THE MATERIAL INCOME OF PLANTS 313 



mixed, especially in the upper part, with more or less organic matter, 

 the offal of antecedent animal and plant life. The soil particles 

 are of various sizes and kinds, and the soil is often named accordingly. 

 Thus there are gravelly, sandy, clayey, and humus soils according to 

 the amount of gravel, sand, clay, or humus present. An indefinite 

 variety of mixtures also occurs, as in loam, with appropriate descriptive 

 names. The texture of the soil depends chiefly upon the size of the 

 individual particles; but when very fine, and especially when repeatedly 

 wetted and dried, these often become aggregated into compound grains, 

 as is obvious in clay. The sort of rock from which the soil was made, 

 the size of the particles, their state of aggregation, and the proportion 

 and character of organic matter, determine the relation of water to the 

 soil, and so the freedom and extent of its movement. 



Soil water. — Of the water which falls upon the surface as rain all may 

 percolate into the soil, or part may run off. The character of the soil 

 and of the vegetation on the surface, the slope, the rate of precipitation, 

 and the existent water content, determine the fate of the falling water. 

 A loose dry soil of level surface, a soil cover of leaves or grass, and a 

 gentle rainfall, tend to reduce the run-off to a minimum. The water 

 which percolates into the soil enters the spaces between the soil particles, 

 which it fills more or less, driving out the air and adhering in the form 

 of films to the component particles, when it does not fill the spaces com- 

 pletely. The thicker the films, the less firmly the molecules more distant 

 from the surface of the soil particles are held; so that gravity suffices 

 to carry down to lower and lower levels a certain amount of the perco- 

 lating water. This may drain away as subterranean streams or may 

 remain, saturating the soil at a certain level and forming thus the " water 

 table," approximately parallel to the surface and at a variable distance 

 from it. 



Capacity of soils for water. — When all the water that will sink to 

 the water table in a well-drained soil has drained out of the upper regions, 

 an amount varying according to the physical characters of the soil re- 

 mains, adhering to the grains. The smaller spaces are still filled; the 

 larger contain bubbles of air which have come in from above as the water 

 sank. If the soil particles be vfery small and close together, a greater 

 quantity of water will be held than in a loose, coarser soil. 



This seems anomalous, but as the amount of water adhering to the surfaces 

 will be almost proportional to the surfaces themselves, it may easily be comprehended 

 by calculating the area of 1000 spheres each i mm. in diameter, which could 



