156 COLLEGE BOTANY 



of the plant whicii we have studied in the preceding chapters. 

 For the present we will consider those plants only that live with 

 their roots in the soil and secure their water from that source. 



Character of the SoiL — In speaking of the soil, we usually 

 refer to the layer of loose material covering the surface of the 

 earth, which in most places is only a few inches in depth. It is 

 very co'mplex, both structurally and chemically, and varies 

 g-reatly in different localities. In brief, it may be said to consist 

 of powdered rock, containing a great deal of decayed plant and 

 animal material, which is usually spoken of as humiis, plus 

 water and air. It also contains a great many microorganisms, 

 some of which are very closely associated with plant growth 

 (page 204:). 



It will be readily seen that the physical character or struc- 

 ture of the soil depends on the kind and fineness or coarseness 

 of the powdered rock; that its chemical character depends on 

 the kind of rock and amount of humus; that the amount of 

 water in the soil depends not only on the amount of rainfall 

 but also on the structure of the soil and amount of humus it 

 contains; that the amount of air in the soil also depends on 

 the structure and the humus. The rock is ground into powder 

 by the action of water, frost and other natural factors, and 

 the mechanical forces exerted by growing plants, the burrow- 

 ing of small animals and the action of acids formed by the 

 decaying of organic materials. This powder is carried by 

 the action of wind, water and other natural agencies and mixed 

 with powder from other localities. Therefore, a soil is fre- 

 quently a mixture of materials of different kinds and from 

 different sources, and varies in accordance with their origin and 

 their physical and chemical properties. A very iine soil will re- 



