120 MINERAL SALTS ABSORPTION IN PLANTS 



in the conducting elements against existing concentration gradients, 

 in the same way as in vacuoles (Table 12). Efficient active accumu- 

 lation in the stele necessitates the existence of a permeability barrier 

 in the root corresponding to the tonoplast in a cell. This barrier 

 may be located, as Priestley (1920) suggested, at the endodermis, 

 where the presence of the Casparian bands prevents leakage of 

 aqueous solutions inwards or outwards along radial and transverse 

 walls. If this is the case, the endodermis may function as a secretory 

 layer comparable to those encountered in animals (e.g. amphibian 

 skin, and gut epithelium). After being actively transported across 

 the endodermis, ions might leak passively through the walls of any 



Table 11. Possible Analogies between Parts of a Vacuolated Cell and 



Multi-cellular Root. 

 (From: SutcliflFe, 1959). 



intervening cells into the cavities of the xylem elements. Alter- 

 natively, the secretory mechanism may be located at the surface of 

 living cells adjoining the xylem vessels, and the closely packed cells 

 of the stele may present a sufficient resistance to allow development 

 of a concentration gradient between xylem sap and medium. This 

 situation needs further investigation. The establishment of a 

 concentration gradient between the xylem sap and external medium 

 is responsible for movement of water by osmosis, across the root, 

 leading to the development of root pressure. As may be expected, 

 root pressure diminishes when salts are withheld from the medium, 

 or when the roots are placed under conditions of reduced meta- 

 bolism. The existence of root pressure testifies to the importance of 

 active processes in the transport of salts into the stele. 



Active transport of salt into the xylem sap has another feature 

 in common with vacuolar accumulation, namely the selectivity 



