THE VEGETABLE CELL. 81 



in water, those with earths and metallic oxides little or not at all so. 

 On the other hand, their compounds with the alkalies and ammonia 

 readilj form double salts with the earths aad metallic oxides (apocrenic 

 acid is penta-basic, crenic tetra-basic) ; the alkalies are therefore not 

 only a means of rendering these acids readily soluble, but they assist in 

 conveying the earths into plants by absorption. 



Alumina plays a special part in reference to crenic and apocrenic 

 acids, since it forms perfectly insoluble compounds with them, in wliich 

 the acids are preserved from decomposition, and cannot be washed away 

 by water, yet they are not thereby completely withheld from plants, 

 since these compounds are capable of decomposition by ammonia, which 

 is thus a means of conveying these compounds into plants very gradually, 

 by continuous decomposition. 



Most important as the above described relation of the humous acids 

 to ammonia is, since their great affinity for it places them in a condition 

 to attract this body, so important to vegetation, from the air and from 

 tbe animal substances decomposing in the soil, and prepares them for 

 absorption by the roots, yet they acquire still more importance from 

 the fact that, according to Mulder's researches, the continuous decom- 

 position of the humous sul^stances is connected with formation of 

 ammonia, since the oxygen of the air is used for the higher oxidation 

 of the rest of their substance. The evidence that niti^ogen is also con- 

 veyed to plants in tliis way, lies in an experiment of Mulder's (" Fhys. 

 Chemistry''), according to which, young Bean-plants which were raised 

 in an atmosphere free from ammonia, in ulmic acid prepared from sugar 

 free from ammonia, and in wood-coal, with water free from ammonia^ 

 yielded, on analysis, twice or thrice as much nitrogen as the seeds fi*om 

 which they were raised. 



That the solutions of humous substances in water are absorbed by the 

 roots as ^uch, and not the products of their decomposition, it would cer- 

 tainly be difficult to prove, since these subhtances cannot be demonstrated 

 to exist as such in the plant, but undergo a transformation directly they 

 are absorbed. But in spite of the opposite results obtained by Hartig 

 (Liebig's '^^ Agricidtural Chemistry,'' 1 ed.) and linger {^^ Flora'' 1842, 

 241), after Saussure's experiments (Liebig ^'-Annal." xlii. 275), Johnson 

 {^^ Mitth. d (Econ, Ge' sells, zu Petersburg," 2 heft 162, extracted in "Wolfi's 

 '^Chem. Forschungen," 202), and Trinchinetti's (^^ JSul facoUa assorhente 

 della radici," 55), the assumption of such absorption is the less unsafe, that 

 it has been long demonstrated, that roots have the power of absorbing 

 dissolved vegetable substances, e.g., tannic acid, narcotic extracts, &c. 

 {See Mulder, " Fhys. Chem.") 



The inorganic compounds vp-hich are taken up by plants as food, 

 and which furnish them with the four principal elementary bodies 

 which tliey require for their formation, are water, carbonic acid, 

 and ammonia. 



As the absorption of watery fluids Ixas already been discussed, 

 I now turn to the consideration of carhoniG acid. This, it is well 

 known, exists universally diffused in atmospheric air and in water. 

 Simple experiments prove that plants do not absorb the carbonic 

 acid dissolved in water, with the latter, by means of their roots; 



