THE CHEMISTRY AND PHYSICS OF THE CELL 37 



pressure of 3075 millimeters of mercury at 14 (about sixty 

 pounds to the square inch). To produce osmotic pressure it is 

 not necessary that the membrane be absolutely impermeable to 

 any of the substances it may only be relatively less permeable 

 for the solute than for the solvent. If, for example, we fill a 

 parchment bag with concentrated sugar solution, tie up the top 

 tightly and throw into water, it will swell up rapidly and 

 eventually burst. But if the parchment is in the form of a 

 tube, open at the top, and the lower end is placed in water, the 

 amount of fluid inside the tube will increase at first, but event- 

 ually the sugar will diffuse out to such an extent that the solu- 

 tion is of the same concentration inside and outside of the tube, 

 and the column of fluid will again become of equal height on 

 both sides. These results indicate that the water passes through 

 the membrane more rapidly than does the sugar, but that event- 

 ually the sugar can all pass through. 



Exactly similar conditions exist in cells, particularly plant 

 cells. The typical cell of plant tissues consists of a cellulose 

 wall, lined internally by a layer of protoplasm which inclqses a 

 mass of aqueous solution, the cell sap, containing sugar and 

 various other solutes. The cellulose wall is readily permeable 

 by water and by most solutes, whereas the protoplasmic layer 

 inside it behaves like a semipermeable membrane which per- 

 mits water to pass through readily but hinders greatly the pas- 

 sage of most solutes ; that it is somewhat permeable is attested 

 by the fact that the cell sap contains solutes derived from the 

 external fluids. As a result of this arrangement there is a con- 

 stant tendency for the cavity of the cell to be distended by 

 water and for the solutes within it to exert their considerable 

 pressure upon the cell wall. Because of the strength of the 

 cellulose layer the cell can withstand great pressures that would 

 tear apart the tender protoplasmic layer that really determines 

 the osmotic conditions, just as in the experimental membrane 

 the clay cylinder supports the delicate precipitation membrane. 

 It is the osmotic pressure that causes the rigidity or turgor of 

 plant cells, and explains the ability of a tender green shoot to 

 hold itself upright or horizontal in the air ; and it is the force 

 that enables growing roots to lift great stones or tear apart 

 rocks in whose clefts they grow. If plant cells are placed in 

 distilled water, the pressure may rise to such an extent that the 

 cells burst, and it was through studying this phenomenon that 

 Pfeffer worked out the basis of our present knowledge of 

 osmotic pressure. If the cell is placed in a solution of greater 

 concentration than its cell sap, the pressure outside will be 



