144 



TRANSLOCATION IN PLANTS 



and a low concentration at the other. This work was 

 reported at the Baltimore meeting of the Botanical Society 

 of America in 1918, but it was never published. At that 

 time I suggested that a similar mechanism might account 

 for root pressure. A comparable mechanism is also 

 described by Blackman (1921), and Ursprung and Blum 

 (1925) have described a method whereby a unilateral 

 suction tension resulting from such a condition can be 

 measured in living plant cells. 



The basic principle of unilateral flow is indicated by 

 Miinch in a diagram similar to Fig. 9. Membrane A 



Fig. 9. — Diagram to demonstrate basic principle of osmotic flow. A, osmotic 

 membrane with high concentration; B, osmotic membrane with low concentra- 

 tion connected by open tube T. Feathered arrows indicate flow of solution 

 from cell or part of cell with high concentration to that with low. Plain arrows 

 indicate direction of flow of water. (From Miinch.) 



contains a solution of high osmotic concentration. The 

 tube T is filled with water as is also membrane B. Both 

 membranes A and B dip into water W. Because of the 

 steep diffusion gradient across the membrane at A, water 

 moves in by osmosis developing a pressure in A. This 

 develops a pressure throughout the system, and, since 

 solutes are lacking in B, or their concentration is low in 

 that region, there will be little or no resistance to the 

 diffusion of water through the membrane B to the external 

 water. As a result there will tend to be a mass flow of 

 solution from A through the tube toward B. As the solute 

 concentration in B rises, there will be a rising resistance 

 to the diffusion of water from B to W, and the pressure 

 in the system will increase. Such a gradual rise in pressure 



