Book I. FOOD OF THE VEGETATING PLANT. 229 



oxygen, and nitrogen, are the principal ingredients of plants ; while the other ingredients 

 contained in them occur but in very small proportions. It does not however follow, that 

 these ingredients enter the plant in an uncombined and insulated state, because they do 

 not always so exist in the soil and atmosphere ; it follows only that they are inhaled or 

 absorbed by the vegetating plant, under one modification or another. The plant then does 

 not select such principles as are the most abundant in the soil and atmosphere ; nor in 

 the proportions in wliich they exist ; nor in an uncombined and insulated state. But 

 what are the substances actually selected ; in what state are they taken up ; and in what 

 proportions ? In order to give arrangement and elucidation to the subject, it shall be 

 considered under the following heads : Water, Gases, Vegetable Extracts, Salts, Earths, 

 Manures. 



1522. Water. As water is necessary to the commencement of vegetation, so also is it 

 necessary to its progress. Plants will not continue to vegetate unless their roots be 

 supplied with water ; and if they be kept long without it, the leaves will droop and 

 become flaccid, and assume a withered appearance. Now this is evidently owing to the 

 loss of water ; for if the roots be again well supplied with water, the weight of the plant 

 is increased, and its freshness restored. But many plants will grow, and thrive, and 

 eifect the developement of all their parts, if the root be merely immersed in water, 

 though not fixed in the soil. Tulips, hyacinths, and a variety of plants with bulbous 

 roots, may be so reared, and are often to be met with so vegetating ; and many plants 

 will also vegetate though wholly immersed. Most of the marine plants are of this de- 

 scription. It can scarcely be doubted, therefore, that water serves for the purpose of a 

 vegetable aliment. But, if plants cannot be made to vegetate without water ; and if 

 they will vegetate, some when partly immersed without the assistance of soil, and some 

 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 

 eighteenth 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 sufiicient to form all the different substances contained in vegetables. 

 Du Hamel reared in the above manner plants of the horsechestnut and almond to some 

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

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

 whether they 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 were 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 vege- 

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

 in water do yet augment the quantity of their carbon. 



1 523. Gases. When water was found to be insufficient to constitute the sole food 

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

 vital energy of the plant was believed to be 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 conjecture is 

 founded on no proof, it is consequently of no value. It must be confessed, however, 

 that atmospheric air is indispensably necessary to the health and vigour 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 be 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 be 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 the 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. 



1524. The effect of the application of carbonic 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 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 



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