228 TRANSLOCATION IN PLANTS 



developed in those vascular tissues demanding most 

 effective transport, and that, so far as we know, they 

 always contain living protoplasm; all point strongly to 

 the conclusion that the sieve tubes take part in transport 

 and that the living contents play a significant and active 

 role. This evidence would strongly favor the proto- 

 plasmic streaming hypothesis, or the Van den Honert 

 modification, as contrasted with those of Miinch or Crafts. 

 None of the hypotheses that have been suggested to 

 account for transport through the phloem seems adequate, 

 and it has not even been definitely established whether 

 transport is or is not unidirectional at any one time and 

 place. 



SUMMARY 



26. Because of the differences between structure and contents of the xylem 

 tissues and those of phloem tissues, one would hardly expect methods that 

 are effective in causing flow through the one to be effective in causing move- 

 ment through the other. Differences in pressure applied externally at the 

 ends of the tracheids or vessels of the xylem, either increased pressure at 

 one end or reduced pressure or tension at the other, are effective in causing 

 flow of solutions through the system, because it consists of rather thick- 

 walled cells that resist collapse and contain httle or no solid material that 

 would obstruct the flow. With the sieve tubes of the phloem, on the other 

 hand, the tubes are usually narrower, are less rigid, have rather frequent 

 cross walls with but minute openings, and contain protoplasm and other 

 solid or semisolid materials that would seem likely to obstruct free flow. 

 Methods that have been found to be effective in causing flow through the 

 xylem have not been found effective in causing flow through the phloem. 

 This difference is probably related to differences in structure and content 

 of the two types of tissues. 



27. It had been suggested that diffusion accounted for transport of solutes 

 through the phloem, but at an early date diffusion alone was demonstrated 

 to be inadequate. It had also been suggested that moving protoplasm 

 which had been widely observed might account for transport, but this sug- 

 gestion was not widely accepted because observations indicated that proto- 

 plasmic streaming does not occur in sieve tubes when they are mature and 

 supposedly active. Rather indefinite appeals were made to turgor pressures 

 as the cause of flow, but no clearly defined hypothesis was formulated to 

 explain the mechanism of flow. Several had recognized the possibility that 

 an explanation of movement from cell to cell involved a transport through 

 surface membranes which often show a very low permeability to sugars and 

 which therefore seem to introduce added difficulties to finding a satisfactory 

 explanation of movement. This difficulty has been met by some through 

 suggesting that movement takes place through plasmodesma thus avoiding 



