34 BULLETIN 56, UNITED STATES NATIONAL MUSEUM. 



With the exception of the palo verde and other green-barked 

 trees and shrubs alhided to below, the desert vegetation appears dull 

 and dusty, and the plants often bear pulpy leaves and exude gums 

 and resins, ajDparently for the purpose of retarding evaporation and 

 preventing the loss of sap when wounded. The foliaceous leaves are 

 usually of small size, and the leaves and joints of the agaves, cacti, 

 and other tleshy plants are covered with waterproof dermal struc- 

 tures, which, if broken, cause the plants to wilt immediately. Other 

 plants in which the leaves are very minute are covered with green 

 bark, Avhich doubtless exercises the physiological function of leaves. 

 Among these bright green trees and shrubs are species of E plied ra^ 

 Holacantha. Canotia, Ktjdberlhiia^ Paroscla^ and Menodord. 



Aside from the trees, which are treated of in detail further on, 

 yuccas, agaves, nolinas, and dasylirions are the most character- 

 istic forms of coarse vegetation over large portions of the boundary 

 area. Five arborescent species of the yucca and one Xolhia are con- 

 sidered as trees. The others have no distinct caudex and cover the 

 ground with bristling tufts of green, which are grateful to the eye 

 and afford shelter to the birds, mammals, and lower forms of life, 

 though the bayonet-pointed yuccas, the spine-armed agaves, and the 

 serrate-leaved dasylirions are often a torment to man and beast in 

 traveling over such a country. Of the acaulescent yuccas, the narrow- 

 leaf yucca, or soap weed {Yucca glauca Nuttall), covers large por- 

 tions of the eastern plains, and the plateau yucca, or Spanish 

 dagger {Yucca haccata Torrey), occupies the broken country, its 

 range extending from southwestern Texas and southern New Mexico 

 across the Colorado Plateau, while on the Boundary the range of the 

 splendid Whipple yucca is confined to the mountains of the Pacific 

 Coast Eange. All of these yuccas bear panicles of lily-like whitish 

 flowers. 



The nolinas, commonly called " bear grass," grow with the beau- 

 tiful dasylirions, on the slopes leading up to the mountains. The 

 range of the saw-tooth dasylirion {Dasylirion wheeleri Watson) 

 does not extend westward beyond the Elevated Central Tract. The 

 Texas nolina {Nolina texana Watson), which ranges west to the San 

 Luis Mountains, and the single-stem species {N. Undheimeri) , which 

 extends as far as the Western Desert, have a very short but thick 

 caudex or trunk, which is concealed by the numerous narrowly linear, 

 serrulate leaves. The former has numerous flower stems, which are 

 shorter than the leaves, while the latter has a single stout stem from 

 four to eight feet in height. 



Agaves or century plants do not grow on the desert plains, 

 fhough one species {Agave palmeri Engelmann) appears as soon as 

 the foothills are reached, while another {Agave applantn parryi) 

 was found in the greatest abundance on the summit of Hachita 



