EFFECT OF UUAINACE. 0.5 



water comes in contact with tlie roots of tlie plants, and 

 can give up potasli to tlicni, it is clear that the constituents 

 of the solution moving about in the soil have but a very 

 trifluig share in the process of nutrition, while the absence 

 from it of ammonia and phosphoric acid is of itself suffi- 

 cient to prove that these materials in the soil cannot 

 change their place. The ground must contain a certain 

 amount of moisture to be able to furnish food to plants ; 

 but it is not necessary for their growth that the water 

 should be free to move about. It is Avell known that 

 stagnant water in the soil is injurious to most of the cul- 

 tivated plants ; and the favourable effect upon their 

 growtli produced by draining just depends on this, that 

 an outlet is opened to the water moving by the force 

 of its own gravity, and the earth is moistened by that 

 water only which is retained by capillary attraction. 



If we regard the porous earth as a system of capillary 

 tubes, the condition which must render them best suited 

 for the growth of plants is unquestionably tliis, that the 

 narrow capillary spaces should be filled with water, the 

 wide spaces with air, and that all of them should be 

 accessible to the atmosphere. In a moist soil of the kind, 

 afibrding free access to atmospheric air, the absorbent 

 root fibres are in most intimate contact with the earthy 

 particles ; the outer surface of the root-fibres may here 

 be supposed to form the one, the porous earthy particles 

 the other wall of a capillary vessel, the connection between 

 them being efiected by an exceedingly thin layer of water. 

 This condition is equally favourable for the absorption of 

 fixed and of gaseous elements of food. If, on a dry day, 

 a wheat or barley-plant is cautiously pulled up from a 

 loose soil, a cylinder of earthy particles is seen to adhere 

 like a sheath round every root-fibre. It is from these earthy 



