CH. II.] THE EOOT. 81 



found in many plants. Iron is present in all soils, 

 in all natural waters, and in all plants. Manganese 

 is found in some soils, is soluble in water containing 

 acids, &c., and is found in a few plants. But none 

 of those substances in a state of purity, either simply 

 or combined, have ever been found capable of perfect- 

 ing a plant through all its stages of growth, when 

 moistened only with distilled water ; the contraiy is 

 the case, however, when the water contains in solution 

 vegetable or animal matters, as the dimg of animals. 

 Now these matters contain carbon, hydro^ren, oxvgen, 

 nitrogen, and various salts : the three first are abso- 

 lutely necessary for the existence of all plants, every 

 part of which is chiefly composed of them; nitrogen 

 is found in most plants ; and the importance of salts 

 to vegetation is demonstrated by the facts, that clover 

 will not flourish where there is no sulphate of lime ; 

 that nettles follow the footsteps of man for the nitrate 

 of potass, which always abounds near the walls of his 

 habitation, and that marine plants linger for the com- 

 mon salt of their native haunts. Salts of some kind 

 or other are found in every species of plant, but none 

 of which the constituents have not also been detected 

 in soils. During decay, vegetable and animal matters 

 exhale various gases. Carbonic acid, hydrogen, car- 

 buretted hydrogen, ammonia, &c. are of the number, 

 all of which have been applied to the roots of plants 

 with great benefit by Sir H. Davy and others. 



a 



