THE METHOD OF MOVEMENT 171 



surface of the receiving cell is in excess of the maximum 

 osmotic pressure in the phloem, water will be drawn across 

 the living cell into the phloem instead of being forced out of 

 the phloem. 



Although Crafts claims an advantage for his hypothesis 

 because of the greater cross-sectional area allowed for 

 transport, and because it is supposed not to require that 

 the receiving cells have lower turgor than the supplying 

 cells, the hypothesis still retains most of the serious weak- 

 nesses of the Miinch hypothesis. These have already 

 been discussed under 1, 2, 4, 5, 6, and 7 of Sec. 30 and need 

 not be repeated. The modifications proposed, although 

 they are supposed to overcome some of the weaknesses of 

 the Miinch hypothesis, because the proposed channel for 

 transport is larger than that of the Miinch scheme and the 

 receiving cells can have a higher turgor than the supplying 

 cells, really result in loss of much of the simplicity of the 

 Miinch hypothesis and introduce other serious difficulties. 

 Crafts' scheme also seems to be based on more unproved 

 assumptions than that of Miinch. These major unproved 

 assumptions are: (1) that the supplying cells can in some 

 way introduce sugars into the phloem in such a manner 

 that they will develop a pressure gradient leading to the 

 receiving cells ; (2) that the sieve tubes are completely per- 

 meable and offer a minimum of resistance to flow of solu- 

 tion through the lumen and across walls at all points, and 

 yet that phloem is so enclosed by cambium and phellogen 

 as to prevent leakage; and (3) that the receiving cells can 

 absorb sugars against a gradient with such rapidity as to 

 lower greatly the concentration within the wall outside of 

 the living membrane. Because of the many weaknesses 

 in the hypothesis of Crafts and because the necessary 

 assumptions are not merely unproved* but seem highly 

 improbable, I at no time have accepted his hypothesis and 

 still feel that the hypothesis, even as modified to include 

 transport through the cell lumina, is untenable. 



* Phillis and Mason (1933) have, however, demonstrated accumulation 

 of sugar in the phloem against a concentration gradient. 



