56 TRANSLOCATION IN PLANTS 



ment through the phloem, for it was claimed that the 

 phloem was inadequate to carry solutes at normal rates. 

 That the phloem can carry these solutes upward has been 

 conclusively demonstrated in the experiments on the 

 behavior of defoliated shoots where first one tissue, the 

 phloem, and then the other, the xylem, have been cut. 

 The experiment on equalizing the transpiration stream 

 also clearly indicates that nitrogen moving into a defoliated 

 region of the stem enters chiefly by way of the phloem 

 and not the xylem. At that time it also seemed to me that 

 the only adequate mechanism that might account for 

 transport was the rotational streaming of the phloem cell 

 contents. If then the phloem was demonstrated as 

 capable of carrying materials both upward and down- 

 ward, it seemed obvious that the movement in both 

 directions was and could be simultaneous. With the 

 mechanism for unidirectional flow proposed by Miinch 

 (1927) and Crafts (1931), however, it is clear that the proof 

 of movement in both directions through the phloem is not 

 proof of simultaneous movement in both directions. 

 Although evidence presented in Chap. V seems clearly to 

 show that hypotheses proposed by Miinch and by Crafts 

 are untenable, the upward movement of sugars and of 

 inorganic constituents through the phloem into defoliated 

 regions, and the downward movement from the leaves to 

 the roots do not offer conclusive proof of simultaneous 

 movement in both directions. The experiment devised 

 to equalize the transpiration stream through defoliated 

 regions, therefore, does not conclusively prove that tran- 

 spiration does not control upward .movement of nitrogen 

 into leaves. It is possible, as far as these particular 

 experiments show, that nitrogen and nutrient salts are 

 carried into the leaves with the transpiration stream and 

 are retransported from the leaves through the phloem into 

 growing shoots or defoliated regions. 



In spite of the fact that the data from these experiments 

 with defoliated shoots seem less conclusive than when first 

 proposed there still remains the extensive and clear-cut 



