106 TRANSLOCATION IN PLANTS 



mechanism that would bring a backward flow through 

 certain of the channels while water is being lost from the 

 leaves and absorbed from the roots. Even if such a 

 mechanism exists and were demonstrated, it would have 

 to be so coordinated with permeability or other changes 

 that sugars would be secreted only into the downward 

 moving streams and these would have to flow continuously 

 or, if some accident should start the stream running in the 

 wrong direction, some regulatory system would have to 

 come into action immediately to unload the sugars from 

 the reversed stream. With such a system one would be 

 almost forced to call upon hormones or something even 

 more subtle than these to aid in the regulation (see Sec. 17). 

 As stated in the preface, it is possible that many of the 

 advocates of this hypothesis no longer favor it, yet in view 

 of the fact that many investigators, including Dixon, 

 MacDougal, Mason and Lewin, and Arndt, advocated it 

 even after the publication of my paper which demon- 

 strated continued transport after the complete severance 

 of the xylem, and in view of the fact that evidence of the 

 sort used to support this hypothesis is still used to support 

 the contention that the xylem is the channel chiefly con- 

 cerned in upward transport, it has seemed advisable to 

 discuss this evidence in the detail here presented. 



SUMMARY 



15. Attempts of many sorts were made by Birch-Hirschfeld to bring 

 about a rapid movement of lithium nitrate or eosin through phloem and 

 parenchyma tissues. The materials were introduced through incisions of 

 various sorts or by applying strong solutions or crystals to the surface, and 

 attempts to bring about movement involved physical manipulation and 

 high rates of transpiration. These materials, however, could be caused to 

 move through phloem or parenchyma at only very slow rates, rarely over 

 5 cm. in 24 hr. The movement in killed tissues was slightly faster than in 

 living tissues. The cross-sectional area of the sieve tubes of the petiole of 

 a bean leaf was measured, and, assuming a moderate rate of photosynthesis, 

 calculations as to rates at which pure sugar or sugar solutions must move 

 to empty the leaf in 24 hr. gave figures over a hundred to several thousand 

 times the rates at which she was able to force water through the phloem. 

 On the other hand, when lithium salts or eosin were introduced into the 

 xylem, they were quickly carried throughout the plant, not only toward the 

 transpiring leaves but also backward to leafless parts. Since the phloem 



