BOTAXICAL LABORATORY IN THE DESERT. 335 



while still of a distinctly desert character. Since it is the chief ob- 

 ject 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 (Cerens 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. 



Ephedra. 



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 



