THE FOOD OF VEGETABI.F^. 



115 



tlie same circumstimce, with flie addition of a 

 quantity of lime, the atniospliere was contamin- 

 ated, and tlie giai)cs did not ri|)en. Oxygen, 

 therefore, is essential to the development of tlio 

 vegetating ])lant ; and for this pui-pose it is ab- 

 sorbed by the stomata of the leaves. 



Saussure having suspended a plant of the 

 cactKs opuntia, after sunset, in a receiver con- 

 taining forty-eight cubic inches of atmospheric 

 air deprived of its carbonic acid, but of wliich 

 six cubic inches were displaced by the leaves, 

 found early next morning, after making the 

 necessary corrections relative to change of tem- 

 perature and pressure, that the atmosphere of 

 tlie plant had diminished in volume four cubic 

 inches. The remaining air when examined, con- 

 tained but 1^3 of oxygen, though before the in- 

 troduction of the cactus, it had contained ,^,\, of 

 the same gas. It follows, tlierefore, tliat the 

 diminution of quantity had affected the oxygen 

 only. But the oxygen did not exist in tlie at- 

 mosphere of the plant under any combination 

 whatever; for the application of lime water gave 

 no indications of the presence of carbonic acid. 

 The oxygen of the atmosphere, therefore, must 

 have been abstracted by the leaves of the cactus. 

 From which it also follows, that the leaves of 

 vegetating plants do actually inhale oxygen, at 

 least in course of the night. 



Similar experiments on vegetating plants gave 

 -similar results, but the quantity of oxygen ab- 

 stracted, was not always in the same proportion. 

 In the present case it was very considerable, 

 amounting to three-fourths of the volume of the 

 leaves, while in other cases, it was often not 

 more than one-half of their volume. 



Nitrogen. Though niti-ogen gas constitutes 

 by far the greater part of the mass of atmos- 

 pheric air, it does not seem capable of affording 

 nutriment to plants; for as seeds will not ger- 

 minate in it, so neither will plants vegetate. ■ It 

 was regarded, however, as constituting a vegeta- 

 ble food by some of the earlier pneumatic che- 

 mists, particularly by Priestley, who found, as it 

 seems, that some sprigs of mint on which he had 

 made the experiment, vegetated better iu phlo- 

 gisticated air than in cither deplilogisticated or 

 common air; and hence, he inferred that phlo- 

 gisticated air, the nitrogen of modern chemists, 

 serves as a vegetable food. In this opinion he 

 was followed by Ingenhoutz, whose experiments 

 appear to have given a similar I'esult; contra- 

 dicted, however, by the result of the experi- 

 ments of Seuebier, Woodhouse, and Saussure, 

 on the same subject. 



Branches of poplar, (^pojMlus nigra,) and 

 willow, (^salix alba,) whose loaf-buds were just 

 ready to open, were introduced by Saussure into 

 an atmosphere of nitrogen both in the shade and 

 sun. They effected no farther development of 

 parts, but were found to be in a state of putre- 



faction, after a period of five days; but in an 

 atmosphere of common air they readily effected 

 their development, and continued to vegetate for 

 many weeks. Hoses and lilies gathered two or 

 three hours before their expansion, and treated 

 in the same manner, gave similar results. 



It must he admitted, however, that many 

 plants will continue to vegetate for a time in an 

 atmosphere of nitrogen gas, when their leaves 

 have been previously developed; but they are 

 such plants only as present a great extent of sur- 

 face, and consume but little oxygen in the shade. 

 A plant of the cactus opuntia, nourished witli 

 water, and placed in an atmosphere of nitrogen 

 gas exposed to the influence of the sun, was 

 found capable of supporting vegetation for the 

 space of three weeks; but it was greatly injured 

 by the experiment, and in the shade it lived 

 only five days. A plant of the sedum tellephium 

 when treated in the above manner, gave a similar 

 result; and yet these plants vegetated to an inde- 

 finite time in an atmosphere of common air. 



From the above experiments, it seems to fol- 

 low that nitrogen gas, at least in its pure state, 

 is unfavourable to vegetation; but particularly 

 in the shade. And yet there are some plants 

 that seem to succeed equally well in an atmos- 

 phere of nitrogen gas, as in an atmosphere of 

 common air. A plant of the lythrmn salicaria, 

 selected for the purpose of experiment, was put 

 into a receiver containing sixty-five cubic inches 

 of nitrogen gas, of which it displaced about one- 

 eighth of a cubic inch. It had its roots immersed 

 in about an ounce of water, and was exposed to 

 the rays of the sun, when it grew and became 

 so luxuriant, that it was more than once neces- 

 sary to remove it into a larger receiver. But 

 this luxuriance of growth seems incompatible 

 with the previous conclusion. At the end of 

 two months, however, when the experiment was 

 stopped, the receiver was found to contain the 

 same quantity of nitrogen gas as at the begin- 

 ning. The plant could have derived no nutri- 

 ment, therefore, from its atmosphere. But this 

 was the case also in all of the preceding exam- 

 ples. There was no diminution in the original 

 quantity of nitrogen introduced into the receiver. 

 It follows, therefore, that nitrogen gas, at leiist 

 in its pure state, is not only incapable of afford- 

 ing a vegetable aliment, but is not even absorbed 

 into the plant. But nitrogen is found in almost 

 all vegetables, particularly in the wood, in ex- 

 tract, and in their green parts. It is probable, 

 therefore, that their nitrogen is derived from tho 

 extractive principle of vegetable mould. 



Hydrogen gas. A plant of the epilobium hir- 

 sutiim, which was confined by Priestley in a re- 

 ceiver fiUed 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 



