THE CELLULAR CONSTRUCTION OF THE PLANT 25 



is struck between internal and external influences, and the condition 

 becomes quiescent. The explanation of these facts is found in changes 

 of the internal turgor of the cell. In normal life, the protoplast exer- 

 cises a stretching influence upon the cell-wall, keeping it tight and 

 tense like the outer cover of a pneumatic tyre. 



The state of turgor thus seen in the hving cells of plants originates 

 Irom the fact that the cell-sap is a solution of salts and sugars, etc., and 

 that the living cytoplasm, which completely surrounds it, behaves as a 

 semipermeable membrane : that is, it exercises control over the passage 

 of those substances outwards, though it allows the passage of water 

 inwards. The cell-wall is, however, a permeable membrane, allowing passage 

 to both. As the solutes in the cell-sap tend to absorb \\ater so long as 

 this is available (as indeed it is normally in the living plant), water will 

 then pass into the cell-cavity, increasing the volume of the cell-sap, and 

 of the cytoplasm round it, and the cell-wall will be stretched. This 

 goes on till the power of absorption is balanced by the resistance of the 

 distended cell-wall. Measured in terms of atmospheric pressure the turgor 

 of a living cell is commonly the equivalent of 5 atmospheres, and may in 

 some cases be as high as 25. What happens when a living cell is plasmolysed 

 is that instead of pure water bathing the outside of the cell, a salt-solution 

 is presented, which exercises a counter-osmotic influence to that of the solutes 

 in the cell-sap ; and if the outer influence be stronger than the inner, then 

 water will flow out through the cytoplasm and plasmolysis results. But the 

 film of cytoplasm, being the controlhng Hmit between the two solutions, it 

 will contract away from the cell-wall, which is readil}^ permeable. This 

 continues till a balance is struck between the external solution and the now 

 concentrated cell-sap. ISIeanwhile the stretching influence of the cytoplasm 

 on the cell-wall is relaxed when plasmolysis occurs, and the cell as a whole 

 shrinks. 



The turgor of the cell depends upon the ability of the cytoplasm 

 to control the passage of soluble substance inwards or outwards, and 

 to select those which shall be absorbed or retained. It may be 

 ranked as a vital phenomenon. If the cell be killed, by high tem- 

 perature or by poison, that power is lost. This may be beautifully 

 shown in preparations of cells in which the cell-sap is coloured, 

 if they are mounted alive in water. The tissues of the Red 

 Beet, or the superficial cells of the Rhubarb leaf will serve. So 

 long as the cells are alive they retain control over the dissolved 

 colouring matter, even after plasmolysis. When they die that 

 control is lost, and the colouring matter diffuses out freely into the 

 surrounding water. Such control is thus sJiown to be a property of the 

 living cytoplasm. 



The control over soluble substances by the living cell is physio- 

 logically most important. But not less is the mechanical importance 



