30 Tin: (iiEMisruY wn rinsicH of the cell 



through slowly or not at all, tliere will soon be an unequal condition on 

 the two sides of the membrane, for tlie diffusible substance would ac- 

 cumulate in equal amounts on each side, while the non-diffusible would 

 remain where it was. On one side there would then be more material 

 exerting osmotic pressure than on the other, and if the membrane 

 were flexible, it would bulge toward the opposite side. The pressure 

 is supposed to be due to the bombardment of the containing walls by 

 molecules or ions of the substances in solution, and hence the more 

 molecules and ions in solution, the more pressure. When equal num- 

 bers of particles are on each side of the partition, the pressure is 

 equalized. It is quite possible to have membranes readily permeable 

 to one substance and almost entirely impermeable to another; such 

 membranes are called semipermeable. To produce osmotic pressure 

 it is not necessary that the membrane be absokitely 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 in- 

 crease at first, but eventually the sugar will diffuse out to such an 

 extent that the solution is of the same concentration inside and out- 

 side 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 

 eventually the sugar can all pass through. 



Exacth' similar conditions exist in cells, particularly plant cells. 

 The typical cell of plant tissue consists of a distinct wall, usually 

 cellulose or ehitin, lined internally by a layer of protoplasm which 

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

 various other solutes. The outer wall is readily permeable by water 

 and by most solutes, whereas the protoplasmic layer inside it behaves 

 like a semipermeable membrane, which permits water to pass through 

 readily but hinders greatly the passage of most solutes; that it is 

 somewhat permeable is attested by the fact that the cell sap contains 

 solutes derived fnmi the external fluids. As a result of this arrange- 

 ment there is a constant tendency for the cavity of the cell to be 

 distended by water and for the solutes within it to exert their con- 

 siderable pressure upon tlie cell wall. Because of the strengtli of the 

 cellulose layer the cell can withstand great pressures that would 

 tear apart tlie tender protopla.smic layer that really determines the 

 osmotic conditions, just as in the exjierimental membi-ane the clay 

 cylinder supports the delicate i)recipitation membrane. It is the 

 osmotic pressure that causes the rigidity oi- turgor of plant cells, 

 and explains the ability of a tender gi-ecii shoot to hold itself up- 

 j'iglit oi- horizontal in the air; and it is the foi-ce that enables growing 



