THE METHOD OF MOVEMENT 147 



29. Points Favoring the Hypothesis of Miinch. — There 

 are a number of points, both theoretical and experimental, 

 that strongly favor the hypothesis developed by Miinch, 

 but there are also several serious weaknesses. One highly 

 intriguing suggestion proposed by Miinch is that the 

 plasmodesma allow for rapid movement from living cell 

 to hving cell of those solutes, especially sugars, to which 

 the surface membrane is highly impermeable. This same 

 point was previously suggested by Kienitz-Gerloff (1891) 

 and by Mangham (1917), and, though it is an important 

 one, its acceptance does not necessitate the acceptance of 

 the hypothesis, that the movement is a unilateral mass 

 flow through either these plasmodesma or the larger pores 

 of the sieve plates. 



That materials can for a short time be forced through the 

 phloem by internally developed pressures seems definitely 

 proved by the fact that phloem contents will rapidly exude 

 from a cut stem. This has been reported by a number of 

 investigators, especially Hartig (1860, 1861), Munch 

 (1930), and Crafts (1931). The exudation from Cucurbita 

 that has been widely observed, though it comes from sieve- 

 tube-Uke cells, comes, at least in part, from tissues very 

 similar to' latex tissues. Unless Crafts is correct in suggest- 

 ing that the material moves within the walls, which is 

 doubtful, this exudation demonstrates that the pores of the 

 sieve plates are sufiicienty large to allow for a flow. It is 

 also possible, however, that the flow from the cut phloem 

 is strictly abnormal and due solely to opening the tissues 

 and thus developing an abnormally steep pressure gradient. 



Perhaps the strongest direct experimental evidence for 

 this hypothesis is that exudation from the phloem has been 

 demonstrated, and it has been reported that exudation soon 

 ceases if a second cut is made through the phloem above 

 the first one, that is between the first cut and the supplying 

 region, while exudation may continue for some time if the 

 second cut through the phloem is made below. The 

 second cut, if made above the first, may stop the flow at a 

 distance up to 1 or even 5 m. (Hartig, 1860; Biisgen and 

 Munch, 1929, p. 134; Munch, 1930, p. 124). 



