170 TRANSLOCATION IN PLANTS 



plasmodesma and sieve pores. Crafts, because he finds 

 the plasmodesma and sieve pores so small, interprets the 

 flow as coming from the walls or from the completely 

 permeable sieve tubes. It seems to me, however, that it 

 may be strictly abnormal exudation of cell contents through 

 breaks in the surface layer or through plasmodesma that 

 have become greatly distended as a result of cutting. 

 Such cutting would release the external pressures which 

 normally would prevent the distention of thin-walled cells 

 having high osmotic concentrations. Miehe (1901) reports 

 having observed nuclei to pass through plasmodesma 

 when immature parenchyma or epidermal tissues are 

 wounded. It is conceivable that this movement may 

 have resulted from sudden release of pressure and disten- 

 tion of plasmodesma. 



Although Crafts claims that, according to his proposals, 

 there need be no pressure gradient between supplying cell 

 and receiving cell, as is required by the Miinch hypothesis, 

 his hypothesis does require a pressure gradient through the 

 phloem, and no mechanism is obvious that will account for 

 secretion into the permeable sieve-tube system at the sup- 

 plying end and removal from the walls at the receiving 

 end. For the latter he says ''The ability of living non- 

 photosynthesizing cells to produce a low concentration 

 within their walls is all that is necessary." Too much 

 space would be necessary adequately to discuss the essen- 

 tials and the difficulties involved in a system that would 

 be effective in introducing sugars into the phloem and in 

 maintaining a pressure gradient that could cause a flow in 

 the proper direction through the phloem which has a com- 

 pletely permeable sieve tube system, but which must 

 prevent leakage through the walls or through accidental 

 breaks. Special difficulties are also involved in accounting 

 for absorption of the sugar through their surface membranes 

 by the living cells at the receiving end, and since a unidirec- 

 tional flow of solution is assumed, there must be some means 

 also for getting rid of the water that is carrying the sugar. 

 Unless the pressure on the solution in the phloem at the 



