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 light. Roots are said to be negatively heliotropic, while 
stems are positively heliotropic. A érotropism, 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 
