UPWARD TRANSPORT OF NITROGEN 53 



definite conclusions can be drawn with reference to increases 

 or decreases in these Ligustrum twigs. 



Several have suggested that most of my data, showing 

 that ringing causes a retardation of upward transport 

 both of carbohydrates and of nitrogen and ash constituents, 

 can be explained on the grounds that the rings have caused 

 plugging or other morbid changes in the xylem, and that 

 they are not to be interpreted as demonstrating that upward 

 transport takes place through the phloem. As is pointed 

 out in Sec. 25, however, there is abundant evidence that 

 in many of the experiments there was no sign of any inter- 

 ference to flow through the xylem. When the ring wounds 

 are thoroughly protected by melted paraffin, as was the 

 practice in all of these experiments, the xylem does not 

 become plugged. Since with these woody plants ringing 

 has consistently interfered with upward transport of 

 nitrogen and ash constituents, even when no plugging has 

 taken place, the interference cannot be ascribed to blocking 

 the xylem. 



Although the xylem has not become blocked, it is true 

 that, in many cases at least, the amount of transpiration 

 has been less from the leaves of the ringed stems. No 

 extensive data are available showing how much transpira- 

 tion is lessened by ringing. A few measurements with 

 standardized cobalt chloride paper have shown that the 

 leaves of ringed branches of Ailanthus were losing about 

 one-half to one-third as much water at midday as similar 

 leaves of check branches. These measurements were made 

 about three weeks after the rings were made, and yet at 

 this time the xylem of the ringed stems was not plugged 

 but carried dyes past the ring as readily as did that of 

 unringed stems. 



This reduction in transpiration from ringed shoots seems 

 to be due largely to the effects of the ringing on the leaves 

 and perhaps chiefly to the effect of the high carbohydrate 

 and low nitrogen on the stomates. To test this point 

 Miss Pleasants has made a number of observations and 

 found that the stomates on the leaves of ringed branches 



