H 



PHYSIOLOGY. 



to move toward the center of the cell. When the salt solution 

 is removed and the thread of spirogyra is again bathed with 

 "water, the movement of the water is inward in 

 the cell. This would suggest that there is some 

 substance dissolved in the cell-sap which does not 

 readily filter out through the membrane, but draws 

 on the water outside. It is this which produces 

 the pressure from within and crowds the mem- 

 brane out against the cell wall again. 



Fig. ii. 

 Spirogyra before 

 placing m salt solu- 

 tion. 



Fig. 13- 



Spirogyra from salt 



solution into water. 



Fig. 12. 

 Spirogyra in if, salt solution. 



31. Tumescence.— Were it not for the resistance which the 

 cell wall offers to the pressure from within, the delicate proto- 



