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a point is reached when no more water can be withdrawn 
without 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than these water may be withdrawn, but in decreasing 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 soil, 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 greater. 
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 
