20 THE STORY OF PLANT LIFE 



A soil consists of small or large particles which 

 touch one another, at one or more points. There 

 are spaces between, or pores, in which water or air 

 circulate as in the intercellular spaces in plants. 

 The water coats the particles as a film, and the air 

 circulates between. The films can become extended 

 and water may flow between the particles. There 

 is a line where the water is more continuous — the 

 water-table. The soil above contains only hygro- 

 scopic water which adheres to the films, and is not 

 capable of withdrawal. Where the water forms a 

 thicker film, and can be withdrawn, it is called capil- 

 lary water, and this is the source of the water which 

 plants absorb. 



The porous passages in the soil give it a degree of 

 capillarity so that water can circulate freely through 

 these passages, and upon their size and mode of 

 communication or accessibility depends the available 

 supply to the plant. In close soils the passages are 

 small and easily clogged ; in more open soils where 

 the grains are coarse there is a freer communication. 

 Water is drawn up from the water-table by capillary 

 motion or suction, as in a sponge, and raised to the 

 surface. But the more open soils have fewer passages 

 and less water is retained. The capacity of the soil 

 for water is limited by super- saturation when all the 

 passages are filled up, or by its limitation alone to 

 hygroscopic water. 



The measure of the water capacity of a soil is the 

 amount retained after the excess is drained off. Clay 



