UPWARD TRANSPORT OF NITROGEN 81 



lower branches and leaves three days before the experiment 

 began, would certainly tend toward severe starvation of 

 the roots and would thus favor leaching of nitrogen into 

 the transpiration stream. This would be especially marked 

 in plants with ringed stems. This special set of conditions 

 might, therefore, account for the data they obtained. 

 Such a condition might also account for their failure to 

 find a clear-cut diffusion gradient for any particular form 

 of nitrogen. Much of the nitrogen may have been moving 

 through the xylem for the reasons stated above. Some 

 may have been moving to the leaves through the phloem 

 or xylem before being converted into organic forms. 

 Some may have been absorbed by the living cells along the 

 path and there converted into organic nitrogen. Their 

 assumption that all synthesis must have taken place in 

 the leaves and that upward transport is exclusively through 

 the xylem has but little supporting evidence. 



It is possible that herbaceous or annual plants carry 

 relatively more of their mineral nutrients through the 

 xylem than do woody perennials. I have found strong 

 tests for nitrates in the sap bleeding from stumps of Iresine 

 plants, yet Thomas (1927) has obtained evidence that in 

 apples little or no nitrate is present in the xylem. Wormall 

 (1924) also found but little in the sap of the vine. Eckerson 

 (1924) found the roots or stems of tomato to be capable 

 of reducing nitrates to organic forms which might be 

 expected to be carried more readily in the phloem. Mason 

 and Maskell (1931, p. 149), however, found organic nitrogen 

 in tracheal sap of cotton in about the same concentration 

 as inorganic nitrogen, and Anderssen (1929) found all the 

 nitrogen in the sap of pear stems to be organic. From 

 this, one might even contend that nitrogen is transported 

 upward chiefly as organic nitrogen which possibly leaches 

 into the transpiration stream more readily than nitrate. 



In all of my experiments on the effect of ringing on 

 nitrogen and ash transport I have worked with woody 

 plants, the roots of which are perennial and at all times 

 are likely to contain a fair supply of carbohydrates. 



