The Root and the Soil. 67 



further proof that roots search for air as well as water. 

 The total absence of live rootlets in the puddled clods of 

 badly-tilled fields shows that roots will not penetrate soil 

 from M^hich the air has been expelled by undue compres- 

 sion while wet. Plants in over- watered greenhouse pots 

 sometimes send rootlets into the air above the soil to 

 secure the oxygen from which their roots have been 

 deprived. 



91. The Ideat Soil for Land Plants must contain enough 

 plant food and water to fully supply the plants, and yet 

 be so porous that air can circulate through it and come 

 in contact \\ith the roots. Each particle of such a soil 

 is surrounded by a thin film of water, while between the 

 particles are spaces connected with each other, and filled 

 with moist air that is in communication with the air 

 above the soil. The root-hairs (101) apply themselves 

 intimately to the wet surfaces of the soil particles, or 

 reach out into cavities filled with saturated air, and are 

 thus able to draw in the well-aerated soil -nater, with its 

 dissolved food constituents, in sufficient quantity to re- 

 store the loss from transpiration (75) and to distend the 

 newly-formed cells (63). 



92. The Soil is a Scene of Constant Changes. The part 

 of the soil in which the roots of plants grow is the field 

 of most potent vital and chemical activities. The dead 

 remains of plants and animals it chances to contain are 

 undergoing decomposition during the warm season, by 

 serving as the feeding ground of myriads of microscopic 

 plants (bacteria). Through their agency nitric acid, 

 which supplies the higher plants with their most valu- 

 able food element — nitrogen (255), is formed in the soil. 



