ROOTS 



77 



FIG. 75. Roots of scarlet - runner bean 

 marked with lines one millimeter apart 

 and photographed after forty -eight 

 hours. 



half to one inch long, mark as delicately as possible in India 

 ink with a soft camePs-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- 

 terial, which may be 

 mixed with a greater or 

 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 



