122 MINERAL SALTS ABSORPTION IN PLANTS 



over sodium when both ions are supplied in the external medium 

 (Sutdiffe, 1957). 



D. Upward Transport of Salts 



In the discussion so far it has been assumed that inorganic 

 solutes transported, by whatever mechanism, into the stele, are 

 carried upwards in the transpiration stream. Evidence supporting 

 this contention may be summarized as follows: 



(i). Tracheal sap has been analysed and shown to contain 

 appreciable amounts of salt. Since the sap moves upwards, it is 

 inevitable that salts are carried along with it into the shoot. 



(ii). Ringing experiments have shown that when a section of bark 

 is removed there is little or no direct interference with movement of 

 solutes into the shoot. Removal of a section of xylem severely 

 limits or entirely prevents longitudinal transport (Clements and 

 Engard, 1938; Phillis and Mason, 1940). 



(iii). An objection can be raised against ringing experiments that 

 the tissues remaining may behave abnormally after the operation. 

 However, with the aid of radioactive isotopes, the upward transport 

 of mineral salts in xylem can be demonstrated without disturbing 

 the longitudinal continuity of any tissue. Stout and Hoagland (1939) 

 separated lengths of bark from the wood in willow and geranium 

 plants by inserting pieces of parchment through longitudinal 

 incisions in the bark. After feeding salts labelled with radioactive 

 isotopes to the roots, radioactivity was detected soon afterwards 

 in the wood of the treated region, but not in the phloem. In places 

 where bark and wood remained in physical contact, rapid transfer of 

 isotope from one tissue to the other occurred. 



(iv). If longitudinal transport takes place in the transpiration 

 stream, the rate at which salts are transferred from roots to leaves 

 should be related to the intensity of transpiration. In fact, under 

 conditions favouring rapid transpiration, salts move much more 

 quickly through the stems of tomato plants, than when trans- 

 piration was low. On a bright sunny day, radioactivity was detected 

 in the tip of a tomato plant over 6 ft tall, 40 min after labelled 

 phosphate had been supplied to the roots (Arnon et al. 1940), 

 whereas under less favourable conditions such movement may 

 require several hours. 



