UPWARD TRANSPORT OF NITROGEN 79 



chiefly through the phloem. The work of Osterhout 

 (1922), Hoagland (1923), and others indicates that living 

 cells can absorb nitrate and other ions and concentrate 

 them within the cells, even without change to organic 

 forms. One would expect, therefore, that if nitrogen were 

 deficient, the living cells would have greater ability to 

 retain the nitrate and prevent it passing through them 

 into the xylem. In fact, any reaching the xylem would, 

 in all probability, be quickly absorbed by the neighboring 

 living cells. If, on the other hand, nitrates or other 

 nutrients are in excess in the soil, the living cells of the 

 root might soon have their accumulation capacity satisfied, 

 in which case they could not prevent the excess from passing 

 into the xylem and the transpiration stream. There seems 

 little doubt that, if solutes get into the transpiration 

 stream, they will be carried with it unless removed. If 

 certain nutrients, as for example nitrogen or potassium, 

 are present in such amounts that the living cells of the 

 roots are in a sense saturated, there seems no reason why 

 they may not pass into the transpiration stream. Here 

 they may be carried to the leaves or may be removed by 

 the living cells along the path if the concentration within 

 the cells is not in equilibrium with the xylem solution.* 



* Professor Hoagland has kindly shown me some of his findings that have 

 not yet been published. These suggest a very different interpretation. His 

 findings indicate that when roots are most rapidly accumulating bromine or 

 potassium against a diffusion gradient, they are at the same time secreting 

 these same elements into the xylem. At least they appear in the guttation 

 water or in that which exudes from a cut stump and seems to come from 

 the xylem. Whether this is an active secretion from healthy cells or a 

 release of solutes, chiefly inorganic, from maturing cells is not clear. The 

 latter explanation is in hne with the suggestion of Priestley (1920, 1929) 

 that maturing xylem cells release their contents, although Priestley suggests 

 that the solutes are chiefly organic. The large amounts obtained, however, 

 and the speed of release to the xylem would seem to preclude the possi- 

 bihty of their all having been released from maturing or dying cells. If 

 active secretion is established the suggestion that the living cells of the root 

 would be hkely to remove certain ions from the xylem and thus tend to 

 prevent their transport with the water is unsound. It is still likely, how- 

 ever, that living cells along the path above the absorbing roots will remove 

 some of these ions from the xylem water. The polar secretion of ions, how- 

 ever, is not indicated in experiments with Nitella (Hoagland, 1923; Hoag- 



