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-hke 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 ceU 

 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 ceU-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 reversed, water is withdrawn from the cell, and the 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 coUapses (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 



