C84 



HISTORY OF THE VEGETABLE KINGDOM. 



opuntia. These plants were confined in glass 

 vessels of atmospheric air, and kept for eighteen 

 or twenty hours in the shade, or in perfect dark- 

 ness ; but, early in the morning, the vessels were 

 taken out and exposed for four or five hours to 

 a bright sunshine ; after such exposure, tlie air 

 was examined, and was then found to have suf- 

 fered no change whatever, either in purity or in 

 volume. 



By other experiments, tYie autnor next pro- 

 ceeds to show that, though the air, when thus 

 exposed to light, had recovered its original com- 

 position, it must, during the experiments, have 

 undergone successive changes of deterioration 

 and renewal. If a substance, as moistened quick- 

 lime, wliieh strongly attracts carbonic acid, 

 were placed in the vessel with the growing 

 plants, the volume of air was observed to dimi- 

 nish, even although the apparatus were placed 

 in sunshine : the air, too, when analysed on the 

 fifth or sixth diiy of the experiment, afforded 

 only ri7„, or had lost five per cent of oxygen gas; 

 whilst similar plants, confined in another vessel, 

 but without lime, produced no change, either in 

 the purity or volume of their atmosphere. Now, 

 the diminution of volume, in the experiment 

 with lime, shows that there had been an attrac- 

 tion, and consequently a formation of carbonic 

 acid gas ; for the lime which produced the dimi- 

 nution, acted only on that gas. Tlie experi- 

 ment, it is added, shows farther, that the fonna- 

 tion of carbonic acid gas is necessary to vegeta- 

 tion, even in sunshine, and that the reason why 

 we do not perceive its production by the plants 

 which vegetate without lime in common air, is, 

 oecaiise they then decompose it in proportion as 

 they form it with the surrounding oxygen. 



This inference, respecting the sinmltaneous 

 formation and decomposition of carbonic acid, 

 derived from experiments made witli common 

 air, is supported by others, in which an artificial 

 atmosphere, containing about seven per cent of 

 carbonic acid was employed. Plants of the same 

 species as those before mentioned were made use 

 of, the same periods of alternate exposure in the 

 shade and in sunsliine were observed, and tlie 

 same times allotted for the duration of tlie expe- 

 riments. The total volume of air, at the end of 

 the experiments, had undergone little variation, 

 but its composition was greatly changed. The 

 carbonic acid gas which was added to the atmos- 

 phere had more or less completely disappeared, 

 and its place was supplied by an increase of oxy- 

 gen gas, so as to raise its proportion from twenty- 

 one to twenty-four or twenty-six per cent. In 

 these experiments, therefore, not only was the 

 carbonic acid naturally formed by the vegeta- 

 tion of the plants decomposed, but the excess 

 of that gas which was added to the atmosi)here 

 underwent the same change ; and the proportion 

 of oxygen gas was consequently increased by 



five or six per cent beyond that which occurred 

 in the experiments with common air. 



From the results of these experiments, we 

 learn that plants, like seeds, require the presence 

 of oxygen gas in the atmosphere in which they 

 grow, and like them, also, convert a portion of it 

 into an equal volume of carbonic acid gas. This 

 conversion is alike effected by their growth in 

 the shade and sunshine. In the fonner case, 

 however, the presence of this acid gas may be 

 readily detected in the residual air by the usual 

 tests; but, in the latter, it escapes detection, 

 because it is then decomposed as soon as formed, 

 by the joint agency of the plants and solar 

 light. Under a bright sunshine, therefore, the 

 two processes, by which carbonic acid is alter- 

 nately formed and decomposed, go on simul- 

 taneously ; and their necessary operation, in as 

 fiir as regards the condition of the air, is that of 

 counteracting each otlier. Hence, though both 

 may be continually exercised in favourable cir- 

 cumstances, the effects of neither on the atmos- 

 phere can be ascertained by ordinary means ; and, 

 consequently, though in the experiments of De 

 Saussure with common air, the production and 

 decomposition of carbonic acid by plants in sun- 

 shine must have been continually going on, yet, 

 in all the analyses which he made, the air was 

 found unchanged, either in purity or in volume; 

 in other words, the processes of formation and 

 decomposition of this acid gas exactly counter- 

 balanced each other. 



Of the two processes which have been now 

 described, each may be considered as in its 

 nature and purpose quite distinct from the other; 

 hence, their effectsmay be readily distinguished; 

 neither do they necessarily interfere, when 

 actually working together. The first, or deteri- 

 orating, process, in which oxygen gas is con- 

 sumed, goes on at all times and in all circum- 

 stances, when vegetation is active. It requires 

 always a suitable temperature in which to dis- 

 play itself; and when that temperature falls 

 below a certain point, which is very variable in 

 regard to different plants, the process is more or 

 less completely suspended, again to be renewed 

 when the tempemture shall again return. This 

 conversion of oxygen into carbonic acid is as 

 necessary to the evolution of the seed as to the 

 gi'owth of the plant, and is all that is required 

 for germination ; but the plant requires some- 

 thing more, for, if light be excluded, vegetation 

 proceeds imperfectly, and the plant does not 

 tlien acquire its proper colour, and otlicr active 

 properties which it ought to have. The chief 

 organs by which the consumption of oxygen 

 gas is effected are the leaves, and its purpose, in 

 great part at least, seems to be that of producing 

 some necessary change in the sap during its 

 transmission through those organs, on its way 

 from the vessels of the wood to tliose of the 



