desert are found not only many plants but large numbers of trees, which are in many places so close together as to form thickets and miniature forests. (See Pls. VI, A, and XXI, A.) In contrast to the treeless plains of New Mexico and southeastern Arizona, this is an arboreal desert, where large shrubs and trees give the country a deceiving verdure that is in marked contrast to its ability to support animal and human life. To the seasonal concentration of the rainfall with accompanying high temperature is due the exceptional character of this flora. The predominant plants-creosote bush, palo verde, and the great cacti— range southward into Mexico, and hence this type of vegetation is called the Sonoran or Sonora-Arizona type. THE TREES The common desert trees (five in number) are mesquite, palo verde, palo fierro, catsclaw or acacia, and ocotillo. Mesquite is by far the most abundant and has a wide range outside of the area. The mesquite is a thorny legume, resembling the eastern honey locust. The flowers are greenish yellow, and the seeds are borne in pods 3 to 4 inches long. Near Tucson the common species is Prosopis velutina; on the drier alluvial slopes and the rocky slopes of the foothills this is a small and inconspicuous shrub, but on the flood plain of Santa Cruz River it grows into a tree 20 to 40 feet high and 6 to 14 inches in diameter. The large size of these trees seems to be associated with shallow ground water, to which the roots penetrate to obtain moisture. Along Gila River mesquite is found in great thickets or in more or less regularly spaced clumps of large trees. Many limbs and trunks are almost prone, and this sprawling habit is in marked contrast to the character of the mesquite in other localities. Near Casa Grande Ruins, where the depth to ground water is between 30 and 40 feet, the plain is covered with a forest of mesquite composed of clumps similar to the one shown in Plate XXIII, A. In sandy areas on the flood plain or the terraces south of Gila River mesquite is usually found capping and protecting the sand dunes. The most remarkable forests of mesquite, however, occur in the interior valleys of the Papago country, where floods spread out in thin sheets over the so-called adobe flats. The mesquite are found here as lines or strings of trees along the channels or spaced at intervals so as to look like orchards. The trees are from 4 to 12 inches in diameter and from 15 to 40 feet high and have an upright habit. Apparently they derive moisture largely from flood water, and lines of dying and dead mesquite are common features along abandoned flood-water channels. (See Pl. VIII, A.) The wood rots slowly, and dead mesquite is the common source of firewood for the camper. It is excellent fuel, though it has a disagreeable odor when burning-by no means, how |