40 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 
Guterrezia euthamie is also frequent. ‘The replacement of Prosopis by 
Larrea and of Atriplex nuttallit by Atriplex polycarpa, the coming in of 
cacti and the appearance of Yucca and Gutverrezia, are some of the most 
distinctive marks of the vegetation of the Maricopa sand as here observed. 
(6) The Maricopa gravelly loam, forming the slopes and terraces above 
the Maricopa sand, is distinctively the home of the creosote-bush and a 
larger number of cacti. With these were found at Pima and near Solo- 
monsville Kerberlinia spinosa, Yucca elata, Acacia constricta, and some 
others, and on the flat terraces Fouquierta splendens. The habits of 
Guherrezia euthanuie, which also occurs here, are the reverse of those of 
Fouquieria as to local choice of habitat. It is hardly seen except on the 
slopes, which at Solomonsville were yellow with it for miles, while on 
reaching the top of the terrace hardly a specimen could be found. 
Some of the species that have been named are far more restricted as 
regards limitation to a special habitat than others are. As examples of 
those more closely restricted in their choice may be named Atriplex 
polycarpa, which belongs definitely to the Maricopa sand and is scarcely 
found beyond it, and Atriplex nuttall, which grows altogether upon the 
Maricopa silt loam, or at most extends into the Maricopa sandy loam. 
The creosote-bush is limited, almost strictly, to the Maricopa gravelly 
loam and the Maricopa sand, ocotillo to the Maricopa gravelly loam, 
and the various species of cacti to this and to the Maricopa sand. 
The mesquite, on the other hand, although attaining its best develop- 
ment on the Maricopa loam, ranges from the Gila fine sandy loam to the 
gravelly loam of the mountain slopes, and Aérzplex canescens has an even 
wider range, since it occurs also on the Pecos sand. A number of species 
grow on any soil whatever, from the shifting river-sand to the heaviest 
silt loam, apparently with almost no limitations, even as regards the 
amount of alkali in the soil. Arrow-weed (Pluchea sericea) is one of 
these, and cocklebur and some other introduced species exhibit the same 
indifference to physical and chemical peculiarities of the soil. 
The details that have been given point to certain important conclusions: 
(1) It is evident, in the first place, that there is a remarkable cortre- 
spondence between topographic features as the result of physiographic 
processes and the local distribution of plants in the Gila Valley. The 
tiver-banks, the flood-plain, the long slopes approaching the mountains, 
and the steeper slopes and terraces above are severally the habitats of 
different and well-marked associations of plants. Here, as well as in 
the eastern United States, it may be said that physiography furnishes 
an efficient basis for an ecological classification of the vegetation. (Com- 
pare Jennings, 1908.) ? 
(2) But such studies as have been conducted here make it plain that 
soil conditions, differing greatly in the different physiographic areas, are 
the real determining factors governing the local distribution of plants. 
