THE VITAL PROPERTIES OF THE CELL 



130 



inducing a flow of water towards the exterior. " Essentially 

 osmosis consists in this, that two fluids simultaneously pass 

 through a membrane in opposite directions ; with regard to an 

 endosmotic equivalent (a term expressing the proportionate inter- 

 change, upon which there is frequently too much stress laid), this 

 cannot be spoken of in such cases where only water is diosmosed 

 through a membrane " (Pfeffer V. 23). 



On account of their fragility and small size, experiments upon 

 osmosis can only be made in animal cells with great difficulty. 

 Hence the osmotic processes have been investigated chiefly by 

 botanists in plant cells, which are much more suitable, and our 



Fig. 59. 1. A young, at most half-grown, cell from the cortical parenchyma of the flower 

 peduncle of Cephalaria leucantha. 2. The same cell immersed in a 4 per cent, solution. 3. 

 The same cell in a 6 per cent, solution. 4. The same cell in a 10 per cent, solution (Nos. 1 

 and 4 are taken from nature, Nos. 2 and 3 are diagrammatic ; all in optical longitudinal 

 section). 7i Peripheral layer ; p protoplasmic coating of wall; 7c nucleus; c chlorophyll 

 granules; s cell sap; e salt solution which has penetrated into the interior. After de 

 Vries (V. 36). 



knowledge has been especially advanced by the following experi- 

 ments. 



If plant cells containing a large sap space are placed in a 5 to 

 20 per cent, solution of a suitable salt, or of sugar or glucose 

 (Fig. 59), they are seen to diminish somewhat in size from having 

 given up water from the interior to the exterior; in consequence, 

 as this process of water abstraction proceeds, the protoplasmic 

 coating becomes separated from the cellulose membrane, which, on 

 account of its greater firmness, is unable to shrink any more 

 (de Vries V. 36). 



