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 (subsoil). 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 is, in the general re- 

 gion of the root-hairs, 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 (Fig. 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 



