CHAPTER XI 

 THE OSMOTIC QUANTITIES OF PLANT CELLS 



The Plant Cell as an Osmotic System. — We have already become ac- 

 quainted with the typical plant cell as a three dimensional structure, limited 

 by cell walls; these walls in turn being lined with a laj'er of protoplasm 

 which encloses a vacuole filled with cell sap. The protoplasm is normally 

 held against the cell wall by a pressure exerted by the cell sap. The cell 

 walls of most living cells are elastic within limits, but possess sufficient tensile 

 strength so they do not commonly rupture. The cytoplasm, although usually 

 possessing a high degree of fluidity, is usually much less elastic than the walls. 

 The cell wall membranes of most living plant cells appear to be quite freely 

 permeable to water and to most substances dissolved in it. The cytoplasmic 

 membranes, on the contrary, almost invariably possess the property of dif- 

 ferential permeability. Water and certain solutes pass through them much 

 more readily than certain other solutes. 



Plasmolysis. — If a plant cell which is at least partially distended is 

 immersed in a hypertonic ^ solution a characteristic series of changes takes 

 place in the appearance of that cell. The first detectable occurrence is a 

 gradual shrinkage in the volume of the entire cell due to outward osmosis, 

 and a consequent reduction in the pressure exerted by the cell sap against 

 the protoplasm and cell wall. This shrinkage in volume can be detected in 

 many cells by actual measurement under a microscope, although there are 

 some types of plant cells in which little or no change in volume occurs under 

 such conditions. There is a lower limit to the elasticity of the cell wall, 

 however, and when this is attained no further contraction in the volume of 

 the cell will occur. Since the cell wall is quite freely permeable to the water 

 and the solutes of the surrounding solution, and hence the hypertonic solution 

 is in contact with the outer surface of the protoplast, the cell sap will continue 

 to lose water by outward osmosis, just as if no cell wall were present. Hence 

 the protoplasm will continue to shrink in volume after the contraction of the 



^ With reference to a. given solution, in this case the cell sap, a hypertonic 

 solution is one with a higher osmotic pressure than the reference solution. 

 Similarly an isotonic solution is one with an osmotic pressure equal to that of the 

 reference solution, while a liypotonic solution is one of lesser osmotic pressure. 



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