

of the Organic Systems of Vegetables. 95 



The phenomena here adverted to, although produced in a 

 great measure by the same organs, are nevertheless the results 

 of different functions ; the one being the effect of the respira- 

 tion, the other of the digestion of the vegetable, as already 

 stated. Hence it is not that plants at one time respire carbonic 

 acid and convert it into oxygen, and at another respire oxygen 

 and convert it into carbonic acid thus breathing differently at 

 different times, and undoing by night what they had done by 

 day but that the respiration of plants, always and at every 

 time, by day as well as by night, in the sunshine as in the 

 shade, convert oxygen into carbonic acid, which process seems 

 essential for the maintenance of their vital irritability, for, if 

 deprived of it, they die. This doctrine was first, I believe, 

 adumbrated by Darwin, who guessed, although he did not 

 give any experiments to substantiate or even to illustrate his 

 speculation, that the oxygen restored by plants to deteriorated 

 atmospheric air was derived from a source independent of their 

 respiration ; and this, he fancied, was the decomposition of 

 water by the assimilating powers of the plant, by which the 

 hydrogen was retained to form its peculiar proximate prin- 

 ciples, as oil, resin, &c., while the oxygen was liberated. 

 This, doubtless, in some measure is the case ; but, as Ellis 

 (some of whose experiments I have repeated) decidedly shews, 

 the chief restoration is effected by the decomposition of the 

 carbonic acid present, so that when that gas is wholly with- 

 drawn, very little if any oxygen is produced. Ellis, likewise, 

 as well as Darwin, very philosophically distinguishes between 

 the processes by which these different results are produced, 

 although the distinction seems since to have been too generally 

 lost sight of. Thus Thompson observes, it is * pretty clear 

 that the leaves of plants absorb oxygen, and the whole series 

 of chemical experiments on plants led to the supposition, that 

 this absorption was confined to the night;' and, after a re- 

 ference to the labours of Saussure and others, he continues : 

 * During the night plants absorb oxygen, and form with it 

 carbonic acid; a portion of this gas is sometimes emitted, 

 together with a little azote, but the greatest part is retained 

 and decomposed by the leaves during the day.' ' Plants,' he 

 continues, ' will not live without this nightly inspiration, even 



