194 Mineral Nutrition of Plants 



A diagram of the spatial relations of different regions in a root tip 

 of tobacco is reproduced from Esau {23) as Figure 2. In this figure, the 

 endodermis is indicated only at the levels where Casparian strips are 

 present. The endodermis, as a cell layer, however, becomes defined be- 

 fore the sieve tubes differentiate. This is clearly shown in the upper of 

 two photomicrographs of a root tip by the same author and reproduced 

 here, as Figure 3 (possibly unusual to a certain extent due to a virus in- 

 fection). 



Anatomical features of roots relative to migration and accumulation 

 of inorganic solute have been discussed in detail by Prevot and Steward 

 (6/). Their studies on distribution of accumulated inorganic solute will 

 be referred to later, but their anatomical diagram is reproduced at this 

 point (Figure 4). The importance of the cortical tissue is emphasized. 



MODES OF INORGANIC SOLUTE INFLUX 



Several modes of inorganic* solute entry into cells may be distin- 

 guished. These are of passive and active nature depending upon 

 whether metabolism of the organism is involved directly or indirectly 

 in the migration (//, 59, j6, 92). Four principal means of movement 

 are at present recognized. These include simple and Donnan diffusion, 

 exchange adsorption, and active or metabolic accumulation. These have 

 been schematically represented in Figures 5 and 6 (//, 92). Detailed 

 discussions of these mechanisms have been published elsewhere (2, 11, 

 42,53,54,76,92). 



Diffusion and Donnan equilibria 



The most obvious mode of migration is that by diffusion. Where- 

 ever a difference of concentration (or better, activity) of a solute exists 

 between two regions, there will be a tendency for movement toward 

 the lower values approaching equality of concentration. This mode of 

 flux across the differentially permeable cytoplasm may be very slow, 

 at least with cells of certain species. The net flux of sodium into 

 Halicystis ovalis is a case in point (Table I). Obviously, for inorganic 

 solutes, a continuous aqueous medium is essential. It was recognized 



*The movement of organic ions through protoplasm interposed between two 

 solutions needs detailed study. 



