CHAP. X. DIGESTION. 315 



nor heat will disengage it ; but it appears to incoi-porate itself 

 wdth the tissue, since solar light readily disengages it. The 

 inference therefore is, that it is absorbed at night, and com- 

 bines with the carbon already existing, forming carbonic acid, 

 and that the latter is decomposed by the sun, as has before 

 been shown." 



It has been ascertained from other experiments that a small 

 quantity of carbonic acid is perpetually evolved by leaves 

 both day and night. Some observations by Burnett upon this 

 subject are detailed in the "Journal of the Royal Institution," 

 and have led their ingenious author to the opinion, that under 

 the name of respiration two distinct phenomena are con- 

 founded ; and that while respiration, properly so called, which 

 consists in the extrication of carbonic acid, is incessantly 

 in action, digestion, which is indicated by the decomposi- 

 tion of carbonic acid and extrication of oxygen, takes place 

 exclusively in daylight. " Hence," he says, " are we not jus- 

 tified in concluding that the production of oxygen, and its 

 converse, the formation of carbonic acid, are the imvarying 

 results of two different functions ; viz. this of I'espiration, that 

 of digestion ; and that both are vegetative actions dependent 

 upon vitality ? To conclude : the formation of carbonic acid 

 is constant both by day and night, during the life of the vege- 

 table; it is equally carried on whether in sickness or in 

 health; it is essential to its existence for the sustentation of 

 its irritability ; for, if deprived of oxygen, and confined in car- 

 bonic acid gas, plants, like animals, quickly die. Tliis func- 

 tion, which is performed chiefly by the leaves and petals, 

 though also in a less degree by the stems and roots, like the 

 respiration of animals, is attended with, and marked by, the 

 conversion of oxygen into carbonic acid ; it is the respiration 

 of plants. 



" Again : vegetables, at certain times and under certain cir- 

 cumstances, decompose carbonic acid, and renovate the atmo- 

 sphere by the restoration of its oxygen; but this occasional 

 restoration is dependent, not upon the respiratory, but the 

 digestive, system : it in part arises from the decomposition of 

 water, but chiefly from the decomposition of carbonic acid, 

 absorbed either in the form of gas or in combination with 



