PRACTICAL EXERCISES 21 



especially the case in the region near the apex of the pro- 

 thallus. 



c. Irrigate a fresh specimen with a 2j per cent, solution of 

 common salt, and watch the result : it will be seen that the 

 protoplasm contracts, often taking the form of an almost 

 spherical ball, thus separating from the cell-walls with which it 

 was originally in contact : the latter will now appear as a 

 continuous network of partitions dividing the whole prothallus 

 into a number of chambers. 



If this preparation be examined under a high power, a 

 number of delicate protoplasmic filaments may be seen con- 

 necting the outer surface of the contracted protoplasm with 

 the cell-wall : this indicates that the two bodies are not 

 merely in apposition in the living cell, but are closely con- 

 nected. 



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 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 such observations as these it is concluded that the 

 passage of substances in solution into or out of the protoplasm 

 is controlled by the primordial utricle so long as the cell is 

 living. 



III. These osmotic properties of the cell can be easily 

 studied in cells which have coloured cell-sap, such as those of 

 the garden Beet : this will at the same time serve as a first 

 exercise in cutting sections from a solid mass of tissue. 



Cut a tranverse section (p. 6) of a piece of a fresh Beet-root 

 sufficiently thin to be transparent, but of such thickness that at 

 least some of the cells shall remain uninjured, and mount in 

 water : observe 



I. The thin cell- walls. 



