116 TRANSLOCATION IN PLANTS 



selenium or tellurium, these elements were found in abun- 

 dance in the xylem, while they were almost absent from the 

 phloem and other tissues. In the discussion that followed 

 the talks on translocation before the British Association 

 Meeting in Toronto, in 1922, Prof. Overton cited similar 

 experiments with potassium salts as definitely estabUshing 

 the xylem as the channel for transport. There are, 

 however, very serious criticisms to this method. The 

 region of deposition or accumulation of a substance may 

 give little or no indication as to what tissues carry the 

 material. For a time it was thought that carbohydrates 

 were carried in the starch sheath or endodermis, for it 

 formed a continuous chain of cells which was well filled with 

 starch. But this notion was abandoned when Heine (1885) 

 demonstrated by modified ringing experiments that this 

 tissue was not concerned in transport. Bokorny (1890) 

 thought the deposition of iron in the walls of thick-walled 

 cells of both xylem and the sclerenchyma cells of the 

 phloem, and its absence in other cells, proved that water 

 and solutes were carried in the walls of these cells and not 

 through the lumen. Scott and Priestley (1928), and 

 Scott (1928) have more recently considered that the micro- 

 chemical detection of dyes or inorganic ions in the walls is 

 evidence that the materials move along the walls and not 

 through other parts of the cell. The finding of deposits of 

 iron or dyes or any other substance in a cell or on its walls 

 may be proof that these elements had reached that point, 

 but it does not prove that those walls or cells were the 

 channels through or along which the material was being 

 carried. In fact, the very accumulation in the walls or in 

 the cells might be considered to indicate that they are not 

 effective channels of transport, for the fixation on or in a 

 nonmoving body would tend to interfere with further 

 movement. 



23. The Occurrence of a Substance in a Tissue as a 

 Criterion of Its Movement through That Tissue.— The 

 complete absence of a substance from a possible conducting 

 channel is obviously conclusive proof that that substance 



