468 Comparative Physiology. 



likely be reduced to carbonic acid first, and the decomposition of 

 humic acid will furnish the same products as that of carbonic 

 acid. It is perhaps however too much to assert that binary- 

 compounds alone are capable of aifording nourishment to plants ; 

 it has not yet been proven that the simple elements will not 

 unite unless in a newly decomposed or nascent state, though un- 

 doubtedly the most suitable. Oxygen is absorbed in greater 

 quantity than needed for respiration. Liebig says it is essential 

 to vegetable life, and gives as the reason why stagnant water at 

 the roots destroys plants, that it has lost its oxygen. Nitrogen 

 is also soluble to a certain extent in water, and likely is partly 

 furnished to the plant in this form. There are other binary 

 compounds also, besides carbonic acid, water, and ammonia, 

 which may constitute food. Air is found in all the vessels of 

 plants, sometimes containing more oxygen and sometimes more 

 nitrogen than usual, evidently showing it has been acted on by 

 the organs of the plant; and in this way both oxygen and nitrogen 

 may be supplied. The fact that beans and other crops carry 

 oif much nitrogen, without exhausting the soil, seems conclusive 

 on the point that the nitrogen of the air is absorbed by plants ; 

 and is allowed by Boussingault and others. Sulphuretted and 

 carburetted hydrogen, especially the latter, which Dumas de- 

 scribes as being generally equal in quantity to the carbonic acid 

 of the atmosphere, are probably made available, especially in the 

 volatile oils formed in leaves, fruits, and barks. Inorganic com- 

 pounds, though consisting of more than two elements, are yet 

 generally combined in binary forms, as acids with bases, &c., which 

 are not decomposed into their elements before combining, as in 

 organic combinations. Nitric acid, combined in quaternary forms 

 of binary combinations with alkaline bases, as nitrates of ammo- 

 nia, potash, soda, and lime, supplies undoubtedly sources of food 

 also. The ammonia itself is generally absorbed in a quaternary form, 

 as carbonate and humate of ammonia. The ternary compounds of 

 gum, sugar, starch, &c., of plants, when they become or are ren- 

 dered soluble in the water of the soil, will certainly be as avail- 

 able for food as the same substances stored up in the plant. It 

 does not appear necessary that they should all be reduced into 

 their elements ; if soluble before being so, they may be decom- 

 posed in the plant ; and so also with organic substances in general, 

 whether ternary or quaternary. It simplifies and makes the 

 subject easier of comprehension, to endeavour to confine the 

 substances used as food to the fewest numbers possible; but in 

 attempting this we may err in restricting the operations of nature 

 too far, which are not generally confined to one particular 

 method. 



" It is a general law of vitality, that the materials of nutrition 

 can only be introduced into the living system in the fluid state ; 



