IIG 



HISTOKV OF THE VEaETABLE KINGDO.M. 



the time and propei- pabulum of the plant. But 

 the experiments of later phytologists, do not at 

 all countenance this opinion. Saussure introduced 

 a plant of the lythrum salicaria, into a receiver 

 containing sixty cubic inches of hydrogen gas, 

 and exposed it to the sun. Its vegetation was 

 perhaps somewhat more vigorous than that of 

 plants confined in an atmosphere of nitrogen ; 

 l)ut it had abstracted no nourishment from its 

 atmosphere, nor effected any material change 

 upon it. For at the end of five weeks of ex- 

 periment, when its atmosphere was fired by the 

 electric spark along with the proper quantity of 

 oxygen, the result was the formation of water. 

 The volume of its atmosphere was indeed di- 

 minished during the period of its vegetation; 

 but this is to be accounted for by another cause, 

 as will appear in the course of tracing the pro- 

 gress of vegetation. 



The conclusion, therefore, must be, that hy- 

 drogen is unfavourable to vegetation, and does 

 not serve as the food of plants. But hydrogen 

 is contained iii 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 suificient 

 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 compon- 

 ent parts. 



Carhordc oxide. When plants were confined 

 by Saussure in atmospheres of carbonic oxide, 

 they required nearly the same condition to sup- 

 port vegetation, and exhibited nearly the same 

 phenomena as in nitrogen. Such as were de- 

 prived of their green parts died in the course of 

 a few days. The vegetation of peas whose leaves 

 were completely developed, was languid in the 

 sun, and did not succeed at all in the shade. 

 The epilobium Mrsutum, lythrum salicaria, and 

 polygonum persicaria, vegetated indeed, as in 

 common air; but at the end of six weeks of ex- 

 periment, they had neither decomposed the oxide 

 constituting their atmosphere, nor diminished 

 its quantity. It cannot, therefore, be regarded 

 as favourable to vegetation. 



Vegetable extract. ■ All vegetables, after they 

 have flourished their allotted time, at last suffer 

 decay, and moulder into the elements, out of 

 which they were originally formed. In this 

 process of decay, part of their substance escapes 

 as gaseous matter, and part returns to the earth. 

 This latter part has been called vegetable mould, 

 and consists of carbon, tannin, or vegetable ex- 

 tract, and a portion of the earths and alkalicd. 

 Without a certain portion of this vegetable ex- 

 tract, no soil is fit for the nourishment of the 

 liigher classes of vegetation ; although some 

 lichens, and other cryptogamic plants, will grow 



from the bare and barren rock, or in pui'e sanJ. 

 It has become a question in what manner those 

 saline and earthy particles are taken up by 

 plants, and whether it is necessary that the sub- 

 stance absorbed, should be either in a gaseous 

 state, or in solution in water. To determine 

 this, 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 ex- 

 pressed and filtered fluid, yielded by evapora- 

 tion 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 fi'om a 

 well cultivated corn field, 10,000 parts of fluid 

 yielded 4 parts of extract. Such was the result 

 in these particular cases. 



But the quantity of extract that may be se- 

 parated from pure mould formed by nature upon 

 the surface of the globe, is not, in general, very 

 considerable. After 12 decoctions, all that could 

 be separated from mould of this sort, was about 

 ■J-j of its weight ; and yet this seems to be more 

 than sufficient for the purposes of vegetation: 

 for a mould containing this quantity was found 

 by experiment to be less fertile, at least for peas 

 and beans, than a mould 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 mould 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 mould 

 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 con- 

 tains 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. 



But it seems also probable, that a small quan- 

 tity of carbonic acid gas enters the plant along 

 with the extractive principle, as it is known to 

 contain this gas also. The mould analysed by 

 Saussure, was quite dry before the commence- 

 ment of the experiment, and the water employed 

 to moisten it contained no carbonic acid. But 

 the solution contained some; for when it was 



