Plant Associations at the Desert Laboratory. 5T 



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 Riverj in Arizona, they are 

 areas of low ground, insufficiently drained, in which have ac- 

 cumulated alkali salts, especially salts of sodium, and which are 

 conspicuously marked both by the presence of various species 

 of Chenopodiaceae ("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, Suoeda 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 A triplex. Thus Airiplex lentiformis grows 

 luxuriantly, even where the so-called black alkali, chiefly sodium 

 carbonate, forms a heavy crust, and both Atriplex Nuttallii and 

 A. elegans are of frequent occurrence in low spots where the ac- 

 cumulation 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 genetically, it can not be referred to the more 

 restricted association of salt-bushes which have the salt-spots 

 as their habitat. Thus a zonal arrangemert, often well marked, 

 is produced, in which the center may be quite bare of plants, 

 while around it, in successive concentric zones, are (1) Suoeda 

 Moquini and Atriplex Nuttallii, (2) Airiplex 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 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 inter- 

 mediate in amount in the intermediate zones. 



In view of their persistency in such habitats, and in a de- 

 finite order corresponding to amounts of alkali, it seems well- 

 nigh self-evident that Suoeda and several species of Atriplex must 

 be regarded as true halophytes or "salt-loving" forms; that is, 

 as having become especially adapted to soils containing a large 

 percentage of alkali salts. It is true that in the vicinity of irri- 



