10 TRANSLOCATION IN PLANTS 



In 1920 I reported some experiments which led me to 

 suggest that both upward and downwprd movements of 

 sugars occur chiefly in the phloem and that probably very 

 little upward transport takes place through the xylem. 

 In the same year, 1920, Madam Birch-Hirschfeld published 

 results of experiments which led her to think that the 

 phloem was not adequate for carrying foods downward. 

 A backward flow of introduced solutes readily took place 

 through the xylem, however, and this tissue, she suggested, 

 might serve for the backward movement of photosynthate 

 from the leaves. In 1922 Professor Dixon read a paper 

 at the British Association meetings in which he reempha- 

 sized the probable inadequacy of the phloem as a tissue for 

 transport, and definitely proposed the hypothesis that the 

 xylem is chiefly concerned not only in upward transfer but 

 also in the backward transport of solutes. 



It is interesting therefore to realize that, previous to the 

 period 1920 to 1922, it was supposed that it was definitely 

 known what tissues were concerned in solute movement, 

 while at that time three distinct hypotheses were advocated ; 

 the older one in which there were supposed to be two 

 channels for solute transport, one, the xylem, for upward 

 transport and the other, the phloem, for downward trans- 

 port; and two more recent hypotheses proposing only one 

 tissue to be chiefly concerned in solute transport in both 

 directions, the one proposing that this transport takes 

 place in both directions through the xylem, and the other 

 that it takes place chiefly through the phloem. A fourth 

 interpretation has more recently been proposed by Miinch 

 (1926) and by Crafts (1931) that certain solutes, chiefly 

 organic, are carried either upward or downward through the 

 phloem but not in both directions simultaneously through 

 the same region, while other solutes, chiefly mineral salts, 

 are carried through the xylem in the transpiration stream. 

 Maskell and Mason (1929, 1931) also have suggested that 

 organic materials move in both directions through the 

 phloem but that salts move up through the xylem. It is 

 obvious that the problem as to the tissue or tissues con- 



