Hoagland — 98 — Plant Nutrition 



introduced phosphate occurred not in the regions of 

 highest transpiration, but in the young still growing 

 fruit. Comparatively little radioactive phosphate ac- 

 cumulated in the fully ripe fruit. Within the fruit 

 itself the distribution was differential, and in general 

 in accordance with the concept of potentiality for rapid 

 accumulation of solutes possessed by growing cells 

 with a high rate of metabolism. The highest concen- 

 tration of phosphate was found in developing seeds. 

 (See plate 19). 



Mason and Phillis (1937, 1940) have proposed 

 that something like a circulation of mobile inorganic 

 nutrients can take place in the plant by which the 

 solutes are carried in the wood to foliar regions, from 

 which they may be reexported through living cells, 

 even back to the roots. The supposition is that along 

 the path of reexport some leakage into the xylem 

 vessels may occur and the solutes be carried again 

 upward. It is, of course, possible for previously ac- 

 cumulated solutes to move out of leaf cells to other 

 regions of the plant. Experiments with radioactive 

 elements give direct evidence of this — some backward 

 movement as far as the roots may occur. Neverthe- 

 less, a circulation theory by itself seems too simplified 

 and does not emphasize sufficiently the role of active 

 processes in accumulation of solutes in both root and 

 shoot. 



General Discussion : — The question of secondary 

 movements of salt in the living cells of the phloem is 

 part of the general problem of translocation in this 

 tissue. The theories evolved to explain this type of 

 translocation are various and still highly controversial. 

 Their detailed consideration falls outside the scope 

 of my discussion. We need note now only that all 

 theories at some point invoke the activities of living 

 cells in the movement of solutes against gradients or 

 in accelerating the processes of diffusion. The latter 



