50 THE BIOLOGY OF FLOWERING PLANTS 



the turgor pressure of the cell sap ; it is an expression by the 

 solvent of the osmotic pressure which is due to the solute. 

 As a result the cell becomes distended and stiff, or turgid. 

 When the cell wall is extended to its maximum water of 

 course ceases to enter, and the whole of the osmotic pressure 

 is used up as turgor pressure acting against the cell wall. 

 When the cell wall is not fully turgid, only a part of the 

 osmotic pressure is used up as turgor pressure, and the 

 balance is available to draw in water. The actual force — 

 suction force — available for drawing water into the cell is 

 therefore the difference between the osmotic pressure and 

 the turgor pressure ; or rather, it is this difference less the 

 opposing forces which tend to retain water in the soil. 



A root hair freely supplied with water would soon become 

 fully turgid, and no suction force would exist were it not 

 that water is constantly passing from it into the root ; a 

 small water deficit is therefore maintained, and this suffices 

 to keep up a flow of water into the plant. This case is 

 realised in water plants. In land plants the deficit is likely 

 to be larger ; the more difficult the water supply, the greater, 

 within the limits of the available osmotic pressure, will be 

 the suction force, and if, as probably happens in very dry 

 soils, the water deficit becomes so great that the cell borders 

 on plasmolysis (shrinking of the plasma lining from the 

 wall), the full osmotic pressure will be available as suction 

 force. 



In dry soils forces other than osmotic pressure are active, 

 both in the cell and in the soil. It has already been 

 noted that the mucilaginous cell wall unites with the col- 

 loidal constituents of the soil ; and thus the relatively 

 great forces of imbibition of these colloids must come into 

 play, if not continually, at all events when the soil moisture 

 has been reduced to a certain point. The extent of these 

 forces, so far as the cell is concerned, is quite unknown. 



Osmotic Pressure o! Root Cells.— As to the osmotic 

 forces at the disposal of the cell we have a good deal of 

 information. Measurements have usually been made of 

 the osmotic pressure without reference to the suction force. 



