Chapter 2. 
ROOTS AND ROOT NUTRITION. 
Absorption by roots and root pressure. Physical and chemical 
nature of soil. Manures and their importance. Bacteria in 
soil. Root tubercles of Leguminous Plants. Rotation of crops. 
Trenching and ridging. 
We have seen that the sap is raised in plants in part 
by the transpiration of water vapour which takes place 
through minute pores which are found scattered over the 
surface, particularly the lower surface, of leaves. In 
addition to this suction exerted by the leaves there exists 
a definite upward pressure of the sap by the roots known 
as root pressure. ‘Vhis upward force is due to the fact that 
the roots of plants are covered in by a porous membrane, 
and that the cells of the root contain a cell sap of greater 
concentration than the water contained in the soil. When- 
ever such a condition of things occurs a physical law 
determines that water passes through such a permeable 
membrane from the less dense to the. denser liquid. 
Water is thus attracted into the cells of the root and, 
causing them to swell, it forces the liquid up certain con- 
ducting channels of the root and into the stem. 
If, during a period of active root absorbtion, we cut 
a plant above the surface of the soil we find that water 
will be forced out of the exposed wound and, fixing a 
glass tube tightly in the place of the stem which has been 
removed, we find that the liquid which exudes can be 
forced up to a considerable height. 
' Particularly in spring when root absorbtion is very 
active the pressure exerted by the roots may be very con- 
9 
