SOME RELATIONS BETWEEN SALT PLANTS AND SALT-SPOTS. 

 By William Austin Cannon, Desert Laboratory, Tucson, Arizona. 



ONE of the characteristic environmental factors which desert plants 

 must successfully meet is the high salt content of the soils. This is 

 especially true of such areas as have poor surface drainage and where 

 water is removed only by evaporation, leaving the salts behind and forming 

 the highly saline areas commonly known as salt-spots. In the salt-spots the 

 most important salts are those of sodium. From these conditions it appears 

 that such perennials as live in areas where the salt content of the substratum 

 is relatively high must not only be able to extract water from a relatively 

 highly concentrated soil solution, but must be able to endure the salts of what- 

 ever kind as such. In addition to these conditions the salt plants live in a 

 climate otherwise very arid. The physiological activities of halophytes are 

 accordingly of great interest in that they appear to include an especially high 

 osmotic efficiency as well as immunity to the salts of sodium. Their diver- 

 gence in these regards from many desert plants, and especially from meso- 

 ph)rtes, is thus very great. 



VEGETATION OF A SALT-SPOT NEAR TUCSON. 



A salt-spot of considerable extent lies on the edge of the flood-plain of 

 the Santa Cruz river along the old Fort Yuma road about four miles north- 

 west of the city of Tucson. The area is more or less sharply set off from the 

 surrounding bottom land by the surface depression, but especially by the 

 high salt content of the soil and by the halophytic character of its perennial 

 plant covering. The vegetation of the non-salt lands adjoining the salt-spot 

 is made largely of Prosopis velutina (the mesquite), two or three Acacias, 

 Atriplex canescens, Bigelovia hartwegii, Koerberlinia spinosa, Suceda suffru- 

 tescens and Zizyphus lycioides. Of these plants Prosopis and Bigelovia occur 

 also along the washes which traverse the salt-spot. Among the plants which 

 are typical of the salt-spot proper are at least four species of Atriplex, namely, 

 canescens, elegans, nuttallii and polycarpa. In addition to these there may 

 be found species of Lycium and small specimens of Suceda (Dondia). 



Generally speaking, the salt plants have a well-marked zonal distribu- 

 tion. As one enters the spot he encounters Atriplex canescens and no other 

 salt bush. But associated with A. canescens are small specimens of Prosopis 

 and Bigelovia in abundance. This zone is referred to in this paper as the 

 canescens zone. As one passes through this zone on his way to the more cen- 



