PHYSIOLOGICAL 391 



inside the pot rises above the level of the pure water outside, and 

 accordingly there is a higher pressure inside than out, as expressed 

 by the extra "head" of water. This "osmotic" pressure finally 

 reaches a constant value; which depends on the strength of the 

 solution, and the nature of the dissolved substance. It is now 

 generally recognised that the external surface of the cell has to 

 some extent the properties of a semi-permeable membrane ; and we 

 therefore find within the cell an osmotic pressure, if the medium 

 in which the cell is bathed is a relatively weak solution. The pressure 

 within the cells may serve to make them rigid by turgor, as is seen 

 in many plant tissues. If the external medium of the cell be a rela- 

 tively strong solution, water will pass from within outwards, and 

 the cell will shrink. These alterations of osmotic pressure are 

 important in connection with the rapid movements of some of the 

 parts of plants, such as the leaves of the Sensitive Plant. It must be 

 noted that the external membrane of the cell is peculiar in its 

 behaviour when compared with artificial semi-permeable membranes. 

 We cannot at present quite re-describe in terms of artificial semi- 

 permeable membranes what the living cell is able to do in the way 

 of allowing certain dissolved substances to pass through, while 

 barring the way to others, for the livingness of the membrane makes 

 a difference which can as yet neither be explained nor ignored. 



Secondly, there is within the cell a pressure — normally much less 

 mportant than the first — which is due to the tendency of colloidal 

 jellies, such as form the framework of the cell, to take up water by 

 "imbibition". This is comparable to the swelling up of dry gelatine 

 in water and to the swelling of the jelly that buoys up the frog's 

 spawn in the pond. 



Thirdly, the cell has an internal pressure due to surface tension. 

 There is always a tension at the surface where a liquid meets another 

 liquid with which it does not mix, or at the surface where it is in 

 contact with the air; and the effect of this tension is to make the 

 surface smaller. It is this force of surface tension that makes water 

 fall from a slightly open tap in droplets instead of in a continuous 

 thin stream. At the surface of the cell there is a tension, the effect 

 of which is to make it contract ; and this is the third way in which 

 a pressure is maintained within the cell. 



THE ENVIRONMENT OF THE CELL: THE PERMEABILITY 

 OF ITS MEMBRANE. — So far we have been considering the 

 cell as a little world in itself, but this view is to be corrected by 

 the picture of the cell as an area in the larger world of the body. 

 So we naturally pass to the relations between one cell and another, 

 and to the relations between a cell and the ambient fluids of the 

 body, notably — in Vertebrates — the lymph. 

 The property of forming a film at its surfaces is one of the most 



