124 TRANSLOCATION IN PLANTS 



through the xylem has not demonstrated, however, that the 

 xylem is the chief channel for transport. One can expect 

 that any living cell will lose some of its contents to a bathing 

 water solution, whether the latter is or is not in a conducting 

 channel. That the transport of such leachings in the 

 transpiration stream is normally of great consequence has 

 not been demonstrated. 



24. Evidence from Movement Observed after Cutting or 

 Blocking One and then Another of the Possible Conducting 

 Channels. — Although the methods of experimentation and 

 criteria described in Chap. IV and in Sees. 20 to 23, when 

 uncritically used, point strongly toward the xylem as the 

 effective tissue for transport, evidence from experiments 

 reported in Sees. 4 to 9, that is, when first one tissue and 

 then the other is cut or blocked, indicates that the xylem does 

 not carry the solutes under natural conditions, but that the 

 phloem can and does carry them. 



Of course, one could contend that cutting first one tissue 

 and then the other might upset normal translocation so 

 that materials might be forced into new channels in order to 

 pass the point of operation. It should be obvious, however, 

 that movement through the phloem when the xylem is cut 

 cannot be explained in this way, for certainly if the normal 

 channel is the xylem, it should continue movement when 

 the phloem alone is cut and the xylem is uninjured. I have 

 heard it frequently stated verbally and have seen it at least 

 once in a pubhcation (Mason, 1922) that both tissues must 

 be necessary or that the phloem must be present to help 

 the movement through the xylem. The continued move- 

 ment when the xylem is completely removed, however, 

 seems definitely to refute such an explanation. 



Although it has been conclusively demonstrated, by 

 experiments where the xylem has been completely severed 

 or the phloem has been isolated from the xylem, that the 

 phloem is capable and adequate, for carrying both inorganic 

 and organic solutes in either direction, these same experi- 

 ments do not conclusively demonstrate a simultaneous 

 transport in both directions. Until this point is cleared up 



