BOTANICAL LABORATORY IN THE DESERT. 337 



the peculiar massing of branches, nor are their ultimate articles so 

 readily detached. Perhaps more interesting than all these is the little 

 half-vim- Opuntia leptocaulis, with stems, of scarcely the thickness of a 

 lead pencil, interweaving among the branches of some companion plant, 

 usually the creosote bush. It is never to my knowledge found grow- 

 ing except in the immediate proximity of another plant, and thus often 

 escapes detection. It is not very well supplied with spines, and might 

 ■asily be destroyed by trampling if away from the company of a plant 

 of sturdier growth. Perhaps this is the reason why it usually is so 

 found — it is stamped out elsewhere. Or possibly it needs the partial 

 shade afforded it. This is a question worth an answer. 



If you travel for a distance in any direction, other plants will be 

 found. Among these are sweet-smelling acacias, ungraciously, but, I 

 fear, not undeservedly, called cats-claws, with their finely divided 

 leaves and small yellow pompons of flowers. The low leafless shrub 

 Ephedra, with its vertical green stems which look like the scouring 

 rush and is as rough and hard to the touch, is another — a typical desert 

 plant if ever there was one. This plant is a relative of our yew, but 

 is possessed of very unique characters, the description of which would 

 take us too far into details. 



Here and there a 'salt' or 'alkali' spot is to be found. Here grow 

 IV w enough plants, and these such as can endure the hardships of a 

 most unfavorable soil, as for example Atriplex and Dondia. Near the 

 water courses, from which for the greater part of the year water is 

 conspicuously absent, one finds, on the other hand, larger shrubs and 

 very small trees of Acacia, mesquite (Prosopis velutina) and a species 

 of palo verde (Parhinsonia) . Ten miles south of Tucson, near to the 

 mission of San Xavier del Bac, the river bottom is occupied by a veri- 

 table forest composed wholly of large mesquite, and this the Papago 

 Indians of the region draw upon chiefly for wood. I should mention 

 the presence in parts of the mesa of the many low shrubs which are 

 noticeable chiefly for their inhospitable thorniness. The palo christi 

 (Koeberlinia) is an extreme type. For a crown of thorns no better 

 material could be imagined than this. 



On approaching the rocky slopes leading to the higher elevations, 

 a different vegetation is met — different in species, but not in general 

 character. At closer range a distinctively green note in the coloring 

 is appreciated, which at a greater distance was a uniform brown. It 

 is the palo verde (ParJcinsonia microphylla) , so called in the Spanish 

 on account of the uniform green color of all its members, which gives 

 the impression of verdure. This is a small, somewhat gnarled tree, re- 

 lated to our locust, of the general appearance of a trimmed orchard tree. 

 It is usually leafless, although the younger shoots are sometimes sup- 



