24 A TEXTBOOK OF PHYSIOLOGY 



contents. The solution of cane-sugar has therefore a greater osmotic- 

 pressure than the cell sap; it is termed a hypertonic solution. If, 

 instead of sugar, another substance, such as potassium nitrate (1 

 gramme-molecule per litre), be used, the solution formed is so strongly 

 hypertonic that the plasmolysis is very marked (Fig. 14, C). 



Plasmolysis may also be readily demonstrated by taking shavings 

 from a beetroot, carefully washing these and immersing them for 

 a time in 5 per cent, sodium chloride. The appearance under the 

 microscope before and after is very characteristic. The red corpuscles 

 of the blood behave in a similar manner. The delicate membrane 

 surrounding the corpuscle is permeable to water, but impermeable 

 to many dissolved substances. In this case, however, there is no rigid 

 cell wall forming the outer membrane. If, therefore, water passes 

 into such a cell, it will first swell up, and then burst, thus allowing the 

 contained red pigment to escape, a process known as the laking of 



FIG. 14. To SHOW THE EFFECT OF PLASMOLYSIS IN TRADESCANTIA DISCOLOR. 



(After De Vries.) 



h, Cell wall; k, nucleus; a, plastids; s, stream lines in protoplasm; p, protoplast. 



blood, or hsemolj-sis. A solution from which water passes into the 

 corpuscles is known as hypotonic. A solution from which water 

 passes in and out of the corpuscle in equal amounts is known as 

 isotonic. The concentration of sodium chloride required to form a 

 solution isotonic with nearly all mammalian bloods is 0-9 per cent.; a 

 solution of this 'concentration is termed " physiological saline " or 

 " physiological salt " solution. 



Hypertonic solutions diminish the volume of the corpuscle owing 

 to water passing out of them. It is suggested that this passing out of 

 fluid from living cells as the result of the action of hypertonic solu- 

 tions may affect the activity of such cells. Thus, it is possible that 

 in many plant cells the formation of starch from sugar only takes 

 place when the sugar concentration reaches a certain limit. Indeed, 

 it is found that with cells having a sugar concentration short of 

 this limit the formation of starch can be induced by producing plas- 

 molysis with a solution of potassium nitrate; this by withdrawing 



