52 



Plant Physiology 



the root there is a tendency to suppress hair pro- 

 duction. 



34. Soil particles. The extent of the root-system 

 previously discussed enables the plant to come into con- 

 tact with an enormous quantity of 

 soil particles. Each particle of the 

 soil, even air-dried soil, is invested 

 with a film of water, and the amount 

 of water which the soil may contain 

 is, to a considerable extent, depend- 

 ent upon the degree of fineness of 

 these particles. It is also dependent 

 upon the amount of organic matter, 

 the so-called humus, as subsequently 

 indicated. 



With respect to the sizes of the 

 particles, a soil may be designated 

 as sand, silt, or clay. These three 

 classes are subdivided into types 

 or grades, depending upon the mi- 

 nuter differences in size of the parti- 

 cles. The following table is generally 

 adopted by soil experts for the pur- 

 classification : 



FIG. 15. Seedling from a 

 sand culture, showing 

 adherence of grains. 



pose of a mechanical 



Coarse sand 

 Medium sand 

 Fine sand 

 Very fine sand 

 Silt 



Fine silt 

 Clay 



2*5 -A inch diam. 

 inch diam. 

 mcn diam. 

 inch diam. 

 i ncn diam. 

 inch diam. 

 inch diam. 



All soils contain, in addition to larger particles, certain 



