40 The Plant World. 



latter has reached a height of several meters, the protruding 

 branches still covering it with an apparently healthy and normal 

 growth. 



All in all, in the success with which its relatively rapid dis- 

 semination is accomplished, in its ready adjustment to widely 

 varying conditions, its utilization of available water, and in other 

 ways, this plant is to be considered one of the best adapted of 

 our desert species, and its wide distribution and present success- 

 ful invasion of new areas mark it, not as a decadent species, but 

 as one in pristine vigor, from which apparently several others 

 are in process of evolution. 



Associated with the mesquite are a number of characteristic 

 species, some of which are confined to the flood-plain, while others 

 extend with it beyond these limits. Of the latter Acacia con- 

 stricta is a conspicuous representative. Its habits are essentially 

 those of the mesquite as to water requirements, and it closely 

 resembles this species in its xerophytic structure. The two 

 grow side by side near the river and to the summit of Tumamoc 

 Hill, in precisely the same situations, one being, apparently, 

 the ecological equivalent of the other. Other species behave 

 diff"erently. Acacia Greggii grows with the mesquite in its lower 

 range, but not on the hill above, and the same is true of Condalia 

 lycioides. Both of these exhibit distinctively xerophytic struc- 

 tures, but both are as yet adapted to a somewhat more restricted 

 range of soil conditions than are the mesquite and Acacia con- 

 stricta. Sambucus and Fraxinus velutina, also of this association, 

 are still more limited in their range, growing near irrigating 

 ditches, hardly affecting even the edge of the mesa^like slopes, 

 and structurally are to be thought of as essentially mesophytic. 



Passing through the list of plants belonging to this associa- 

 tion, it is seen that the same relations are maintained — -certain 

 species are strictly confined to the flood-plain, while others 

 occur rather widely beyond it; it is to be noted, however, that 

 even those that range most freely exhibit their best development 

 on the flood-plain and not on the hillsides. The mesquite, 

 which attains the size of a tree on the plain, is a mere shrub on 

 the hill, and Bigelowia Hartivegii, which grows with the luxuriant 

 habits of a weed in the former situation, is scattering and small 

 in the latter. 



