ROOTS 



259 



be determined by using such seedlings as those of peas, 

 beans, and corn. When the young roots have become a 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 reveal the region of 

 elongation and of greatest elongation 

 (Fig. 226). 



143. The soil. The soil is too 

 commonly thought of as merely an ac- 

 cumulation of " dirt," from which 

 plants can obtain " food." It has 

 been pointed out in preceding chapters 

 that ordinary plants do not obtain 

 food from the soil, but that they do 

 obtain certain materials used in food- 

 manufacture. It now remains to con- 

 sider whether the soil is merely "dirt." 



Soil is finely divided rock (mineral) 

 material, which in "rich" soil is mixed 

 with more or less organic material de- 

 rived from the broken-down bodies or 

 waste products of plants and animals. 

 It is the organic material that makes 

 soils dark, and when there is a considerable amount of this, 

 as in the upper soil of forests, the soil is called humus (often 

 " vegetable mould " or " leaf mould "). Soil, therefore, is a 

 mixture of certain mineral (inorganic) and organic materials. 

 These materials must include certain chemical substances 

 that plants need, and in general all soils contain these sub- 

 stances. This is indicated by the fact that almost all soils 

 in nature are covered by vegetation, and even in the desert 

 of Sahara, reputed to have the most " sterile " of soils, the 

 breaking through to the surface of a spring of water results 

 in an " oasis " with luxuriant vegetation. This indicates 



FIG. 225. Longitudinal 

 section of root of ar- 

 row-leaf, showing the 

 branches starting from 

 the vascular cylinder and 

 penetrating the cortex. 



