70 Principles of Plant Culture. 



clayey soil, due to the compacting influeiice of rain, is a 

 great hindrance to its ventilation. Earthworms and other 

 animals that burrow in the soil aid in aerating it. 



94. Hotbeds Require Especial Care in Ventilation 

 (365), since they usually contain large quantities of de- 

 composing organic matter (manure), which rapidly ab- 

 sorbs oxygen from the soil, replacing it with carbonic 

 acid. 



95. 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 im- 

 mersed in water. True, most plants may be grown in 

 "water culture," i. e., with their roots from germina- 

 tion 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 within its 

 cavities from absorbing air. 



96. Potted Plants Require Drainage (412), and the 

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

 through their walls. Potting soil should contain suffi- 

 cient sand and humus (92), so that it does not readily 

 become puddled by watering (31). 



97. Potted Plants should be Watered with Care 

 (218). They should receive sufficient water so that the 

 soil particles are constantly surrounded with a film of 

 water, but not so much that the soil cavities remain 

 filled. 



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

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



