232 



PLANT PHYSIOLOGY 



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solution, water leaves the cell. When the amount in the vacuole 

 reaches a certain minimum, the protoplast withdraws from the 

 cell wall and becomes balled up in the middle of the cell, leaving a 

 space between it and the cell wall. These changes are especially 

 easy to observe in cells with colored cell sap such as the epidermal 

 cells of the primrose (Primula). In this condition the cell is 

 flaccid or plasmolyzed and the state of the cell resulting from this 

 excessive exosmosis is called plasmolysis. If the osmotic pressure 

 inside the cell is greater than that outside, water will enter the 

 cell; while if the greater osmotic pressure is in the liquid on the 



outside, water will leave the cell; and if 

 the osmotic concentration is the same 

 outside as inside, no change in the move- 

 ment of the water will be observed. 



Solution Osmotic Pressure. — By solu- 

 tion osmotic pressure is then meant the 

 pressure which would be exerted by a 

 liquid if it were confined in a vessel and 

 separated from pure water by a mem- 

 brane which is absolutely semipermea- 

 ble, i. e., which lets water enter freely 

 but prohibits the solutes from passing 

 out. Thus we say that a 1% sucrose 

 solution has an osmotic pressure of 0.69 

 atmosphere, because it is capable of ex- 

 erting that much pressure per square 

 Fig. 12.- -If the glass tube inch wheR la(jed [n ft container wh i c h 

 is closed by a semipermea- ^ 



ble membrane with a 1% is permeable to water and not to sugar. 



sugar solution on the inside This can be illustrated further by sup- 



and pure water on the out- pogmg the gugar soIution to be in a 

 side, then a torce ot lU.oo _ . , . 



lbs. is required to keep the cylinder at the top of which is a piston. 



piston, P, from rising, i. e., The bottom (Fig. 12) is closed by a semi- 

 from being forced up, if it has permea ki e membrane. If the cross sec- 

 an area ot one square inch. .-,,,, r 



tion of the chamber has an area ot one 



square inch, it would take 0.69X15 lbs. or 10.35 lbs. to keep the 

 piston from being forced upwards. 



If two solutions have the same osmotic pressure they are said 

 to be isotonic. If they are different, the lower one is said to be 

 hypotonic in reference to the other and the higher one is hyper- 

 tonic in reference to the lower. 





