Hoagland — 90 — Plant Nutrition 



in the nutrient solution, to what extent can the shoot 

 deplete the root of its previously accumulated salt? 

 With bromide as the test ion, it has been shown that 

 the growing shoot can gradually withdraw nearly all 

 the bromide previously stored in the root cells. This 

 was demonstrated in independent experiments by 

 Steward, Prevot and Harrison (1942) and by 

 Broyer and myself. Potassium can be reduced to a 

 very low level of concentration, although naturally 

 not all the potassium can be withdrawn from the root 

 system. In a sense a competition for solutes occurs 

 between the root and the shoot. 



Why ions that have been accumulated by root cells 

 and which can not be removed from the healthy cells 

 by leaching with distilled water should nevertheless 

 move out of these cells in the intact plant and upward 

 to the shoot, is far from receiving a satisfactory ex- 

 planation. The whole process involves a polarized 

 movement of salt. Prevot and Steward have re- 

 marked in an earlier paper: 



"The elucidation of those factors which in the 

 intact plant cause this ready removal of electrolytes 

 from root cells which have already accumulated them, 

 and which in excised roots would necessitate drastic 

 treatment with perhaps even irreversible changes, rep- 

 resents one of the most difficult and certainly one of the 

 most fundamental of the outstanding problems." 



As a sidelight on this problem the recent data 

 of Steward, Prevot and Harrison (1942) on longi- 

 tudinal distribution of potassium, rubidium and brom- 

 ide ions in barley roots are worth citation. The 

 spectroscopic technique was used in determining rubi- 

 dium and potassium. In the excised root, the ac- 

 cumulation of these ions is most rapid at the apex 

 and decreases at points farther removed from the 

 apex. When the root is attached to the growing 

 shoot the gradient tends to become reversed. Steward 

 and his collaborators think that this may mean that 

 the most active secretion of salt into the stele takes 



