Book I. 



FOOD OF THE VEGETATING PLANT. 



161 



even when totally immersed, so as that no other food seems to have access to them ; does 

 it not follow that water is the sole food of plants, the soil being merely the basis on 

 which they rest, and the receptacle of their food ? This opinion has had many advo- 

 cates ; and the arguments and experiments adduced in support of it were, at one time, 

 thought to have completely established its truth. It was indeed the prevailing opinion 

 of the seventeenth century, and was embraced by several philosophers even of the eight- 

 eenth century ; but its ablest and most zealous advocates were Van Helmont, Boyle, 

 Du Hamel, and Bonnet, who contended that water, by virtue of the vital energy of 

 the plant, was sufficient to form all the different substances contained in vegetables. 

 Du Hamel reared in the above manner plants of the horse-chestnut and almond to some 

 considerable size, and an oak till it was eight years old. And, though he informs us 

 that they died at last only from neglect of watering : yet it seems extremely doubtful 

 whether tliey would have continued to vegetate much longer, even if they had been 

 watered ever so regularly ; for he admits, in the first place, that they made less and less 

 progress every year ; and, in the second place, that their roots Vv'ere found to be in a 

 very bad state. The result of a great variety of experiments is, that water is not the 

 sole food of plants, and is not convertible into the whole of the ingredients of the veget- 

 able substance, even with the aid of the vital energy ; though plants vegetating merely 

 in water, do yet augment the quantity of their carbon. 



727. Gases. When it was found that water is insufficient to constitute the sole food 

 of plants, recourse was next had to the assistance of the atmospheric air ; and it was 

 believed that the vital energy of the plant, is at least capable of furnishing all the dif- 

 ferent ingredients of the vegetable substance, by means of decomposing and combining, 

 in different ways, atmospheric air and water. But as this extravagant conjectui*e is 

 founded on no proof, it is consequently of no value. It must be confessed, how^ever, 

 that atmospheric air is indispensably necessary to the health and vigor of the plant, 

 as may be seen by looking at the different aspects of plants exposed to a free circulation 

 of air, and plants deprived of it : the former are vigorous and luxuriant ; the latter 

 weak and stunted. It may be seen also by means of experiment even upon a small 

 scale. If a plant is placed under a glass to which no new supply of air has access, it soon 

 begins to languish, and at length withers and dies ; but particularly if it is placed under 

 the exhausted receiver of an air-pump ; as might indeed be expected from the failure of 

 the germination of the seed in similar circumstances. The result of experiments on this 

 subject is, that atmospheric air and water are not the only principles constituting the 

 food of plants. But as in germination, so also in the progress of vegetation, it is part 

 only of the component principles of tlie atmospheric air that are adapted to the purposes 

 of vegetable nutrition, and selected by the plant as a food. Let us take them in the 

 order of their reversed proportions. 



728. The effect of the application of cajhonic acid gas was found to be altogether prejudicial in the pro- 

 cess of the germination of the seed. But in the process of subsequent vegetation its application has been 

 found, on the contrary, to be extremely beneficial. Plants will not indeed vegetate in an atmosphere of 

 pure carbonic acid, as was first ascertained by Dr. Priestley, who found that sprigs of mint growing in 

 water, and placed over wort in a state of fermentation, generally became quite dead in the space of a day, 

 and did not even recover when put into an atmosphere of common air. Of a number of experiments the 

 results are — 1st, That carbonic acid gas is of great utility to the growth of plants vegetating in the sun, as 

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

 sphere, at least v/ithin a given degree, forwards vegetation ; 2d, That, as appUed to the leaves and 

 branches of plants, it is prejudicial to their 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. 



729. 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 de- 

 prived 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 tire sun, ripened Avithout effecting any material alteration in its 

 atmosphere. But when a bunch was placed in the same circumstances, v/ith 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 the night. 



730. Though nitrogene gas constitutes by far the greater part of the mass of amospheric 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, such as the vinca minor, lythrum salicaria, inula dysenterica, 

 epilobium hirsutum, and polygonum persicaria, that seem to succeed equally well in an atmosphere of 

 nitrogene 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 veget- 

 able mould. 



731. Hydrogene gas. A plant of the epilobium hirsutum, 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. 

 Our conclusion from various experiments is, that hydrogen is unfavorable to vegetation, and docs nut 

 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 tlic state of 

 gas, they may at least acquire it in the state of water, which is indisputably a vegetable food, and of 

 whicii liydrogen constitutes one of tlie component parts. 



