PHYSICAL FORCES INVOLVED IN BIOLOGICAL ACTIVITIES 177 



In the discussions of osmotic pressure there has been constantly in 

 mind the action of solutions upon microorganisms. Either a cell-wall 

 or membrane exists as a distinct structural part as in the yeast cell or 

 the protoplasm comes in contact with its surrounding medium without 

 any distinctive cell-wall or membrane as in the amoeba. Whether 

 there is a layer of protoplasm on the outer surface of the amoeba which 

 has the functioning capacities of a distinctive cell-wall may not be easily 

 asserted for there is evidence pointing to the two possibilities. Inas- 

 much, however, as the passage of materials into the substance of the 

 cell is really that of diffusion or a modification of it, and species and 

 varieties respond differently to this diffusion, it is easily seen that 

 every species at least must be considered by itself in this respect and 

 values likewise determined. 



-p 



FIG. 106. Plasmolysis in cells (After DeVries from Macleod). 



It is well known that water will pass into some cells and cause them 

 to swell or fill out when apparently the substance of the cell or its fluid 

 content is more concentrated than the surrounding medium. On the 

 other hand, when the medium without is more concentrated than the 

 cell-contents, water flows from the cell toward the more concentrated 

 solution outside of the cell and accordingly the cell shrinks. This is 

 many times made evident by the contraction of the protoplasm. This 

 process in which the water is abstracted from the cell through osmotic 

 pressure is known as plasm oly sis. 



COLLOIDS AND CRYSTALLOIDS 



Since the time of Thomas Graham who established these two 

 classes of substances there has been a growing interest in them. At 

 present, however, instead of dividing substances into two classes 



placing one substance in one class, as colloids, and another distinct 

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