6 DESERT BOTANICAL LABORATORY 



of the White Sands. The bottoms occur at nearly the same level as 

 the surface of the plain surrounding the sands, the dunes are irregular 

 heaps and ridges of white gypsum sand, rising to a maximum height 

 estimated at 60 feet. When moist the sand is of a slightly yellowish 

 or buff color, when dry almost pure white. When taken in the hand 

 it has not the sparkling effect of silicious sand, but its grains are dull 

 even in sunlight. As compared with silicious sand it may be likened 

 to corn meal, the other to granulated sugar. The separate grains can 

 be rubbed between the fingers to a fine white powder. Except on 

 steep slopes the dunes of the White Sands form an excellent surface for 

 walking, comparable in hardness with a sandy seabeach wet by an 

 outgoing tide. These hard surfaces are covered everywhere with rip- 

 ple marks caused by the wind. 



The most characteristic plant of the dunes is the threeleaf 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 pro- 

 tected above from the rain by the close covering of the branches and 

 leaves, and the sand in the column itself bound together by the long 

 penetrating roots. An incrustation, apparently of gypsum, is often 

 found on dead roots. One of these columns was about 15 feet high 

 fi-om its base to the summit of the protecting bush and about 8 feet 

 in diameter at the base (Plate IV) . 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 char- 

 acteristic aroma of the crushed twigs. This, mixing with the sand, 

 forms hard honeycombed masses sometimes three inches in diameter. 



Other characteristic woody plants of the dunes are Atriplex canes- 

 cens^ two species of Chiysothamnus ^ and Yucca radiosa. The under- 

 ground trunks of the Atriplex often attain a diameter of four inches, 

 those of the Yucca six 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 fifteen 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 lime 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 



