OSMOTIC iMji:ssri;K 31 



roots to lift <>reat stoiu's 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 throuf^h 

 studyin<>: this ])lu'n()nieiion that Pfett'er worked out the basis of our 

 present knowledge of osmotic pressure. If the cell is placed in a so- 

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

 will be greater than that inside and the protoplasmic membrane will 

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

 and perhaps 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 cell dies the turgor is lost 

 because the membrane becomes permeal)U\ and so pressure soon be- 

 comes the same on both sides of the cell wall. 



I21 animal cells the wall is not so highly developed as in plants, 

 nor is it backed up by a rigid material like cellulose ; indeed, for 

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

 appears to be naked. Nevertheless the behavior of the animal cells 

 indicates that they do possess what resembles a cell wall, in that 

 they behave when in solutions as if the^^ were surrounded by a dif- 

 fusion membrane. The degree to which phenomena of this nature 

 are shown varies with different cells; with red corpuscles, for example, 

 the osmotic pressure influences are very marked, as shown by the 

 wrinkling or crenation of the corpuscles when they are placed in 

 fluids of higher concentration than the blood plasma, and by their 

 swelling and disintegration with escape of the hemogiobin (hemoly- 

 sis) when they are put into distilled water or solutions of less con- 

 centration than the plasma. Other tissue cells seem to undergo more 

 or less alteration from changes in the osmotic pressure in the fluids 

 surrounding them. The diffusion membrane that surrounds the cell 

 is generally not well defined, and for most cells seems to be but a 

 surface condensation of the protoplasm, perhaps formed through the 

 effects of surface tension. It seems probable that this surface dif- 

 fusion membrane contains a large proportion of cell lipoids, i. e., 

 cholesterol and lecithin (for the red corpuscles this is practically 

 certain) ; hence substances soluble in lipoids penetrate the cell read- 

 ily, while to many substances insoluble in lipoids the cell is nearly 

 or quite impermeable (Overton). Probably the wall of the aninuil 

 cell is not so nearly semipermeable as is that of the plant cell, for 

 nowhere in the animal body do we get such turgor in the cells as 

 we see in plant tissues. Lacking a cellulose wall, animal cells could 

 not develop such an internal pressure without rupturing, and such 

 a process of rupturing {plasinorrhexis, plasmoptysis) does not seem 

 to be a normal occurrence in animal tissues. AVe shall be most 

 nearly correct, probably, if we look upon the animal cell as possess- 



