86 BRITISH PLANTS 



descend, until, at the level of the ground-water, all the 

 pore-spaces are filled with water. Towards the top, the 

 films become very thin and stretched, and if the section 

 is sufficiently thick, the topmost layers may be perfectly 

 dry. As the films become more attenuated, it is increas- 

 ingly difficult to withdraw water from them, and at last 

 a point is reached when no more water can be withdrawn 

 v/ithout rupturing them. This last layer is very resis- 

 tant ; it is difficult to break, and constitutes' what is 

 known as hygroscopic water in soils apparently dry. 

 Hygroscopic water is of no service to plants because it 

 cannot be drawn from the films. From all films thicker 

 than these water may be withdrawn, but in decreasing 

 quantity as we approach the hygroscopic limit. Such 

 water is called capillary water, and this alone is avail- 

 able to plants. When all the pore -spaces are filled 

 with water the soil is said to be saturated, or water- 

 logged ; if drainage is possible and the excess allowed to 

 drain away, the water that is left remains as films round 

 the particles, the rest of the pore-spaces being filled with 

 air. When during a shower water percolates through a 

 sandy soU, it passes down to the water-table through the 

 water-films. 



1. The Capillarity of the Soil. — The pore-spaces in the 

 soil form a series of irregularly branching tubes. These 

 tubes are everywhere in communication with one another, 

 opening above into the air, and reaching below to the 

 ground-water. The power of the soil to absorb and 

 retain water depends upon the number and width of 

 these capillary tubes. In a narrow tube the water ascends 

 to a greater height than in a wide tube, and is more 

 difficult to displace. In soils where the particles are very 

 small and very close together, the capillary tubes are very 

 minute, and much more numerous than in coarser soils, 

 where the air-spaces between the particles are larger. 

 The ascent of water, therefore, is greater in fine-grained 

 soils than in coarse. The capillarity of the subsoil 

 becomes very important when ground-water is present. 

 With the level of the water-table at a certain depth, some 

 soils may be able to make use of it by suction, while 

 others may not. 



2. The Water-Capacity of Soils. — There are two limits 

 to the water-holding capacity of soils. The higher limit 



