CH. II.] THE KOOT. 107 



the roots of incumbent plants abstract the extra dose 

 of oxygen, and reconvert it to the protoxide. Coal 

 ashes, in common with all carbonaceous matters, have 

 the po^wer of strongly attracting oxygen. Every gar- 

 dener may have observed how rapidly a bright spade ; 

 of iron left foul with coal ashes becomes covered with 

 rust, or red oxide. All animal and vegetable ma- | 

 nures absorb oxygen from the air during putrefa^- j 

 tion. If it be inquired, of what benefit this property 

 is to plants, since the gases are frequently presented 

 to them in the atmosphere, it admits the ready an- 

 swer, that they enjoy the additional quantity which 

 is thus collected to the vicinity of their roots, with- 

 out the latter source being diminished; and, that 

 plants are benefited by such additional apphcation to 

 their radiculae, has been proved by the experiments 

 of Mr. Hill, already quoted. The question may also 

 be asked, whether the roots have the power to ex- 

 tract the oxygen from its combination. That they 

 have this power admits of little doubt, since Saussure 

 found that they were able to extract various saline 

 bodies from their combinations ; not only extracting, 

 but selecting in those cases where several salts were 

 in the same solution. Dr. Daubeny, the Oxford Pro- 

 fessor of Agriculture, has also shown that Strontian 

 is rejected by barley, pelargoniums, and the vringed 

 pea. 



Carbonic acid is also of benefit to plants, when ap- 



1 iN 



