152 ROOTS 



the soil to a greater depth. The roots of Willows and Poplars 

 will extend long distances in response to moisture. When these 

 trees grow near a well, their roots often grow down the sides of 

 the well until the water is reached. In seeking water and air the 

 roots of trees and weeds grow into drain tiles and sewers, often 

 clogging them. 



Stems, in general, grow toward the light, while most roots 

 shun the Ught. Roots are said to be negatively heliotropic, while 

 stems are positively heliotropic. A erotropism, growth toward those 

 regions of the soil where air is more plentiful, Chemotropism, 

 growth toward certain substances, and Traumatropism, .growth 

 away from injurious bodies, are other movements of roots. 



The Soil as the Home of Roots 



In the most general meaning of the term, the soil is that upper- 

 most layer of the earth's crust in which, by means of their root 

 systems, plants are able to obtain the substances necessary for 

 growth. However, in agriculture, the term soil is often applied 

 to the layer which is tilled, and the term subsoil to that which 

 lies beneath. Although the term soil is used in different ways, 

 we usually think of the soil as extending down to where the 

 dark color changes to a light, due to the absence of humus. The 

 depth of the soil varies greatly in different localities, ranging 

 from a few inches to several feet. 



As to origin, the soil is fundamentally pulverized rock of which 

 there are a number of kinds, such as granite, limestone, sandstone, 

 shales, etc., each of which gives some special property to the soil. 

 Various agencies, such as wind, water, ice, chemicals, tempera- 

 ture variations, and plants are active in breaking all rocks into a 

 pulverized form. They may be very finely pulverized into clay, 

 as the silicates are, or left in the form of fine sand, coarse sand, 

 or gravel. 



The rock constituents of any bit of soil, even of the finest clays, 

 are exceedingly various in size and shape as a microscopical 

 examination shows. The irregularity in size and shape makes 

 it impossible for the particles to pack closely, and thus insures 

 the open spaces which are estimated to be from 25 to 50 per cent 

 of the volume of cultivated soils. {Fig. 136.) The spaces are 

 exceedingly important, for they permit the circulation of water 



