132 TRANSLOCATION IN PLANTS 



stained and others that the transport has been along the wall itself. Such 

 deposits clearly demonstrate the arrival of the material, but they do not 

 demonstrate that those cells or tissues are concerned in transport. In 

 fact, deposits in nonmoving parts of the cell seem more strongly to point to 

 ineffectiveness of that tissue as a path of transport. 



23. Sugars, organic acids, organic forms of nitrogen, and many inorganic 

 elements have been found in the sap exuding from cut stems, or in that 

 forced out of the xylem by various means. This has been cited as proof 

 that the xylem is the channel through which these materials are naturally 

 carried. These materials, however, seem especially abundant at seasons 

 when the plants are leafless and may largely disappear through absorption 

 by hving cells before there is an active transpiration stream. The finding 

 of these small amounts in the xylem sap probably accounts for the transport 

 that has been demonstrated to take place through the xylem, but it does 

 not prove that the wood is the major path of transport. 



24. Experiments, in which first one possible channel and then the other 

 is cut, result in little or no transport of naturally occurring solutes when the 

 phloem is cut, and transport at approximately normal rates when the xylem 

 is cut. This method of experimentation is not open to the criticisms that 

 apply to methods where attempts are made to force solutions through the 

 tissues that might be supposed to carry the materials and which have been 

 cut open to admit the material used in testing. Although it has been 

 clearly demonstrated that the phloem can carry both organic and inorganic 

 solutes in either direction, it has not been conclusively demonstrated that 

 movement in both directions is simultaneous. 



25. Many have doubted the evidence for transport through the phloem 

 that is based on ringing experiments because they have felt that ringing has 

 resulted in a plugging of the xylem. Although plugging may result from 

 ringing, it has been found that careful protection of the ring wound with 

 melted paraffin may entirely prevent such stoppage. Although definitely 

 sought for, no evidence was obtained indicating that plugging might have 

 been responsible for the responses obtained. Several have claimed that 

 injury to the xylem is unavoidable but the evidence supporting such claim 

 is weak and in several cases is shown to be definitely unsound. Even if 

 partial stoppage were demonstrated always to occur, this can hardly account 

 for the marked reduction in solute transport following ringing, for the 

 stoppage cannot be complete if the parts above continue to receive adequate 

 water. It is rather surprising to find that many individuals unhesitatingly 

 accept the evidence from ringing experiments when it shows a reduced sugar 

 movement, but when similar evidence from similar ringing experiments 

 points to a check in movement of nitrogen or ash, it is assumed to be false 

 and due to plugging of the xylem. 



