82 PLANT LIFE 



required to leave about 1 % in coarse sand. 

 These experiments are of great value in 

 enabling us to see the way to attack many 

 problems of plant and soil relations, and they 

 show that the notion that the plant has forcibly 

 to wrest the water from the soil is a fairly 

 accurate one. 



How, then, does the root-hair do this ? 

 What force can it exercise in the process ? 



Experiments show that a plant cell will, 

 in general, absorb and retain water with 

 considerable avidity. This it does by means 

 of the so-called osmotic pressure exerted within 

 it by various substances, such as sugar, 

 organic acids, and the like, which are dissolved 

 in the watery sap within the cell. For whereas 

 water can pass freely in and out of the cell, 

 the protoplasm either does not allow the 

 dissolved substances to pass out, or it only 

 lets them through very slowly. Without 

 going at all fully into the difficult and complex 

 subject of osmotic pressure in general, it may 

 be remarked that, under these circumstances, 

 water tends to flow into the cell and to such 

 an extent that the cell sap exerts a very 

 considerable pressure. This may easily 

 reach a value equivalent to about eleven 

 atmospheres. It is this circumstance which 

 at least partly accounts for absorption of 

 water from even relatively dry soil by the 

 root-hairs, to make good that which is 

 lost from other parts of the plant. For, as 

 already explained, the parts of the plant above 



