The Root and the Soil. 69 



on the surface of clayey soil, due to the compacting influ- 

 ence of rain, is a great hindrance to its ventilation. 

 Earthworms and other animals that burrow in the soil 

 aid in aerating it. 



95. Hotbeds Require Especial Care in Ventilation (365), 

 since they usually contain large quantities of decompos- 

 ing organic matter (manure), which rapidly absorbs 

 oxygen from the soil, replacing it with carbonic acid. 



96. Drainage Promotes Soil Aeration by forming an 

 outlet for the surplus water that would otherwise fill the 

 cavities. Although moisture is essential to root growth, 

 land plants do not prosper with their roots immersed in 

 water. True, most plants may be grown in " water cul- 

 ture," i. e., with their roots from germination grown in 

 water that is freely exposed to the air; but the roots of 

 land plants soon smother for want of free oxygen when 

 the soil cavities are filled with water/ because the soil 

 tends to prevent the water witl in its cavities from ab- 

 sorbing air. 



97. Potted Plants Require Drainage (412), and the out- 

 side of the pots should be kept clean, to admit air 

 through their walls. Potting soil should contain suffi- 

 cient sand and humus (93), so that it does not readily 

 become puddled by watering (31). 



98. Potted Plants should be Watered with Care (219). 

 They should receive sufficient water so that the soil par- 

 ticles are constantly surrounded with a film of water, 

 but not so much that the soil cavities remain filled. 



99. How the Root-Tip Penetrates the Soil. Darwin 

 made the interesting discovery that the root-tip, in ad- 

 vancing through the soil, does not move in a straight 



