CHAP. X. 
DIGESTION. 
315 
nor heat will disengage it ; but it appears to incorporate itself 
with 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 unvarying 
results of two diiferent functions ; viz. this of respiration, 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. This 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 
