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from the cell-wall. This contractility signifies, that under 

 certain circumstances of which we at present are ignorant, the 

 minute micellae of the protoplasm attract each other with 

 greater force than at other times, and hence the size of the 

 protoplasm diminishes. It has been laid down as a distinction . 

 between the animal kingdom and plants, that the latter are not 

 possessed of motion ; this, however, is not so, for the protoplasm 

 in plants is contractile, exhibiting movements, and probably 

 plasmolysis is an instance of this condition. 



As regards the properties of the cell-sap : it plays a very 

 important part in the physical or mechanical condition of the 

 cell. It is the fluid which keeps the cells in a tense or turgid 

 condition. This condition is one of the most important factors 

 in growth, and in the movements of motile organs in plants. 

 Let us enquire, then, by what means this tension is kept up. 

 When the cell is growing, if the quantity of cell-sap did not 

 increase with it, the cell would become loose and flaccid ; this 

 quality of turgidity depends on the presence of certain sub- 

 stances in solution in the cell-sap. In an ordinary case of osmosis, 

 where a membrane is placed between a strong solution of sugar on 

 the one side and water on the other, the water passes more rapidly 

 through the membrane than the sugar solution does: z'.e., the sub- 

 stance of the sugar has the power of attracting a certain amount 

 of water to itself until equilibrium is established — that is, until 

 the solution of sugar on both sides of the membrane is exactly 

 of equal strength— and then all further current ceases. In the 

 cell-sap certain substances exercise this same power, and attract 

 rather more water into the cell than it can conveniently hold, 

 and hence a distinct and important pressure is set up between 

 the cell-membrane and the cell-fluid. The protoplasm then 

 maintains a hydrostatic tension on the cell, and it does this in 

 virtue of the osmotic properties of these substances held in 

 solution in the cell-sap. What are these substances, then? 

 The cell-sap is a fluid of very low specific gravity, nearly the 

 same as that of water, and if we dry it, the amount of ash after 

 its evaporation is very slight ; and, this being the case, it is 

 difficult to see how the cell-sap exercises this extraordinary 



