78 A TEXT-BOOK OF BOTANY 
that the different soils may supply to the plant, they are 
especially characterized by their relation to water. The 
power of a soil to receive and to retain water is a very 
important consideration in connection with plants. For 
example, it is evident that the receptive power of sand is 
high, and its retentive power is low; while in the case of 
clay the reverse is true. One of the great advantages of 
humus is that its receptive and retentive powers are better 
balanced than in sand and clay. It is easy to devise a 
series of experiments that will show in a rough way the 
comparative receptive and retentive powers of these three 
types of soil. It has been shown also that for any given 
soil, the more finely the particles are divided the better it is 
for plants. When the soil is turned up with plow or spade, 
it is dried by the air and pulverized and so put in better 
condition for plants. 
It is evident that in considering the relation of the soil 
to plants, not only the surface soil must be considered, but 
also the soil beneath (subsow). For example, if humus 
rests on sand, the water will drain away much more rapidly 
than if humus rests on clay. The whole subject of the soil 
in its relation to plants is one of extreme complexity and is 
as yet little understood. 
40. Absorption of water.—To obtain water from the 
soil, the root not only often branches profusely, but also 
develops the root-hairs described above (§ 36). Only in 
the younger portions of the root, that ix, in the general re- 
gion of the root-huirs, is absorption of water effected. The 
root-hairs push out among the soil particles and come into 
very close contact with them, the particles sometimes be- 
coming embedded in the wall of the hair (lig. 76). In this 
way the films of water adhering to each soil particle are 
closely applied to the hair, and water passes from them 
through the wall of the hair into its cavity, and so into the 
plant. The process by which the water passes in is known 
