THM ROOT AND THE SOIL 63 



95. Soil aeration. — 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 culture," if their roots are kept in 

 water that is freely exposed to air from the time the seed 

 germinates, but the roots of plants growing in soil 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. Watering potted plants. — 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- 

 vancing through the soil, does not move in a straight 

 line, but has an oscillating motion, which enables it to 

 take advantage of openings between the soil particles. 

 The force with which the root-tip is pushed forward was 

 calculated by Darwin to be at least a quarter of a pound 

 in some cases, while the increase of the root in diameter 

 may exert a much greater force. The root-tip is pro- 

 tected in its passage through the soil by a thimble-like 

 covering called the root-cap. The root-cap is readily 

 seen without a magnifying glass when a bean plant is 

 grown in water. 



