38 BOTANY OF THE LIVING PLANT 



is eventually reached at which the available osmotic forces are unable 

 to stretch the wall any further. The suction pressure is now nil, and 

 the cell has no capacity for further absorption of water. On the other 

 hand the pressure of the cell-contents on the wall — known as the 

 Turgor Pressure — is now at its maximum, the whole of the osmotic 

 pressure of the cell-sap being exerted on the wall. The cytoplasmic 

 membrane and the wall of such a turgid cell are in a state of inflation, 

 and may be compared with a blown-up football, the membrane 

 corresponding to the bladder, the cell-wall to the case. 



The possible development of turgor pressures of values ranging 

 from 5 to 20 atmospheres within relatively thin-walled cells will 

 naturally occasion some surprise. Actually under normal conditions 

 plant cells are not fully turgid, as is indicated by the swelling usually 

 shown on placing a living tissue in water. Also it should be remem- 

 bered that most cells are exceedingly small, and what would be 

 impossible in a larger structure may be quite possible in a smaller 

 (Chapter X.). 



By command of such high osmotic forces as those above quoted, 

 upon which the growth of the cells depends, the organs of plants are 

 able to overcome considerable resistances (Chapter IX.). Thus the 

 root forces its way into the soil, while the shoots of plants occasionally 

 lift asphalt or flagged pavements. The turgor of the individual 

 cells also gives such degree of firmness and rigidity as is seen in 

 sappy and herbaceous plants, which they lose on wilting or withering 

 or on plasmolysis (Chapter X). Like the football or the pneumatic 

 tyre they are rigid when distended, but flaccid and limp when the 

 pressure falls. In pneumatic tyres it is a pressure of compressible 

 gas that gives the mechanical effect : in the plant cell it is a turgor 

 caused by an accumulation of incompressible liquid. But in both 

 cases the mechanical effectiveness depends upon the resistance of 

 the elastic outer cover. 



The Living Cell and Dissolved Substances. 



It is frequently stated that the cytoplasmic lining of the cell is 

 semi-permeable to the cell-sap solution. It is, however, obvious that 

 the mineral salts of the sap must have entered the cell from without, and 

 unless the cell contains chloroplasts any sugars that are present must 

 also have passed from another cell into the one in question. In fact 

 the physiology of the plant requires that there shall be free diffusion 

 of dissolved substances from one living cell to another. It seems that 



