710 Retrospective Criticism. 



the bottle to the influence of the sun's rays ; and, upon examination some 

 hours afterwards, I found the plants healthy, and the artificial atmosphere 

 in which they were put deprived of its destructive qiiality to animal life ; 

 which pui'ification must have been effected by the inhalation, and conse- 

 quent decomposition, of carbonic acid by the plants ; or it might be partly 

 owing to the expulsion of the oxygen that was contained in the plants. 

 Allowing, then, that plants do derive part of their carbon from the atmo- 

 sphere, in some instances perhaiis the greater part, it still remains to be 

 proved that from it plants generally " absorb the carbon necessary for 

 their support," 1 can scarcely think that these are the sentiments of the 

 learned professor, although the opinion is still further corroborated by 

 •what follows ; viz. " If 3'ou deprive them (plants) of atmospheric air, they 

 languish, their leaves lose their colour, their flowers their scent, and their 

 fruit its flavour, till, by degrees, they pine away and die at last absolutely 

 for want of food." (p. 437.) Now this appears to me to possess a little 

 of that absurdity which would be laid to my charge were I to affirm that 

 the little animal, that had been permitted to fill its belly to satiety, died for 

 want of food, when cruelly confined in the exhausted receiver of an air- 

 pump. 



It would not be requisite to advert to these things, but for the great 

 difference of opinion which exists among our physiological writers; one 

 class looking upon the leaves of plants as so many robbers, wasting, by 

 their exhalations, the nourishment which the stem ought to receive; while 

 others are so convinced of their absorbing and elaborating properties, 

 that they forget their exhaling ones; and, not content with making them 

 lungs, they have elevated them a degree farther, and made them the mouths 

 by which plants absorb their nourishment. 



The most common observer must have remarked the rapid progress of 

 vegetable developement, when the plant is in a green succulent state, and 

 the comparatively small progress it makes after induration has commenced ; 

 but we cannot ascribe this eflect to the " absorption of carbon from the 

 atmosphere ; " and, besides, we find that light and a free current of air are 

 unfavourable; while moisture, shade, and a certain quantity of heat are the 

 most favourable circumstances for elongation of stem. This tends to prove 

 something else than that plants derive their whole support from the 

 atmosphere, or die for want of food when deprived of it. 



If plants derived from the atmosphere the carbon necessary for their 

 support, by the agency of light, then we might naturally conclude that 

 their growth would be greatest during the day ; but the result of a great 

 many observations convinces me that the plants grow most, and fruit 

 swells most, during the night; when, instead of inhaling, plants exhale 

 carbonic acid gas. This is practically observable in hot-houses, and any 

 person may be convinced of its truth by attending, night and morning, to 

 the growth of any quick-growing plant. I found, for instance, that, upon 

 an average, a plant of Erythrina Crista galli grew three times as much in 

 the night as during the day ; the fruit of melons, that increased, upon an 

 average, 1 in. in circumference during the night, grew so little during the 

 da}', that seldom any advance was perceivable, with the exception of one 

 day, when the atmosphere was cloudy and had those appearances which 

 forebode a thunder storm ; encumbers, shaded by their own leaves, grew 

 nearly as much during the day as at night ; but some, which I exposed 

 fully to the light, did not grow in length above half as much during the day 

 as during the night. These results may be accounted for by the diminu- 

 tion of transpiration and the inhalation of oxj-gen gas, which, being neces- 

 sary to the existence of animal life, and essential to the germination of the 

 seed, seems likewise to exert a powerful agency it the after-processes of 

 vegetation. 



