48 THE MECHANISM OF LIFE 



up and fades. When, however, we place the shrivelled flower 

 in water, the contracted protoplasm swells up again and 

 refills the cells, which become turgid, and the flower revives. 

 This phenomenon is due to the fact that vegetable protoplasm 

 holds in solution substances like sugars and salts which have 

 a high osmotic pressure. Consequently water has a tendency 

 to penetrate the cellular walls of plants, to distend the 

 cells and render them turgescent. De Vries has used this 

 phenomenon for the measurement of osmotic tension. He 

 employs for this purpose the turgid cells of the plant 

 Tradescantia discolor. The cells are placed under the micro- 

 scope and irrigated with a solution of nitrate of soda. On 

 gradually increasing the concentration of the solution there 

 comes a moment when the protoplasmic mass is seen to 

 contract and to detach itself from the walls of the cell. 

 This phenomenon, which is known as plasmolysis, occurs at 

 the moment when the solution of nitrate of soda begins to 

 abstract water from the protoplasmic juice, i.e. when the 

 osmotic tension of the nitrate of soda becomes greater than 

 that of the protoplasmic liquid. So long as the osmotic 

 tension of the soda solution is less than that of the protoplasm, 

 there will be a tendency for water to penetrate the cell wall 

 and swell the protoplasm. When the osmotic tension of 

 the solution which bathes the cell is identical with that of 

 the cellular juice, there is no change in the volume of the 

 protoplasm. In this way we are able to determine the 

 osmotic pressure of any solution. We have only to dilute 

 the solution till it has no effect on the protoplasm of the 

 vegetable cells. Since the osmotic tension of this protoplasm 

 is known, we can easily calculate the osmotic tension of the 

 solution from the degree of dilution required. 



lied Mood Corpuscles as Indicators of Isotony. — In 1886, 

 Hambunrer showed that the weakest solutions of various 

 substances which would allow the deposition of the red 

 blood cells, without being dilute enough to dissolve the 

 haemoglobin, were isotonic to one another, and also to the 

 blood serum, and to the contents of the blood corpuscles. 

 This is Hamburger's method of determining the osmotic 



