SALT RELATIONS OF VASCULAR PLANTS 



125 



E. Distribution of Salts in the Shoot 



As the transpiration stream ascends the stem, ions are absorbed 

 from it by the surrounding tissues, notably by the cambium, and the 

 concentration of solutes is reduced. For this reason, leaves which are 

 inserted higher on the plant receive through the xylem less salt for a 

 given amount of water absorbed than do those located lower down. 

 In spite of this, the salt content of the former tends to be greater 

 than that of the latter; Biddulph (1951) demonstrated an approx- 

 imately linear relationship between the logarithm of the con- 



200 



100 



Flowering 



Ripe fruit 



Q. 

 D 

 V> 



C 

 Q> 



E 

 a> 



V 



oi 

 5*. 



621 3 2912 26 I73II428II25I0 31 21 12 9 7 ^ 4 _ 



Aug. Sept. Oct. fJov. Dec. Jon. Feb. Mor. Apr. Moy June July Aug. 



Fig. 41. Composition of xylem sap of an apple tree showing seasonal 



variation in New Zealand O = potassium x x = nitrogen; 



• — #=phosphorus (redrawn from Bollard, 1953). 



centration of phosphate in leaves, and their position on the stem of 

 Phaseolus vulgaris (Fig. 42). Young leaves evidently receive an 

 additional supply of salts, and this comes from older leaves via the 

 phloem. Steward (see Steward and Millar, 1954) has shown that 

 leaves in the same orthostichy tend to function as a nutritional unit, 

 and this is related to the presence of vascular connections between 

 them. The bulk of the salt accumulating in storage organs, such as 

 fruits and tubers, comes indirectly from the leaves via the phloem 

 rather than directly in the transpiration stream. Export of mineral 

 salts from mature leaves can easily be demonstrated with radio 

 active tracers (see below), but even without such techniques it is 



