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In the changes that take place in the composition 

 of the organized parts, it is probable that saccharine 

 compounds are principally formed during the absence 

 of light; gum, woody fibre, oils, and resins during 

 its presence; and the evolution of carbonic acid gas, or 

 its formation during the night, may be necessary to 

 give, greater solubility to certain compounds in the 

 plant. I once suspected that all the carbonic acid gas 

 produced by plants in the night, or in shade, might be 

 owing to the decay of some part of the leaf, or epider- 

 mis; but the recent experiments of Mr. D. Ellis, are 

 opposed to this idea; and I found that a perfectly 

 healthy plant of celery, placed in a given portion of air 

 for a few hours'only, occasioned a production of car- 

 bonic acid gas, and an absorption of oxygene. 



Some persons have supposed that plants exposed 

 in the free atmosphere to the vicissitudes of sunshine 

 and shade, light and darkness, consume more oxy- 

 gene than they produce, and that their permanent agen- 

 cy upon air is similar to that of animals; and this opin- 

 ion is espoused by the writer on the subject I have 

 just quoted, in his ingenious researches on vegetation. 

 But all the experiments brought forwards in favour of 

 this idea, and particularly his experiments, have been 

 made under circumstances unfavourable to accuracy 

 of result. The plants have been confined and suppli- 

 ed with food in an unnatural manner; and the influ- 

 ence of light upon them has been very much dimin- 

 ished by the nature of the media through which it pas- 

 sed. Plants confined in limited portions of atmos- 

 pheric air soon become diseased; their leaves decay, 



