SALT RELATIONS OF VASCULAR PLANTS 



127 



The ability of growing organs to accumulate salts at the expense of 

 non-growing tissues is a well-established phenomenon, but the 

 reason why nutrients are diverted in tliis way is far from understood. 

 Presumably, in the growing tissue, salts are accumulated in vacuoles 

 or utilized in the synthesis of cell constituents to such an extent that 



a. 



E 



c 

 o 

 o 



o 



E 



c 



o 



Fig. 43. The pattern of distribution and redistribution of phosphorus in oat 

 plants with varying phosphorus supply, a. low phosphorus supply; b. 

 medium phosphorus supply; c. high phosphorus supply (redrawn from 



Williams, 1948). 



the concentration of free ions in the cytoplasm is reduced, and this 

 stimulates salt movement towards the "sink" from the ends of the 

 sieve tubes through the cytoplasm of surrounding cells. The 

 mechanism may be fundamentally similar to that involved in the 

 polarized movement of salt across the root cortex towards the stele 

 (see pp. 117f). 



Calcium ions are not transported in the phloem, and it is for 

 this reason that the calcium content of leaves does not decrease 

 before leaf fall, as is the case with other ions. The calcium content of 

 many fruits is relatively low, because of a dependence on supply via 



