DOWNWARD TRANSPORT THROUGH THE XYLEM 103 



instance less than 0.03 per cent, so that a backward flow 

 much in excess of that found would be necessary to carry 

 the necessary amounts of sugar. Even if the sap obtained 

 from the outer layers invariably contained more sugar 

 than the inner layers, this would not offer very strong 

 support for the hypothesis of downward transfer through 

 these tissues. One might expect a higher concentration 

 in vessels which have recently matured, which are closer 

 to active cambium cells containing a high sugar content, 

 and which are also closer than the inner tissues to the 

 sugar-conducting cells of the phloem. MacDougal does 

 not explain how he collects the sap from the separate 

 layers by suction. Presumably the layers are blocked off 

 at the point where suction is applied. Even though suction 

 is applied on the cut end of a given layer, the possibility 

 remains that within the tissues there may be flow from one 

 layer to another. That such flow may occur is indicated 

 by his own observations, as well as those of others, that 

 introduced dyes may pass from one layer to another and 

 even back again. 



Though he is not positive as to the tissues concerned in 

 downward movement of foods and states that the prevalent 

 view that foods move down through the phloem is based 

 on very imperfect evidence, he has no hesitation in saying, 

 "Positive identifications of the route of upwardly moving 

 solution is much more easily obtained." Apparently 

 he takes no stock in the evidence that I have published, 

 but it is rather disappointing that he offers no explanations 

 of the data obtained. Dixon (1924, p. 54) is equally 

 positive in his statement: "Taking aU the evidence into 

 account we may conclude with certainty that the rising 

 water current conveys upward not only dissolved inorganic 

 substances but that it is of importance in transferring 

 organic substances also from the lower parts of the plant 

 to its upper extremities." He also disappoints one in that 

 the only answer he gives to the extensive data on ringing 

 experiments is that ringing may result in plugging of the 

 xylem vessels. It would be interesting to know how he 



