CRITERIA AND METHODS 125 



and the mechanism of transport through the phloem is 

 clearly established, one is not justified in saying that the 

 xylem is or is not necessary for upward transport of solutes 

 of one sort or another. 



25. Possible Injury to the Xylem When the Phloem Is 

 Cut. — A number of botanists have hesitated to accept data 

 obtained from ringing experiments because they have felt 

 that ringing has had an effect other than the mere severance 

 of a possible conducting channel. Dixon (1922, 1924), 

 for example, lightly passes over all of the evidence based on 

 ringing experiments on the assumption that ringing always 

 results in some sort of plugging of the xylem. However, 

 he depends for his evidence solely upon the observations 

 that others have made when ring wounds were not pro- 

 tected. In the earliest experiments that I carried out to 

 determine the channel of upward transfer (Curtis, 1920a) 

 it became evident that conduction of water through the 

 xylem was interfered with by drying or plugging near the 

 ring if this wound were not thoroughly protected by a 

 layer of melted paraffin wax. In all subsequent series the 

 xylem was always protected. 



When the xylem was well protected, no indication of 

 even partial plugging was evident. Various methods have 

 been used to test for plugging. Ringed parts have been 

 sectioned and examined under the microscope and these 

 showed no sign of plugging, whereas unprotected wounds 

 showed visible plugging of the underlying xylem. After 

 the effects of ringing on solute transfer have become evi- 

 dent, as measured by growth responses, carbohydrate 

 analyses, and ash and nitrogen analyses, it was found that 

 introduced dyes readily passed the region of the ring. 

 After the completion of ringing experiments of this sort, 

 the comparative rates of flow of a dye solution through 

 ringed and check stems, when both led from the same main 

 stems forming a Y, have been tested. The dye (acid 

 fuchsin) was found to pass just as rapidly into and through 

 the ringed stem as through the other. If there was any 

 increased resistance in the ringed stem, this should have 



