THE SOIL AND ITS IMPORTANCE TO PLANTS 27 



attraction which is sufficient to Hft water against gravity. 

 The Hfting force will be proportional to the exposed surface of 

 the tube, and therefore where the volume of water is small in 

 relation to this surface (as is the case inside the tube) the water 

 will rise somewhat before the weight of the lifted column counter- 

 balances the pull exerted by the surface attraction. Obviously, 

 the narrower the tube, the higher the column of water will rise, 

 since the volume of liquid to be lifted will be smaller in proportion 

 to the area of the attracting surface. Thus, in any material the 



Fig .15. — The relation between the size of soil particles and the amount of 

 surface which they present. Diagrams of sections through equal volumes of 

 small spheres and of large spheres. The total surface is evidently greater where 

 the size of the particles is less. 



structure of which presents a great amount of surface surrounding 

 small but communicating tubes, pores or other narrow spaces, as 

 in blotting paper, lamp-wicks, and the like, water will evidently 

 be carried to a considerable distance in all directions by capil- 

 larity. Just such a material as this is the soil. The multitude 

 of its tiny particles, packed closely together, form a capillary 

 system which is able to carry water far. This water tends to 

 surround each particle in a thin, capillary film, but if the soil 

 particles are very coarse the film cannot pass around them, and 

 under such conditions the ascent of water necessarily stops. 

 Water moves readily within the films and when those at the top 

 of the ascending column are thinned through evaporation or 

 through the attraction exerted by still higher and un wetted 

 surfaces, the films below are drawn upon, and water passes upward 

 through the whole system. This movement continues until the 

 weight of the water lifted balances the surfaced attraction at the 

 top of the column. The height to which water will rise by capil- 



