Mat, 1893.] YUCCAS OF THE DEATH. VALLEY EXPEDITION. 6<)6 



Pahroc Mountains. — Tolerably common near Paliroc Spring. 



Highland Range. — Occurs sparingly on the west slope. 



Juniper Mountains. — Found sparingly on the west slope of the Ju- 

 niper Mountains between Panaca and Sheep Spring, at an altitude of 

 1,830 to 1,970 meters (6,000 to 6,500 feet). Here it was only in bud May 

 19, though it was in flower on the Beaverdam Mountains and on the 

 south slope of Pine Valley Mountain, Utah, a week or ten days earlier. 



UTAH. 



Santa Clara Valley. — In the Upper Santa Clara Valley, north of St. 

 George, this handsome species occurs in a belt a few miles wide, begin- 

 ning at about 1,150 meters (3,800 feet) and reaching up to 1,460 meters 

 (4,800 feet). 



Beaverdam Mountains. — Common, ranging down to 1,080 meters (3,600 

 feet) on the east slope, and 1,030 meters (3,400 feet) on the west slope. 

 It was beginning to flower May 10, though some plants were only in 

 bud at that time. 



Yucca arboresceus \_=¥ucca brevifolia]. (Plate xiii and frontispiece.) 



Among the many unusual and peculiar modifications of plant life of 

 the desert regions of the southwestern United States, none is more re- 

 markable or striking than the tree yucca (Yucea arborescens). 



Tree yuccas form open forests or groves, usually of small size, but 

 sometimes 15 or 20 miles or more in length, according to the extent of 

 the area suitable to their requirements. The individual trees are well 

 spaced and vary from 6 to 9 meters (20 to 30 feet) in height. They 

 branch in a very peculiar mauner and are abundantly clothed with 

 stiff, spiny leaves set so near together that their bases are in actual 

 contact. As the tree grows the leaves die from below upward, and the 

 dead ones at first point outward at right angles to the trunk, and then 

 downward, their points surrounding the branch or trunk like a belt of 

 bayonets, effectually preventing most animals from climbing up from 

 below. The dead leaves fall off after a year or two, so that the trunks 

 and lower parts of the main branches finally become bare. 



Tree yuccas are abundant about the borders of the Mohave Desert 

 and on many of the included ' lost ranges,' and also in places of suit- 

 able elevation throughout the deserts of southeastern California, south- 

 ern Nevada, western Arizona, and the extreme southwestern corner of 

 Utah. They do not grow in the bottoms of the arid basins, or upon 

 the steep declivities of the mountains, but thrive best on the higher 

 gravel slopes that skirt the deserts and upon the basal slopes of the 

 included desert ranges, always in a definite zone or belt the extreme 

 vertical width of which rarely exceeds 450 meters (1,500 feet), and 

 usually is much less. The altitude, of this belt varies with the base 

 level, but invariably marks the upper limit of the Lower Sonorau 

 zone. 



Looking north w T ard over the Mohave Desert from the summit of Cajon 

 12731— No, 7 23 



