26 



BOTANY: PRINCIPLES AND PROBLEMS 



surface, internal or external, which may be wetted (such as a 

 sponge, blotting paper or coarse fabric) will therefore hold within 

 itself, when thoroughly soaked, a great amount of water which 

 will not drain out under gravity. For exactly the same reason, 

 much of the water entering a soil will fail to percolate through it 

 but will instead adhere in thin films to the surfaces, very great in 

 total area, which are presented by the multitude of soil particles. 

 If the amount of rainfall is small, all of it may thus be retained 



. Air-SpacG 

 .Water- Film 



•Rock-Parficle 



Fig. 14. — Section through soil, much enlarged. A capillary water-film 

 surrounds each particle and fills the narrow spaces between particles. The 

 larger spaces are occupied by air. Much enlarged. 



and none lost through percolation. Each particle in such a moist 

 soil is covered by a thin layer of water (Fig. 14). The films about 

 adjacent particles coalesce, filling the minuter spaces and lining 

 those that are larger, and a continuous film-system is thus set 

 up. It is this film or capillary water which furnishes plants 

 with the great bulk of their water-supply. One of the important 

 objects in manipulating a soil is to increase, by one means or 

 other, this water-holding capacity, and thus to prevent waste 

 through run-off or percolation. 



The principle of capillarity is of further importance in deter- 

 mining all movements of water in the soil other than the down- 

 ward one due to gravity. The familiar fact that when a narrow 

 glass tube is placed in water, the water will rise inside the tube 

 to a point somewhat higher than its level outside, is due to the 

 attraction between the surface of the glass and the water, an 



