MAMMALS OF THE MEXICAN BOUNDARY. 33 



streiim, and thej' are usually flanked by a broader zone of mesquite. 

 The desert willow, hackberry, yewleaf willow, and indigo tree 

 occupy arroyos in which there is a slight amount of moisture; and 

 several green-barked species of Parkiiisonia and Cercidium, known 

 collectively as the " palo verde," together with the Sonora ironwood 

 or " palo de hierro," occupy the edges of arroyos and ravines in the 

 foothills of the Western Desert Tract. 



The sandy deserts of the southwestern interior present a widely 

 different appearance from the grassy prairies of the northern plains. 

 In the former the grasses, although numerous in species, nowhere 

 form a continuous turf over considerable areas, but are broken- up 

 into tufts and clumps, which often require the protection of the 

 shrubs and other coarse plants which make up the bulk of the vege- 

 tation. There are a few large alkali flats that are perfectly bare of 

 vegetation ; and some of the most sandy spots in the deserts are 

 without plants. But four species of plants, the creosote bush, a 

 sage, an ephedra, and a grass, were seen on the interior of the wide 

 Yuma Desert which stretches between the Gila Mountains and the 

 Colorado River. 



Herbaceous plants are abundant in species, though few in individ- 

 uals. Therefore the region, although seeming rich in plant-forms to 

 the botanist, always strikes the traveler as barren from paucity of 

 vegetation. 



Thorny shrubs, ephedras, cacti, 3'uccas, agaves, dasylirions, and 

 nolinas are the vegetable forms that most appeal to the eye, as well 

 as to the sense of touch of the traveler, who also sees an abundance of 

 bare unoccupied soil, in which nothing grows except after occasional 

 rainy periods which may not recur for several years ; for these plants 

 form patches or plant colonies in which prickly mimosas, acacias, 

 koeberlinias, and many other scrubby bushes join forces with the 

 spine-armed cacti and uninviting agaves and yuccas in repelling 

 invasions and, incidentally, give shade, shelter, and stability to the 

 more tender grasses and herbaceous plants. It is fortunate that 

 these impenetrable mats of mixed plant growth are so disposed 

 in tufts, surrounded by bare spaces, because travel would be greatly 

 impeded if the chaparral, were continuous. 



On the lowest plains and vAlleys, usually corresponding to the beds 

 of ancient lakes or seas, the soil is apt to be sandy and vegetation 

 scarce. On approaching the foothills, shrubs, herbs, and grasses in- 

 crease in number and variety ; but the broad slopes surrounding the 

 higher mountains are sometimes covered with grasses, dasylirions, 

 and nolinas, without much shrubbery, until the ravines near the lower 

 timber line are reached. Rocky soil is much richer in plant life than 

 sand or clay, because it retains moisture longer, 

 30639— No. 56—07 m 3 



