ILLUSTRATIONS. 



PLATE. FACING PAGE. 



I. Giant cactus {Cereus giganteus) near Tucson, Arizona (Frontispiece) 



II. Tree yucca ( Yucca radiosa) in the Tularosa desert, New Mexico . 5 



III. The White Sands, Tularosa desert 5 



IV. Sand column in the White Sands ... 6 



V. In the White Sands, Tularosa desert, looking northwest . . 7 



VI. Water hole in the White Sands 7 



VII. Yucca radiosa growing up through a dune thirty feet high ... 7 



VIII. Advancing eastern margin of the White Sands . . 7 



IX. Winter vegetation of desert plain at Tucson, Arizona 12 



X. Cat's claw tree {Acacia gfeggii) n&ax Tucson 12 



XI. Tucson mountain, near Tucson ... 14 



XII. Vegetation on Tucson mountain, near Tucson 14 



XIII. Looking westward from site of Laboratory 14 



XIV. Yucca-like vegetation at Nogales, Arizona 17 



XV. Vegetation of the Sonera desert near Torres, Mexico 17 



XVI. Guarequi (/^e;'Z'///ea 5o«orae) under bushes, near Torres .... 18 



XVII. Tree. ocotiWo {Fouquieria macdougalii) near Torres 18 



XVIII. Papago Indian drinking from a cactus {Echinocactus emoryi) . . 19 



XIX. Yaki Indian family and house, west of Torres 19 



XX. Ollas at a Yaqui Indian house west of Torres 19 



XXI. Desert island, Guaymas bay, Mexico ... 20 



XXII. Cereus pringlei on above island 20 



XXIII. Salt valley, Colorado desert, California 21 



XXIV. Salt encrusted ground near Salton, California ... 21 



XXV. Group of palms, Colorado desert, California . . 21 



XXVI. Belt of palms, Colorado desert, California 21 



XXVII. Vegetation in the Grand Canyon, Arizona 23 



XXVIII. Bright Angel trail. Grand Canyon, Arizona , .23 



XXIX. Alpine desert on summit of San Francisco inountain, Arizona . . 43 



PAGE. 



Fig. I. Rainfall map of arid regions . . . . 28 



Fig. 2. Copy of map of American deserts, 1835 . . 32 



Fig- 3- Copy of map of American deserts, 1859 35 



Fig. 4. Meteorological data from San Francisco mountain, Arizona ... 44 



