28 THE CHEMISTRY AND PHYSIOS OF THE CELL 



crease at first, but eventually the sugar will diifuse 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 

 eventuall}^ tlu; sugar can all pass through. 



Exactly similar conditions exist in cells, particularly plant cells. 

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

 cellulose, 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 readil}- permeable b}' 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 from 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 the cell wall. Because of the strength of the 

 cellulose layer the cell can withstand great pressures that Avould 

 tear apart the tender protoplasmic layer that really determines the 

 osmotic pressure that causes the rigidity or turgor of plant (-('lis, 

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

 right 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 certain 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 pn^sent 

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

 of greater concentration than its cell sap, the pressure outside will 

 be greater than that inside and the protoplasmic meml>rane will be 

 forced away from the cellulose wall, while its central cavity shrinks 

 and i)erhaps disappears entirely, the protoplasm forming a ball in the 

 center. This is practically what occurs when a plant stem is cut 

 and it "wilts" — the water is removed by evaporation, the osmotic 

 pressure outside the cells becomes greater than that inside, and the 

 water passes out. Likewise when a plant c(>ll dies the turgor is lost 

 because the membrane becomes ))ermeable, and so pi'(>ssure soon be- 

 conuis the sanu^ on both sides of the cell wall. 



In animal cells the wall is not so highly developed as in plants, 

 nor is it backed up b}^ a rigid material like celluIos(>; indeed for 

 many animal cells there is no well-defined wall and the piotojilasm 

 appears to be naked. Nevertlu^le.ss the behavior of tlu> animal cells 

 indicates that they do possess what resiMubles a ('(>11 wall, in that 

 they behave when in solutions as if they wei-e surrounded by a dif- 

 fusion membi'aTie. The degi-ee to which piieiioineii;! ot' this iiatui'e 



