MECHANISIVI OF TRANSLOCATION OF SOLUTES 499 



As shown in this figure two differentially permeable membranes, both 

 dipping in water, are connected by a tube to form a closed system. Membrane 

 X is assumed to enclose a stronger solution of sucrose than membrane Y. 

 Water will at first enter both membranes, but the greater turgor pressure 

 developed in X will soon be transmitted throughout the system. This will 

 result in a greater dii^usion pressure in the water in Y than in the pure water 

 in which the membranes are immersed. Water will therefore pass out of 

 the membrane Y, and coincidently there will be a flow of solution along the 

 tube from X to Y. The mass movement of solution from X to Y will con- 

 tinue until the concentrations of the sugar solutions in both membranes are 



Fig. III. Diagram of an osmotic system in which mass flow of solution will occur. 



equal. At this point tlie flow of solution in the tube will stop and a dynamic 

 equilibrium will be established between the solution in the closed system and 

 the circumambient water. 



If such an apparatus could be set up so that the sugar could be utilized 

 or be converted into an insoluble form as fast as it was translocated into Y, 

 and so that additional sugar could pass into solution in X as rapidly as it 

 moved out of that membrane, flow of solution from X to Y would continue 

 indefinitely. 



The Miinch hypothesis assumes that a sj'stem analogous to that just de- 

 scribed accounts for translocation of solutes through the phloem. Fig. 112 

 (from Crafts, 1931) illustrates diagrammatically how it is supposed to operate 

 as applied to the downward translocation of solutes. Cells Lj, L2, and L3 

 represent the green cells of the leaf and correspond to membrane X in Fig. ill. 

 Similarly R^^, Ro, R^ represent root cells which are analogous to membrane Y 

 in Fig. III. The continuous system of phloem connecting leaf and root cells 

 is represented by P. Similarly X represents the xylem and C the cambium. 

 The osmotic pressure of the leaf cells is maintained at a relatively high value 

 as a result of photosynthesis. In the root cells the osmotic pressure is (usually) 

 relatively lower because most of the sugars translocated into them are used 

 in metabolic activities or are converted into insoluble storage forms. Water 

 is supplied continuously to the leaf cells through the xylem. 



