92 BRITISH PLANTS 
restored. In this way, the roots can explore for water 
a larger extent of soil than that actually occupied by the 
root-hairs, the suction extending farthest where the 
“ capillary tubes ”’ are narrowest. ; 
In most cases the particles round which the water- 
films occur are insoluble solids, the films alone containing 
the soluble matters. In clay, however, the particles of 
which consist chiefly of silicate of alumina, the outer 
layers form with water a jelly-like material surrounding 
the grain. This jelly-like form of the silicate has a 
greedy attraction for water, and, when wet, swells like 
mucilage, blocking up the narrow pore-spaces. It is 
partly for this reason that wet clay is greasy to the touch, 
so difficult to dry, so absorbent of solutions, and almost 
impermeable to the passage of water. 
The root-hairs present on the roots of plants are merely 
certain surface-cells very much elongated to increase the 
absorbing surface of the roots. There are no actual 
holes for the passage of water as there are in the leaves 
for the interchange of gases. The water passes through 
the walls of the root-hairs into the interior of the cells by 
a physical process known as liquid-diffusion, or osmosis. 
To reach the cell-sap, it is clear that the water must 
penetrate two very different membranes : 
1. The outer cell-wall, or membrane, formed of cellulose, 
and non-living. This membrane is permeable to water 
and all watery solutions of mineral salts, whatever their 
strength may be. The activity of the diffusion-current 
depends upon the strength of the solution outside the cell 
compared with the strength of the solution inside, and 
it becomes greater as the difference between the concen- 
tration of the two liquids increases. If the soil-water is 
very dilute, there is an active current of water passing 
inwards. If, on the other hand, the external water 
contains a large amount of soluble salts, and its concen- 
tration approaches that of the cell-sap, the quantity of 
water diffusing inwards is very small; and if its concen- 
tration exceeds that of the sap, the current is actually 
reversed, water is withdrawn from the cell, and the 
protoplasm collapses (plasmolysis—Gr. plasma, the proto- 
plasm ; lysis, loosening). The result to the plant is dis- 
astrous. It is for this reason that only very dilute solu- 
tions pass right through to the inside of the cell, and that 
