84 ELEMENTARY SCIENCE 



it depends on the size of the soil grains. Clay has greater 

 retentive capacity for water than sand because its grains 

 are much smaller. Liquids have a tendency to adhere 

 to the surface of substances with which they are in con- 

 tact, even in opposition to the force of gravity. So each 

 soil grain holds to its surface a film of water; this is known 

 as the water of adhesion. Therefore, the greater the num- 

 ber of soil grains in a given volume of soil, the greater the 

 total area, and the greater the amount of water of adhesion. 

 The total surface of a cubic foot of the tiny grains of clay 

 is very much greater than the total surface of a cubic foot 

 of the much larger grains of sand. 



In your study of wells (see page 46), you learned that a 

 layer of clay is often very important as a barrier which 

 prevents the escape of water through it. Now let us note 

 that if this clay is moist, which it inevitably is, if in con- 

 tact with another water-containing stratum, then it be- 

 comes much more impenetrable to water than if it were 

 dry. Due to the minuteness of its grains, and its conse- 

 quent high powers of surface tension, it holds the water 

 that gets into it with greater tenacity, and nothing is more 

 impervious (impassable) to water than a layer which is 

 already filled with water. 



Roots and Water. You have just noted that the fine- 

 ness of soil grains multiplies the water-retaining capacity 

 of soil. Similarly, the increase of water-absorbing surfaces 

 multiplies the water-absorbing capacity of roots. This 

 increase of water-absorbing surface roots secure by means 

 of root-hairs. On the roots of seedlings, such as those of 

 corn or radish, these hairs are so dense that they form a 

 fuzz, as you may have seen (see Fig. 36). When you 



