166 TRANSLOCATION IN PLANTS 



sieve-tube protoplasm is completely permeable, so flow 

 from cell to cell may take place through end and lateral 

 walls, and is not restricted to flow through pores or plas- 

 modesma. For actually causing the flow he proposed an 

 osmotic gradient scheme similar to that proposed by 

 Miinch. His scheme differs, however, in that the materials 

 are not supposed to be forced into the phloem or from the 

 phloem into the receiving cells through the plasmodesma 

 by a pressure gradient. The receiving cells are assumed 

 to be able to remove sugars from the wall through the 

 surface membrane against a concentration gradient. 



The chief advantage claimed by Crafts for the superiority 

 of his hypothesis over that of Munch is that it proposes a 

 large increase in the cross-sectional area through which 

 materials may be forced, and therefore seemingly reduces 

 the pressure gradient necessary to cause the flow. He 

 found that the pores through sieve plates in fresh material 

 are more minute; than commonly observed in fixed material, 

 and that they are completely filled with strands of proto- 

 plasm. Therefore they do not offer tubes for unobstructed 

 flow. Exudation from the peduncle of a cucumber was 

 found to continue at a normal rate even after the sieve 

 tubes were heavily calloused and the sieve pores were 

 apparently closed. It seemed therefore that the exudate 

 could not have come through the sieve pores. He also 

 claims that his hypothesis does not demand a lower osmotic 

 concentration in the receiving cells as does the Miinch 

 hypothesis, for he assumes that the receiving cells can 

 Hiadily absorb solutes through these surfaces against a 

 concentration gradient. 



32. Weakness in the Hypothesis of Crafts.— At first 

 iWM) Crafts proposed that the high water content of the 

 phloem walls should make them suitable channels for 

 transport. The area of the wall available as a channel 

 he assumcKl to be indicated by the water content of the 

 wall, which in his determinations was approximately 50 

 per cent. When one realizes, however, that a tube filled 

 with a 2 per cent agar gel does not allow for ready mass 



