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 



