240 Mineral Nutrition of Plants 



similar reasoning to that suggested for distribution in single cells may 

 be invoked. Other evidence on the transverse pattern of salt accumula- 

 tion among the various cells of the root is wanting. Anatomical con- 

 siderations, along with postulated oxidative levels, suggest accumula- 

 tion gradients of decreasing concentration progressively inward from 

 the external surface. Since all cells of the cortex may be bathed by fluid 

 similar to the external medium of roots (62), the radial gradient across 

 this tissue may be small. Consideration of a controlling influence by 

 the endodermis on rate of translocation and oxidative levels of the 



40 



RueiOlUM 



CONC 



MCM EQUIYS 



Pt* lOOO c«s 

 facsh wcicmt 



30 



—A— *0 MBS. 

 — O— ?S MBS 



20 



7 3 



distance in cms tpom aptx 



Figure 24. Distribution of rubidum in excised barley roots showing the 

 effect of time. Culture medium 0.005M RbBr. — Steward et al. (79) 



