PLANT ASSOCIATIONS AND HABITATS. 13 
(4) SALT=SPOTS; ASSOCIATION OF SALT=BUSHES. 
The description already given holds in all its general features for the 
soil and vegetation of the flood-plain of the Santa Cruz River, but cer- 
tain areas, some of which approach the neighborhood of Tumamoc Hill, 
are so exceptional in character as to call for a special account. 
The so-called salt-spots include areas varying from a few rods to many 
square miles in extent (plate 5), and are widely distributed throughout 
the arid and semi-arid regions west of the Rocky Mountains. As they 
occur in the Santa Cruz Valley, and also in the valleys of the Gila and 
Salt Rivers in Arizona, they are areas of low ground, insufficiently drained, 
in which have accumulated alkali salts, especially salts of sodium, and 
which are conspicuously marked both by the presence of various species 
of Chenopodiaceze (‘‘salt-bushes’’), and also by the absence of many 
species characteristic of surrounding areas. 
In the lowest part of such spots, where the drainage is most defective, 
Sueda moquini is characteristic, and in some cases is well-nigh exclusive 
in its occupation of the soil. Commonly, however, there are associated 
with it a few other plants, especially one or more species of Atriplex. 
Thus, Alriplex lentiformis grows luxuriantly, even where the so-called 
black alkali, chiefly sodium carbonate, forms a heavy crust, and both 
Atriplex nuttalia and A. elegans are of frequent occurrence in low spots 
where the accumulation of alkali has reached a high percentage. 
Beyond this association, and outside of it, Atriplex canescens, in many 
cases, is conspicuously present. Its range, however, extends over the 
flood-plain and even beyond it, so that although closely related geneti- 
cally, it can not be referred to the more restricted association of salt- 
bushes which have the salt-spots as their habitat. Thus a zonal arrange- 
ment, often well marked, is produced, in which the center may be quite 
bare of plants, while around it, in successive concentric zones, are (1) 
Sueda moquint and Atriplex nuttallit, (2) Atriplex polycarpa, (3) Atriplex 
canescens and various other species which belong around or outside the 
limits of the salt-spot proper. 
It has been shown by Cannon (1908) that at the salt-spot on the flood- 
plain of the Santa Cruz, just north of Tucson, where there are three well- 
marked zones of vegetation, alkali salts are most abundant in the inner, 
_least abundant in the outer, and intermediate in amount in the inter- 
mediate zones. 
In view of their persistency in such habitats, and in a definite order 
corresponding to amounts of alkali, it seems well-nigh self-evident that 
Sueda and several species of Atriplex must be regarded as true halophytes 
or “‘salt-loving’”’ forms; that is, as having become specially adapted to 
soils containing a large percentage of alkali salts. It is true that in the 
vicinity of irrigating ditches the species characteristic of salt-spots, and 
