68 Principles of Plant Culture. 



sometimes send rootlets into the air above the soil to 

 secure the oxygen from which their roots have been 

 deprived. 



90. The Ideal 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 with the roots. Bach 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 communica- 

 tion with the air above the soil. The root-hairs (100) 

 apply themselves intimately to the wet surfaces of the 

 soil particles, or reach out into cavities filled with sat- 

 urated air, and are thus able to draw in the well-aerated 

 soil water, with its dissolved food constituents, in suf- 

 ficient quantity to restore the loss from transpiration 

 (74) and to distend the newly formed cells (62). 



91. 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 con- 

 tain are undergoing decomposition during the warm 

 season, by serving as the feeding ground of myriads 

 of microscopic plants (bacteria) (255). Through their 

 agency nitric acid, which supplies the higher plants 

 with their most valuable food element— nitrogen (254), 

 is formed in the soil. The carbonic acid these remains 

 took from the air during growth is also set free to 

 slowly disintegrate the mineral soil constituents, ren- 

 dering these soluble and thus available as plant food. 

 In winter, the frost separates the compacted particles 



