while still of a distinctly desert character. Since it is the chief object of the laboratory to study 'drought-resistant vegetation' it would have been absurd to put the laboratory in an out-and-out desert, and but little better to have selected a semi-arid region with a rich flora. Nor would it have been foresighted to have chosen a locality which might sooner or later be threatened by irrigation. The conditions above stated may, of course, be met in many places, but scarcely better than on the hills west of Tucson, and on the adjacent slope and mesa. The general character of the vegetation here is in the main similar to that of the mesa and rocky ridge of the whole territory between Texas and western Arizona, but is, also, within the limits of distribution of the saguaro or giant cactus (Cereus giganteus). It is, therefore, representative in this important respect, of a very wide stretch of country which is of an undoubtedly arid character, the plants of which are, with the supply of water derived from a meager rainfall and a little snow, able through long periods of drought to sustain their powers of growth unimpaired. A more immediate view of a few of the more striking of these desert plants will here be of interest.
The vegetation of the country about Tucson is very naturally and obviously divided into two formations which occupy, the one the level mesa, the other the rocky hills and slopes. We may consider first the mesa, where the most prominent element is the greasewood, or creosote bush (Covillea tridentata), a plant of rather singular aspect. The average plant in point of size stands two and a half to three feet in height. The numerous branches, which arise together from the soil, spread radially at an angle with its surface, and bear in their upper parts a meager foliage which, in times of drought, is olive-brown in color. The leaves, the form of which is peculiar to the family
