will germinate only after having passed through the stomachs of animals. 

 Certain it is that the undigested seeds are often distributed and the plant 

 established in this way. 



Along the southern border of the Staked Plains there is a gradual transi- 

 tion from the flora, as from the topography, of the Edwards Plateau and a 

 number of the more hardy woody species of the latter region persist for 

 some distance into the plains. Some of the trees and shrubs find their 

 northern limit about the buttes and ridges of Comanchean limestone that 

 appear at intervals through the more recent residual deposits, but others 

 reappear in the canyons or persist about the marginal rim of the elevated 

 plains. Amongst those of the former class appear to be the Riverside 

 Walnut, Western Red-bud, Live Oak, Mexican Buckeye and deciduous 

 Coral Bean (Prosopis affinis) ; more persistent are the Shin Oak, the Red- 

 fruited Juniper, Hackl)erry, Argireta (Mahonia trifoliata)^ Gum-elastic 

 (Btimelia fexana) and wild China {Sapindus Dnimmondii). In flats and 

 depressions shrubby species from the Rio Grande Plain and the more arid 

 regions to the southwest appear. Typical of these are the Creosote Bush 

 (Larrea tridentata), the slender Cactus (Opitntia leptocaidis). Rims micro- 

 plujlla and Koeberlinea spinosa. 



The marginal escarpment of the plateau with its high, perpendicular cliffs, 

 frequently overhanging ledges of cap-rock and deep indenting coves and 

 ravines, affords protection to many plants that are unable to maintain 

 themselves in more open situations; but as most of the ligneous species 

 found here are also common to the canyons, some of which have worked far 

 back into the interior, they will be mentioned in that connection. 



The streams which traverse the upper plains, with their very slight fall 

 and shallow channels, serve to carry off the surplus water after heavy rains. 

 However, owing to the flatness of the country, the generally dry atmosphere 

 and almost constant winds, the amount of water that reaches them is very 

 small in proportion to the area, most of it remaining on the surface and dis- 

 appearing in a short time through evaporation. Under these conditions 

 there is little erosion through the action of the streams in this part of their 



