230 SCIENCE OF AGRICULTURE. Part II. 



applied to the leaves and branches, and whatever increases the proportion of this gas in their atmo- 

 sphere, at least within a given degree, forwards vegetation ; 2d, That, as applied to the leaves and 

 branches of plants, it is prejudicial to vegetation in the shade, if administered in a proportion beyond 

 that in which it exists in atmospheric air ; 3d, That carbonic acid gas, as applied to the roots of plants, is 

 also beneficial to their growth, at least in the more advanced stages of vegetation. 



1525. As oxygen is essential to the commencement and progress of germination, so also it is essential to 

 the progress of vegetation. It is obvious, then, that the experiment proves that it is beneficial to the 

 growth of the vegetable as applied to the root ; necessary to the developement of the leaves ; and to the 

 developement of the flower and fruit. The flower-bud will not expand if confined in an atmosphere 

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

 faded without expanding. A bunch of unripe 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 of a 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 tlie night. 



1526. Though nitrogen gas constitutes by far the greater part of the mass of atmospheric air, it does not 

 seem capable of affording nutriment to plants ; for as seeds will not germinate, so neither will plants 

 vegetate, in it, but for a very limited time, with the exception of the /'inca minor, iy thrum Salicaria, 

 /'nula dysenterica, Epilobium hirsutum, 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. 



1527. Hi/drogen gas. A plant of the Epilbbium hirsiitum, which was confined by Priestley in a receiver 

 filled with inflammable air or hydrogen, 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 of the plant. But the experiments of later phytologists do not at all countenance this opinion. 

 The conclusion from various experiments is, that hydrogen is unfavourable to vegetation, and does not 

 serve as the food of plants. But hydrogen is contained in plants, as is evident from their analysis : and if 

 they refuse it when presented to them in a gaseous state, in what state do they then acquire it ? To this 

 question it is sufficient for the present to reply, that if plants do not acquire their hydrogen in the state of 

 gas, 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. 



1528. 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 then 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 of the 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 dust or 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. 



1529. Saussure 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 days, 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, 

 10,000 parts of a fluid yielded 10 of extract ; and, in a similar experiment upon mould taken from a well- 

 cultivated corn field, 10,000 parts of fluid yielded 4 parts of extract. Such was the result in these par- 

 ticular cases. But 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 yet 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. But 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 yet the only perceptible difference between them is, that the former 

 can imbibe and retain a much greater quantity of water than the latter. From this 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 vegetable 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 the extractive 

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



1 5S0. Salts, in a certain proportion, are 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 



