THE METHOD OF MOVEMENT 145 



was actually demonstrated in a working model I described 

 in 1918. If, on the other hand, the solute is removed in 

 some way when it reaches B, water will continue to pass 

 out readily through the membrane, and solution will 

 continue to flow into B. 



Mlinch applies the same principle of unilateral secretion 

 to the plant. He suggests, for example, that the chloren- 

 chyma cells of the leaf manufacture sugar from carbon 

 dioxide and water, both of which readily diffuse through 

 protoplasmic membranes. The cell membrane is not easily 

 permeable to the sugar, so an osmotic pressure is set up 

 in the cell. Plasmodesma, however, which connect such 

 chlorenchyma cells with other similar living cells and 

 eventually with sieve-tube cells, are assumed to allow for 

 a mass flow of sugar solution into the sieve tubes which, 

 with their larger pores, allow for continuous flow for some 

 distance. That side of a cell abutting on the dead water- 

 supplying cells would not be permeable to sugar; for on 

 that side there would be a hmiting protoplasmic membrane. 

 The comparatively large pores of the sieve tubes should 

 allow for a relatively rapid mass flow of solution through 

 them. The receiving cells, such as the cambium, by 

 removing the sugars from solution through respiration, 

 deposition in cell walls, or other conversion to insoluble 

 material, or to materials of higher molecular weight, would 

 tend to have a low osmotic concentration and would 

 therefore readily give up water through their semiper- 

 meable membranes; very much as B in the diagram, will 

 lose water to W. 



A simplified diagrammatic representation of the system 

 as it may occur in plants as suggested by Miinch is pre- 

 sented in Fig. 10. Miinch gives diagrams and descriptions 

 of the possible working of such a mechanism of transport 

 under three widely differing sorts of conditions: (1) Trans- 

 port under conditions of suppressed transpiration and water 

 absorption; the figure is similar to that here given, but 

 the two cells at the base are omitted and no water is lost 

 from the cell P to the atmosphere. (2) Transport in a 



