136 DIFFUSION AND OSMOTIC PRESSURE 



effective turgor pressure of the mass of liquid within. But 

 since protoplasm and cellulose wall can be parted so readily 

 during plasmolysis, the first alternative is well-nigh untenable. 

 If the wall be forced into the spherical shape by a change in the 

 pressure from within this must be brought about by a change 

 in the volume of the contained liquids. Now, this slight 

 change in volume which might produce a change in the tur- 

 gidity of the cell is most probably due to an alteration in the 

 amount of cell sap within the vacuole. When the surround- 

 ing medium suffers change in concentration, a change in the 

 volume of the vacuole may come about through the proto- 

 plasmic sac either secreting liquid or acting merely as a 

 semi-permeable membrane. 



When filaments are placed in a concentrated solution 

 their behavior suggests at once partial plasmolysis. Water 

 may be extracted, the effective turgor pressure on the walls 

 may be decreased, and by the forces of surface tension and 

 cohesion the protoplasm may tend to round itself up into a 

 sphere. If this be true, we have an explanation of the lateral 

 bulging which accompanies the longitudinal shrinking of the 

 cellulose envelope. If the protoplasm tended to assume a 

 spherical form within the cylindrical wall, the pressure upon 

 this would be decreased first at the angles. At the same time, 

 it would be relatively increased upon the lateral walls near 

 their middle. Thus would come about a bulging of the lateral 

 walls outward, and hence a shortening of the cell and a draw- 

 ing of the end walls toward each other. But the internal 

 pressure is to be counted as almost nothing at the angles, 

 while it is still considerable in the middle of each end wall. 

 So the margins of the end walls would approach the middle 

 of the cell more rapidly than do their central portions, and 

 splitting of the common membrane of two adjacent cells 

 would necessarily ensue. Several facts were observed in the 

 cultures which seem to support some such hypothesis as the 



