THE FARMER'S MAGAZINE. 



113 



portion of earthy matters in plants is considerably in- 

 flnenced by the soil on which they grow ; but whether 

 they derive the whole of these fixed principles from the 

 soil, or form them partly by some unknown powers of 

 vegetation, has not been satisfactorily ascertained. Some 

 would lead us to believe that all the earths are derived 

 from the soil ; others, that a portion at least is derived 

 from vegetation, as in the case of plants wholly removed 

 from the soil. But plants removed from contact of the 

 earth do not arrive at maturity, or produce Iruit, and 

 contain much less carbonaceous matter than others of 

 the same kind. 



Plants decompose common air, and change it into 

 carbonic- acid gas, forming carbon for the use of the 

 plant. In light, oxygen is given out by the leaves of 

 plants, and hydrogen retained ; in the dark, oxygen 

 is inhaled, and hydrogen given out. But coal consti- 

 tutes only one-third — or, according to Lavoisier, 

 28-IOOths of carbonic-acid gas ; and the atmosphere con- 

 tains only a thousandth part of fixed air, or, as before 

 mentioned, according to Lavoisier, none at all. 



In Hassenfratz's theory the difficulty lies in account- 

 ing for the quantity of carbon that trees find in one 

 place, as they have no locomotive powers to go, like 

 animals, in search of food. But both that writer and 

 Ingenhousz are candid enough to admit that both the 

 earth and air may combine in affording materials for the 

 growth of plants. 



It has been long and very generally supposed, by 

 vegetable physiologists, that " humus" audits modifica- 

 tions formed the chief food of plants, and that alkalis 

 were necessary to promote the decomposition. The 

 name of " humus" was applied to the vegetable layer, 

 or mould, arising from the putrefaction of organic 

 substances, the quantity and quality of it indicating the 

 fertility or barrenness of soils. But, properly speaking, 

 humus is only a particular portion of the vegetable layer 

 in a certain state of preservation, and does not extend to 

 the stratum of mould that has generally received that 

 name ; consisting of vegetable matters, partly decom- 

 posed, but not completely disorganized ; resembling a 

 plant without organization pervading and nourishing 

 others. Humus being slightly soluble in water, it was 

 supposed to yield materials capable of being absorbed by 

 the roots of plants, and the soluble parts were called 

 " humic acid," and the insoluble " humin," or coal of 

 humus ; carbon being, as usual, the chief constituent 

 element. But it has been found that humus requires 

 2,500 times its own weight of water to dissolve it — a 

 much greater quantity than nature affords ; and the very 

 trifling portion of alkalis in many soils would be insuffi- 

 cient to yield any great supply of carbon from that 

 source. On lands that arc regularly mown, humus in- 

 creases, and yields as much carbon as manured grounds. 

 It is thence conjectured that the air supplies the carbon, 

 as it has been incontestably proved that plants imbibe 

 carbonic-acid gas, retain the carbon, and emit the 

 oxygen. Water is a prime agent, and the elements of 

 it may be decomposed and assimilated by plants at the 

 same time. From the very small quantity of carbonic- 

 acid gas in the air, and of carbon with the gas itself, it 



may be supposed that the vegetative process possesses the 

 power both of assimilation and of augmentation. Expe- 

 riments made by supplying carbonic acid and water to 

 plants failed, because other necessary^ ingredients were 

 wanting. The decaying vegetable matter, by converting 

 the oxygen into carbonic acid, affords the first food to 

 plants till the leaves are able to perform the functions of 

 inhalation. The carbon is thus derived from two sources : 

 from the humus in the soil in the first place, and then 

 from the atmosphere. 



Ammonia, in different forms, is a powerful promoter 

 of vegetation, and is thought to afford the nitrogen to 

 plants, being itself a compound of nitrogen and hydro- 

 gen; or, ammonia 17 =14 nitrogen x 3 hydrogen. 

 Ammonia exists largely in manures of various kinds, 

 and in plants it forms several colours ; it abounds in 

 the last products of all animal substances, and in rain 

 and in snow-water ; the smell exhaled by the ground 

 after the melting of snow most sensibly showing its 

 presence. The use of gypsum, charcoal, and burned 

 clay arises from their fixing and retaining ammonia'; and 

 oxides and ferruginous matters also possess that property. 

 It has been supposed that manures act only by the 

 formation of ammonia ; but nitrogen, or the product of 

 ammonia, exists sparingly in vegetables, and with the 

 nature of that elementary body, whether it be simple or 

 compound, we are, as yet, very imperfectly acquainted. 

 Plants contain a great preponderance of carbon over the 

 azote, and animal bodies the contrary; and pure 

 vegetable substances are doubted to contain any azote. 

 Such is the uncertainty that attends the most plausible 

 theory on this subject. 



It has been supposed by several eminent physiologists, 

 that plants eject faeces like animals, and that the excre- 

 ments are converted into humus by exposure and 

 cultivation. It has also been supposed that the fecal 

 exudations of one plant supply food to another that is 

 of a wholly different kind, and that the benefits of alter- 

 nate cropping are derived from that source. But no 

 instance is known, in the animal world, of any one indi- 

 vidual deriving its support directly from the excrements 

 of another ; it seems to be repugnant to the laws of 

 Nature, until the substance has undergone many modi- 

 fications, and has been converted by the process of 

 assimilation into animal and vegetable food. The 

 theory is only a modification of the old hypothesis, that 

 plants have a power of choosing and rejecting, and that 

 different plants require different food. The excrements 

 are supposed to restore to the soil the carbon they de- 

 rived from the humus during the early period of growth, 

 and alkalis and ashes may hasten the decomposition ; 

 but it is still uncertain if the exudations be derived from 

 the soil, or from some other source, or formed by the 

 plant itself ; and until that be ascertained no certainty 

 can exist of the justness of the theory. 



Experiment has not yet decided if soluble animal and 

 vegetable matter passes unaltered into plants, and be- 

 comes a part of their organization. If it does pass, it 

 must be by many combined and unseen agencies ; and it 

 is very probable that water, air, and earth, although 

 they do not singly afford the food of plants, all of them 

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