UPWARD TRANSPORT OF NITROGEN 87 



all cases be real but merely apparent and due to the method of expressing 

 composition. Expressing composition changes on a per leaf or per organ 

 basis seems usually preferable for determining absolute changes, but this 

 method has its disadvantages in tending to alter to a greater or less degree 

 the normal changes. Even when demonstrated to be real, an increase of 

 nitrogen in leaves during the day as compared with that at night may be 

 due to the increased carbohydrate or to hght causing greater retention and 

 accumulation, and may thus be entirely independent of transpiration. 



13. Evidence of recent years has clearly demonstrated no predictable 

 relation between transpiration and salt absorption. It is commonly 

 assumed, however, that after absorption, salts are transported in the trans- 

 piration stream. It is pointed out that a failure to obtain increased absorp- 

 tion with increased transpiration is an indication that salts are not carried 

 in the transpiration stream as commonly supposed. Furthermore, a greater 

 accumulation of salts in roots with less in the tops in plants under higher 

 transpiration does not point toward transport with the water. It is proba- 

 ble that observed differences in mineral absorption and distribution are 

 not directly influenced by transpiration, but indirectly, through the influence 

 of transpiration, or the means used in altering it, on some other phases of 

 metabolism. The suggestion that salts carried down to the roots through 

 the phloem tend to have a damping effect on further absorption, and that 

 this accounts for failure of transpiration to increase absorption, points 

 toward a rapid circulation of salts in the plant. The failure of rings to trap 

 large quantities of salts above the cut phloem, however, strongly negatives 

 this suggestion. Certain cases of reported accumulation of salts in leaves 

 under conditions of hot winds are discredited on the grounds that the changes 

 were expressed as percentages of dry weight, and the conditions were such 

 as to cause a marked change in dry weight. 



14. When toxic solutes are supplied to the roots, they are carried with 

 the transpiration stream, probably because they have injured the proto- 

 plasmic membranes and are therefore carried more directly with the water. 

 Living cells are known to be able to accumulate certain ions against a con- 

 centration gradient if the external concentration is weak. It would seem, 

 therefore, that such ions would tend to be absorbed by the living cells of 

 the root and stem, would be transported in them, and not be liberated into 

 the transpiration stream. This would seem especially to apply to nitrogen 

 which may be quickly transformed into organic compounds in these tissues. 

 On the other hand, if these salts are in excess so that the accumulation 

 capacity of the living cells is exceeded, large amounts can be expected to 

 leach into the water-conducting tissues and be there transported. Ringing 

 in such a fashion as to starve the roots will tend to reduce their capacity to 

 accumulate or retain the nitrogen and ash they originally held, and the 

 effect will be to increase leakage into, and transport through, the xylem. 

 The fact that under conditions of root starvation or excess supply in the soil, 

 nitrogen and ash constituents may be carried chiefly in the transpiration 

 stream, while under more normal conditions they may be carried chiefly 

 through the phloem, may account for much of the contradictory evidence. 

 Recent experiments with herbaceous plants indicate a secretion into the 

 xylem by actively absorbing roots. This is contrary to the suggestion that 

 living root cells would prevent entrance into the xylem. 



