180 



BOTANY 



consequence, wilts. Such a flaccid plant plainly shows that the 

 rigidity is not maintained by its framework of cell walls, but by the 

 hydrostatic pressure within the cells, for with a more abundant water- 

 supply it returns to its original condition. 



In addition to loss by evaporation, water can also be withdrawn 

 from cells by the same molecular force which causes the internal or 

 endosmotic pressure. In cases where the cells 

 are surrounded by a solution which exerts an 

 attraction upon water, the turgidity of the 

 cells is proportionally weakened, and may be 

 altogether overcome. On account of the con- 

 sequent PLASMOLYSis, or the contraction and 

 separation of the protoplasm from the cell walls, 

 occasioned by the withdrawal of water, the 

 tension of the cell walls is decreased, and the 

 cell becomes flaccid and collapses (Fig. 174). 



Solutions of non-metallic organic substances possess 

 the same osmotic power when the number of molecules 

 dissolved in the litre is the same ; their " equimolecular 

 solutions" are "isosmotic." On account of their dis- 

 sociation when in solution into ions, metallic com- 

 pounds exert H, 2, 2 times this osmotic effect. These 

 numbers represent the "isosmotic coefficients" of these 

 substances. 



If placed in pure water, however, the previous tur- 

 gescence of the cells can be restored, if their proto- 

 plasm has not been too strongly affected by the action 

 of the solution. If the protoplasm has been killed 

 in the process, it becomes permeable to water, and it 

 FIG. 174. Internodal cell of j g no ] on g er possible to set up an internal pressure. 

 Xitella. F, Fresh and tur- . ,. i j c 



gescent ; p, with turgor Fresh sectlons ot Beet . placed in water, give up none of 

 reduced, flaccid, shorter their sugar or colouring matter ; but after the protoplasm 

 and smaller, the proto- has been killed, the sugar and colouring matter at once 



plasm separated from the escape into the surrounding water, and the sections lose 

 cell walls in folds; .<w, ,, . c , ,., 



their firmness and rijndity. 



lateral segments, (x circa J 



6 \ On the other hand, r ungi, Bacteria, or marine plants, 



when placed in a weaker saline solution or in fresh water, 



have the internal pressure of their cells increased. This may even lead to the 

 rupture of the cell wall (plasmoptysis). 



Through a knowledge of the strength of a solution necessary to produce 

 plasmolysis, a means is afforded of measuring the internal pressure within plant 

 cells. For example, if a solution of saltpetre with an osmotic pressure of 5 

 atmospheres (a 1 per cent solution, according to PFKFFKK'S investigations, gives 

 rise to a pressure of about 3^ atmospheres) is just sufficient to overcome the 

 turgidity of a plant cell (which in the case of stretched elastic cells shows itself by 

 the limit of contraction being reached), then, conversely, the cell sap exerts 

 upon water an equivalent endosmotic pressure. The force required to forcibly 

 stretch a flaccid or plasmolysed organ to its original length furnishes also a rough 

 means of estimating the pressure developed in turgescent tissues ( :t ). 



