DESERT REGIONS OF NORTH AMERICA. I3 
The surface of the dunes is sparkling white, due to the dry condition 
of the gypsum powder, but a few inches beneath it is of a yellowish or 
buff color and is distinctly moist and cool to the touch, even when the air 
is extremely hot. The smallest particles may be crumbled in the fingers, 
and as a consequence the dunes are solidly packed except on newly 
forming steep slopes (plate 1). 
The most characteristic plant of the dunes is the three-leaf sumac 
(Rhus trilobata), which occurs in the form of single hemispherical bushes 
4 to 8 feet high, the lower branches hugging the sand. The plant grows 
vigorously, the trunk at or beneath the surface often reaching a diameter 
of 3 inches. The binding and protecting effect of this bush is often 
shown in a striking manner when in the cutting down of an older dune by 
the wind a column of sand may be left protected above from the sun by 
the close covering of the branches and leaves, and the sand in the col- 
umn itself bound together by the long penetrating roots. An incrusta- 
tion, apparently of gypsum, is often found on dead roots. One of these 
columns was about 15 feet high from its base to the summit of the 
protecting bush and about 8 feet in diameter at the base (plate 2). A 
curious fact brought out in the denudation of the underground trunks 
of this plant by the shifting of the dunes is the abundant exudation of 
a pale amber gum with the characteristic aroma of the crushed twigs. 
This, mixing with the sand, forms hard, honeycombed masses sometimes 
3 inches in diameter. 
Other characteristic woody plants of the dunes are Atriplex canescens, 
two species of Chrysothamnus, and Yucca radiosa. The underground 
trunks of the Atriplex often attain a diameter of 4 inches, those of the 
Yucca 6 inches. A marked peculiarity of the White Sands is that a 
cottonwood is occasionally found in the lower dunes, reaching a foot in 
diameter, but seldom more than 15 feet in height; yet at the same time 
not a mesquite was seen. The mesquite is a tree requiring less moisture 
than the cottonwood. Apparently the presence of an excess of gypsum is 
prejudicial to the growth of the mesquite. 
The bottoms among the dunes have a dense vegetation as compared 
with that of the dunes themselves. It is characterized especially by the 
presence of a grama grass (Bouteloua), forming almost a turf, and by fre- 
quent clumps of Ephedra of a grayish purple color at this season and with 
3-scaled nodes (plate3). These bottoms usually show no signof moisture, 
but in two places we found water-holes, the water so alkaline that the 
horses would not drink it at the endof their first day’sdrive. About both 
holes occurred the salt-grass (Distichlis spicata) and wire-grass (Juncus 
balticus), both of them characteristic of moist alkaline soils (plate 4). 
The relation of Yucca radiosa to the sand dunes is unusually inter- 
esting. A group of four small yucca shoots standing about 3 feet high 
to the tip of the highest leaf was found upon the summit ridge of a 30- 
