32 PRACTICAL BOTANY 



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. 



IV. 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 transverse section (p. 7), 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 under a low 

 power 



1. The thin cell-walls. 



2. The layer of protoplasm (primordial utricle), 

 which lines the cell-wall. 



3. The red cell-sap filling the cavity of the cell 

 (vacuole). Note that the red sap does not escape 

 from uninjured cells. 



a. Examine a similar section which has been dipped 

 for a moment into alcohol and thus killed : the red sap 



