2:)0 



SCIENCE OF AGRICULTURE. Part II. 



applied to the loaves and branches, and whatever increases tin proportion of tin- gas 

 inhere at lead within a given degree, forward* vegetation ; 2d, That, as applied to t 

 branches of plants, It ii prejudicial to vegetation in the shade, If administered in a pro 



in their atnio- 



the leaves and 



proportion beyond 



tl?at"n!'« in. h Uextsbi in atmospheric air ;"ld, Thai carbonic acid gas, as applied to the roots of plants, la 



also beneficial to their growth, at least in the 1 <■ advanced stages m vegetation. 



\ i isential to the commencemei t and progress of germination, so also it Is essential to 



the progress ol vegetation li Is obvious, thi n, tl .1 the experiment proves that it is beneficial to the 

 KTOwthofthevcReUbli I totherool . necessary to the developement ol the leaves; and to the 



Sevelopemcnl ofthe flower and fruit. The Bower-bud will not expand if confined in an atmosphere 

 deprived of oxygen, nor will the fruit ripen. Flower-buds confined ta an abnomhere of pure nitrogen 

 faded without expat ding A bunch of unnpe grapes introduced into a globe of glass which was luted by 

 its orifice to the bough, and exposed to the sun. ripened without effecting any material alteration in its 

 atmosphere but when a bunch was placed in the same circumstances, with the addition ofa quantity of 

 lime the atmosphere was contaminated, and the grapes did not ripen. Oxygen, therefore, is essential to 

 the developement of the vegetating plant, and is inhaled during the night ,..-.. 



1 . 1 Though nUroeen eat 1 stitutes by far the greater part ol the mass of atmospheric air, it does not 



seem capableof affbnfing nutriment to plants; for as -ecus will not germinate, so neither will plants 

 vegetate in it but for avery limited time, with the exception of the Hnca minor, Lythram Salicaria, 

 / ■mil 1 dvsenterica Bpilubium hirsatum, and Polygonum Persicaria, which seem to succeed equally well 

 in an atmosphere of nitrogen gas as in an atmosphere of common air. Nitrogen is found in almost all 

 vegetables, particularly in the wood, in extract, and in their green parts, derived, no doubt, from the 

 extractive principle of vegetable mould. . „„•.„. 



p, 7 Harfr m tat A plant ofthe Epilobium hirsutum, which was confined by Priestley in a receiver 



rilled with Inflammable air or hvdrogen, consumed one third of its atmosphere and was still green. 

 Hence Priestley inferred, that it 'serves as a vegetable food, and constitutes even the true and proper 

 pabulum ofthe plant But the experiments of later phytologists do not at all countenance this opinion. 



gasf they'may at least acquire it In "the state of water, which is indisputably a vegetable food, and of 

 which hydrogen constitutes one of the component parts. 



1 528. Vegetable extract. When it was found that atmospheric air and water are not, 

 even conjointly, capable of furnishing the whole of the aliment necessary- to the de- 

 velopement of the plant, it was men alleged that, with the exception of water, all sub- 

 stances constituting a vegetable food must at least be administered to the plant in a 

 gaseous state. But this also is a conjecture unsupported by proof; for even with 

 regard to such plants as grow upon a barren rock, or in pure sand, it cannot be said that 

 they receive no nourishment whatever besides water, except in a gaseous state. Many of 

 the particles of decayed animal and vegetable substances, which float on the atmosphere 

 and attach themselves to the leaves, must be supposed to enter the plant in solution with 

 the moisture which the leaves imbibe ; and so also similar substances contained in the 

 soil must be supposed to enter it by the root : but these substances may certainly con- 

 tain vegetable nourishment ; and they will perhaps be found to be taken up by the 

 plant in proportion to their degree of solubility in water, and to the quantity in which 

 they exist in the soil. Now one ofthe most important of these substances is vegetable 

 extract. When plants have attained to the maturity of their species, the principles of 

 decay begin gradually to operate upon them, till they at length die and are converted 

 into dustTor vegetable mould, which, as might be expected, constitutes a considerable 

 proportion of the soil. The chance then is, that it is again converted into vegetable 

 nourishment, and again enters the plant. But it cannot wholly enter the plant, because 

 it is not wholly soluble in water. Part of it, however, is soluble, and consequently 

 capable of being absorbed by the root, and that is the substance which has been denomi 

 nated extract. 



15°9 Saiissiirc filled a large vessel with pure mould of turf, and moistened it with distilled or rain 

 water till it was saturated. At the end of five davs, when it was subjected to the action of the press, 

 10,000 parts in weight of the expressed and filtered fluid yielded, by evaporation to dryness, 26 parts of 

 extract. In a similar experiment upon the mould of a kitchen-garden which had been manured with dung, 

 loooo parts of a fluid yielded 10 of extract; and, in a similar experiment upon mould taken from a well- 

 cultivated corn field, 10,01 parts of fluid yielded 4 parts of extract. Such was the result in these par. 

 tuular cases Hut the quantity of extract which may be separated from the common soil is not in general 

 very considerable. After twelve decoctions, all that could be separated was about one eleventh of its 

 weight; and vet this seems to be more than sufficient for the purposes of vegetation : for a soil containing 

 this quantity was found by experiment to be less fertile, at least for peas and beans, than a soil containing 

 only one half or two thirds of the quantity. Kut if the quantity of extract must not be too much, neither 

 must it be too little. Plants that were put to vegetate in soil deprived of its extract, as far as repeated 

 decoctions could deprive it, were found to be much less vigorous and luxuriant than plants vegetating in 

 soil not deprived of its extract : and vet the only perceptible difference between them is, that the former 

 can imbibe and retain a much greater quantity of water than the latter. From tins last experiment, as 

 well as from the great proportion in which it exists in the living plant, it evidently follows that extract 

 constitutes a vegetable food. But extract contains nitrogen ; for it yields by distillation a fluid impregnated 

 with ammonia. The difficulty, therefore, of accounting for the introduction of nitrogen into the vegeta- 

 ting plant as well as for its existence in the mature vi getable substance, is done away ; for, although the 

 plant refuses it when presented in a gaseous state, it is plain that it must admit it along with the extract. 

 It seems also probable that a small quantity of carbonic acid gas enters the plant along with theextractive 

 principle, as it is known to contain this gas also. 



1530. Salts, in a certain proportion, arc found in most plants, such as nitrate, muriate, 

 and sulphate of potass or soda, as has been already shown. These salts are known to 

 exist in the soil, and the root is supposed to absorb them in solution with the water by 

 which the plant is nourished. It is at least certain that plants may be made to take up 

 by the roots a considerable proportion of salts in a state of artificial solution. But if 



