SALT RELATIONS OF VASCULAR PLANTS 119 



the xylem. Salts do not usually pass through the central vacuoles of 

 cortical cells on their way into the stele. The vacuoles seem rather 

 to compete for available salts in the cytoplasm, removing a part of 

 that which would otherwise move into the xylem. Salts accumulated 

 in cortical vacuoles are not however irretrievably sealed off from 

 the rest of the plant, and under conditions of salt starvation they may 

 subsequently be released again and transferred elsewhere (Steward 

 et al. 1942). As may be expected, roots of intact plants which have 

 a low salt content, retain a higher proportion of the salt absorbed by 

 the plant, than do roots whose salt content is high (Broyer, 1950). 

 It has been shown in barley plants that as the external concentration 

 of phosphate is decreased, a smaller proportion of that which is 

 absorbed reaches the shoot (Russell and Martin, 1953). 



Various hypotheses have been proposed to account for the active 

 transport of salts from the medium into the non-living xylem 

 elements. Crafts and Broyer (1938) suggested that salts absorbed 

 actively into the cytoplasm of surface cells, move passively through 

 the symplast (see p. 108) along a concentration gradient. The 

 cytoplasm of living cells in the stele adjoining the conducting 

 elements is perhaps incapable of retaining salts to the same extent 

 as can those situated near the root surface, because of the relatively 

 anaerobic conditions existing in centre of the root, and salts are thus 

 released. Somewhat similar suggestions have been made by 

 Wiersum (1948) and by Lundegardh (1954). There is no evidence 

 for the postulated gradient, nor has it been possible to demonstrate 

 that there is in fact a deficiency of oxygen in the stele of roots. 



Another possibility is that ions are bound to cytoplasmic 

 constituents or accumulated in submicroscopic vesicles, and actively 

 transported by protoplasmic streaming through the symplast to an 

 inner protoplasmic boundary across which they are released, either 

 actively or passively. On the basis of this idea, a series of analogies 

 can be drawn between the salt relations of a single vacuolated cell 

 and the multicellular root (Table 11). In both systems, the 

 mechanisms of movement of ions into the cytoplasm, through it, 

 and out into either vacuole or xylem sap, are possibly the same. 

 Elucidation of the processes involved in the simpler system repre- 

 sented by the parenchyma cells should lead rapidly to greater 

 understanding of the situation in the root. 



Analyses of xylem sap have shown that salts can be accumulated 



