CHAPTER VI 

 THE METHOD OF MOVEMENT THROUGH THE PHLOEM 



26. Peculiarities of the System. — Although it has long 

 been recognized that solute translocation, at least the 

 backward translocation from the leaves, takes place 

 through the phloem, until recently very little attention 

 has been given to a consideration of the possible mechanism 

 of this transfer. Since the structures of the xylem and the 

 phloem tissues are so very different, it is obvious that the 

 methods by which materials are moved through the tissues 

 are also probably different. Any hypothesis explaining 

 the mechanism of solute movement must therefore be 

 adapted to the tissue concerned. 



Because of the rigid walls of the xylem conduits, water 

 and solutions are readily moved through it by differences 

 in tension or pressure initiated at either end of the system, 

 but it appears impossible by similar methods to force 

 solutions, or even pure water, through the phloem at 

 adequate rates, even though the cells form a continuous 

 longitudinal series. This is probably due to the fact that 

 the phloem cells have small lumina with nonrigid, perhaps 

 somewhat elastic side walls; are connected, one cell to 

 the next, by rather minute pores which may be completely 

 filled with protoplasm; contain living protoplasm; and 

 probably have a high content of rather viscous, colloidal 

 material. A pressure or suction applied externally at one 

 or the other end can be expected, therefore, to be ineffective 

 in forcing solution through the system. 



Birch-Hirschfeld (1920), whose findings have already 

 been discussed in some detail in Sec. 16, has demonstrated 

 that it is impossible to force water or solutions through 

 phloem tissues by methods that are easily successful in 

 forcing solutions through the xylem. This experimental 



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