ROOTS rue 
half to one inch long, mark as delicately as possible in India 
ink with a soft camel’s-hair brush a series of equally spaced 
lines, beginning at the tip. Observations at the end of 
twenty-four to forty-eight 
hours will discover the 
region of elongation and 
of greatest elongation 
(Fig. 75). 
39. The soil.— Before 
absorption by roots is con- 
sidered, it is necessary to 
know something of the 
structure of soil. Soil is 
finely divided rock ma- 
Fic. 75.—Roots of scarlet-runner bean 
terial, which may be marked with lines one millimeter apart 
mixed with a ereater or oe after forty - eight 
less amount of material 
(called organic material) derived from the broken-down 
bodies or waste products of plants and animals. However 
fine the particles of soil may be, they never fit together in 
close contact, so that there are open spaces everywhere 
among them. Immediately after a soaking rain these 
spaces are full of water, but if the soil is one that drains 
easily, the water gradually disappears from the spaces, and 
the larger ones are occupied by air. In addition to this 
occasional water, each particle of soil is invested by a thin 
film of water, which adheres to it closely, and which never 
entirely disappears even in the driest soil. The soil water 
is never absolutely pure, but contains dissolved in it cer- 
tain materials obtained from the soil. 
As types of soil, sand, clay, and humus may be con- 
sidered. Humus is a soil in which there is intermixed a 
large amount of decayed plant material; and it is frequently 
called vegetable mold, or leaf mold, the best illustration 
being the upper soil of forests. Aside from certain materials 
