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 
