lO THE PLANT WORLD. 



ON THE ELECTRIC RESISTANCE OF SOLUTIONS 



OF SALT PLANTS AND SOLUTIONS OF 



ALKALI SOILS. 



By Dr. W. A. Cannon. 



A salt spot of considerable extent lies In the bottoms of 

 the Santa Cruz river along the old Fort Yuma road, near the 

 city of Tucson. The spot is rather sharply set off from the 

 surrounding bottom land by the character of its soil and by 

 that of its vegetation although the physiography of the two 

 are apparently not unlike. The natural vegetal covering of 

 the non-alkali lands in the vicinity of the salt spot is made 

 up very largely of such trees and shrubs as Prosopis velutina, 

 Acacia Greggii, Bigelovia Hartwegii, Koeberlinia spinosa, 

 Suae da siiffrutescens, Zizyphus lycioides and A triplex canes- 

 cens. Among the plants which are typical of the salt spot 

 itself are Atriplex canescens, A. polycarpa, A. elegans, A. 

 Niittallii and Suaeda siiffrutescens. In addition to these there 

 are along the washes which traverse the spot Bigelovia and 

 Prosopis. Generally speaking the plants which are character- 

 istic of the salt spot have a well marked zonal arrangement. 

 As one enters the "alkali" area he encounters A. canescens 

 with which are to be found also Prosopis and Bigelovia. 

 This zone is referred to in this paper as the canescens zone. 

 As one passes through this zone on his way to the center of 

 the spot he encounters A. polycarpa with which are species 

 of Lycium and Suaeda also, but non-salt plants as Prosopis 

 are confined to the washes. This is the polycarpa zone. 

 Still nearer the center of the area A. Nuttallii is met and 

 with it A. elegans also occurs and dwarfed specimens of 

 Suaeda. At the very center of the salt spot there are no 

 plants. 



The soil of the river bottoms is mainly adobe; that of 

 the salt spot is a fine sandy loam. Analyses of soil by the 

 Bureau of Soils, U. S. Department of Agriculture, taken 

 from the upper foot at the center of the spot, are as follows : 

 Mechanical analysis, — clay, 33.3 per cent.; silt, 21.5 per 



