4 
80 PLANT LIFE 
work in opposition to the surface-forces that 
tend to retain the water in the soil as a film 
which wets the minute particles of which soil 
is composed. The root-hairs are very closely 
adherent to some of these particles, and 
they wrest the water from the film which 
surrounds them. ‘This disturbs the equili- 
brium of the water as distributed in the soil, 
and it causes a constant flow towards the 
spot whence it is being abstracted. It is to 
this circumstance that much of the drying 
effects of plants on soil is due, for the total 
amount of root-hair surface of a tree is far 
smaller than the area of ground that it will 
drain. Another example of the movements 
of water in soil is seen in the way that it loses 
moisture in dry weather. This is because 
evaporation is going on at the surface of the 
ground, and water is continually passing up- 
wards from the lower levels to replace that 
which has passed into the atmosphere as 
vapour. The resistance to movements of 
water as the films lining the soil particles 
become very thin rapidly increases, and thus 
ordinary ground does not easily become dry 
for a great distance below the surface. Any- 
thing that disturbs the continuity of the soil 
particles also- interposes a further hindrance 
to the movement of water, and this is why a 
garden soil that is kept stirred with a hoe 
withstands drought so much better than one 
that is not cultivated in this way. The 
particles of soil that have been stirred by the 
