THE METHOD OF MOVEMENT 229 



passage through a surface membrane. Others have suggested that it is 

 only the vacuolar membrane of the cell that shows low permeability to 

 sugar, and low permeability of the surface membrane offers no special 

 difficulty. 



28. Mlinch has suggested that there is a unidirectional flow of phloem 

 contents that is brought about by a pressure gradient. This pressure gradi- 

 ent is estabhshed and maintained by differences in osmotic concentrations 

 of supplying and receiving tissues. If within an osmotic system (a single 

 membrane or a group of membranes connected by openings that will allow 

 mass flow of solution) the osmotic concentration is high at one end or on one 

 face and the concentration is low at the other end or on another face, and 

 water is available at the end having a high concentration, there will be 

 absorption of water through the membrane in the region of high concen- 

 tration, a mass flow through the system, and excretion of water from the 

 region of low concentration. In applying this to living plants it is assumed 

 that a semipermeable membrane exists separating living cells from water 

 in water-conducting tissues. Plasmodesma are supposed to allow for flow 

 of solutions from one living cell to another, while sieve pores allow for similar 

 flow through specialized sieve tubes. Therefore, if the supplying cells or 

 tissues have a high osmotic concentration, the receiving cells a low concen- 

 tration, and the plasmodesma and sieve pores allow for mass flow, there 

 should be a transport of solution from the cells with high concentration and 

 high turgor to those with low. 



29. Several factors tend to support the Mlinch hypothesis. The fact of 

 exudation from cut phloem proves the possibility of mass flow through this 

 tissue. The observed cessation of such flow if a second cut is made above 

 the first and the continued flow if the second cut is made below points to 

 a mass flow of the sort postulated. An exudation of water from the cambial 

 surface when left in contact with phloem so that it can receive material 

 from the phloem, is added evidence, but evidence is presented by Weevers 

 and Westenberg denying such an exudation. 



30. There are many weaknesses in the Mtinch hypothesis. (1) The 

 proposed mechanism would not allow for simultaneous movement in both 

 directions through the phloem, yet there is a fair amovmt of evidence point- 

 ing to such a simultaneous movement. (2) Calculations indicate that the 

 pressure gradients available are insufficient to cause a flow of solution 

 through the conducting tissues into receiving cells and exudation of water 

 from the receiving cells. This is true especially where receiving cells are 

 at a great distance from the supplying cells, as is the case in certain trees 

 or vines where the receiving roots may be a great distance from the leaves. 

 (3) Naturally existing osmotic concentration gradients as well as turgor 

 gradients have been demonstrated in many cases to lead in a direction the 

 reverse of that required by the proposed hypothesis. The reduced turgor 

 of leaves during the day should reduce transport, but available evidence 

 indicates greater transport by day. Although it has been suggested that 

 this reduced turgor of the leaves is offset by increased tension in the receiving 

 cells, this would be effective only if the sieve tubes were rigid and could 

 transmit such a tension throughout their length. Wliere tissues are com- 

 peting for organic solutes often those with higher turgor receive the solutes, 



