80 TRANSLOCATION IN PLANTS 



Often under natural conditions these elements, nitrogen 

 and potassium, are deficient in the soil. Under such con- 

 ditions one might expect most of their transport to take 

 place through the phloem. Even ions, such as Ca++ or 

 SO 4 — , that seem not to be concentrated in living cells, 

 would probably not be carried in the transpiration stream 

 because protoplasm is so highly impermeable to them that 

 they would not get through the living layers into the 

 xylem, unless the concentration of salts is so high as to 

 destroy the semipermeability of the cells or they are injured 

 in some other manner. Ions or compounds to which cell 

 membranes are highy permeable and which are not retained 

 by the living cells might then be rather readily carried by 

 the transpiration stream. This may account for the rapid 

 transport of lithium with the transpiration stream. This 

 possibility, that under one set of conditions the transport 

 of a given element, like nitrogen, may take place chiefly 

 through the phloem and under another chiefly through the 

 xylem, complicates the problem greatly and may easily 

 lead to misinterpretations and contradictions. 



The cotton plants used by Maskell and Mason may have 

 been growing in a highly fertile soil with excess nitrogen. 

 The heavy pruning of the plants, by the removal of all the 



land, Hibbard, and Davis, 1926) nor with potato and other storage tissues 

 (Steward, 1932). 



If salts are secreted only from actively growing cells or come from matur- 

 ing cells, it would seem that most actively growing roots would at the same 

 time release the greatest amount of salt to the xylem water. This is in 

 agreement with observations that greater root pressures are associated with 

 conditions favoring more active growth of roots. One could therefore 

 expect that those plants which show most active guttation or bleeding would 

 also carry relatively larger amounts of salts through the xylem. In this 

 connection it is worth noting that Clements (1930, see Table 11) found 

 a very much greater salt movement past the ring on grape stems than on 

 those of plum or raspberry. The copious and long-continued exudation 

 from the xylem of grape is well known. This interpretation would also 

 indicate greater salt transport through the xylem of rapidly growing her- 

 baceous plants, such as Cucurbita, which exhibit vigorous exudation, while 

 in more slowly growing woody plants, which show less frequent and less 

 active root exudation, more of the salts may be transported through the 

 phloem. 



