162 SCIENCE OF GARDENING* Tart II. 



732. 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 alledged 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 in 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. 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 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 four parts of extract. Such was 

 the result in these particular cases. But the quantity of extract that may be separated 

 from 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 that contained 

 only one half or two thirds 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 vegetating 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. 



733. 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 

 salts are thus taken up by the root of the vegetating plant, does it appear that they are 

 taken up as a food ? Some plants, it must be confessed, are injured by the application of 

 salts, as is evident from the experiments of Saussure ; but others are as evidently benefited 

 by it. Trefoil and lucerne have their growth much accelerated by the application of sul- 

 phate of lime, though many other plants are not at all influenced by its action. The 

 parietaria, nettle, and borage will not thrive, except in such soils as contain nitrate of lime 

 or nitrate of potass ; and plants inhabiting the sea-coast, as was observed by Du Hamel, 

 will not thrive in a soil that does not contain muriate of soda. It has been thought, how- 

 ever, that the salts are not actually taken up by the root, though converted to purposes of 

 utility by acting as astringents or corrosives in stopping up the orifices of the vessels of the 

 plant, and preventing the admission of too much water : but it is to be recollected that 

 the salts in question are found by analysis in the very substance of the plant, and must 

 consequently have entered in solution. It has been also thought that salts are favorable 

 to vegetation only in proportion as they hasten the putrefaction of vegetable substances 

 contained in the soil, or attract the humidity of the atmosphere. But sulphate of lime is 



