MICRO-PHYSICS OF THE CELL. 39 



A cell in this state is said to be plasmolytic. The contraction 

 is due to the withdrawal of water from the cell-sap by the strong 

 salt solution, this withdrawal not being compensated for by the 

 entrance of salt solution into the vacuole. The salt solution 

 diffuses through the cell- wall, and occupies the space between the 

 cell- wall and the contracted primordial utricle, but it cannot pass 

 through the primordial utricle to any considerable extent. 



On washing the section with water, the plasmolytic 

 cells gradually reassume their normal appearance. 



From these observations it is evident that the passage of 

 substances in solution into or out of the vacuole is controlled by 

 the primordial utricle so long as the cell is living. 



Plasmolysis can also be well demonstrated on a Fern- 

 prothallus by treating it as above with salt solution ; it 

 will be seen that the contracted primordial utricle is 

 connected to the cell-wall by a great number of delicate 

 protoplasmic filaments. 



III. Optical Properties. 



1. Double Refraction. 



In order to study this subject, apparatus for polarising 

 light must be adapted to the microscope. This consists 

 of two Nicol's prisms, one of which is fitted into an 

 eye-piece, the other being fixed below the stage of the 

 microscope, so that the light which is reflected from the 

 mirror must pass through it : the former prism is termed 

 the analyser, the latter the polariser. 



The sections to be examined may be mounted in 

 water or in glycerine, but the best results are obtained 

 with sections mounted in Canada Balsam. A twig of a 

 tree affords good material for observation. A thin, 

 nearly median, longitudinal section is to be made and 

 mounted : a high power must be used. 



