154 Miscellanies. 
6th. ‘The leaves of the plant either deteriorate or ameliorate the 
air,—increasing or diminishing its oxygen. Both these effects have 
been too generally ascribed to the same cause, viz. the respiration of 
plants, which has been supposed sometimes to form, and at others to 
decompose carbonic acid; but they are, in truth, distinct, and are 
performed by two separate systems—the one being the result of the 
digestive, the other of the respiratory funetion. 
“th. Plants do not at one time respire carbonic acid, and convert 
it into oxygen, and at another respire oxygen, and convert it into car- 
bonie acid,—thus breathing differently at different times, and undoing 
by night what they had done by day,—but the respiration of plants 
at every time, by day as well as by night, in the sunshine as in the 
shade, converts oxygen into carbonic acid. 
Sth. Light, assisted only by the filamentary forms of lifeless matter, 
ig unable to effect those changes which the living plant so quickly 
so certainly induces; nay, experiments show that the decaying leaves 
of plants, and newly turned up mould deprave the air in which they 
may be confined. . 
Oth. Unhealthy plants deprave the air, both in the sunshine and in 
the shade. If the leaves of healthy plants be crushed so as to inter 
fere with the due performance of their functions, they deteriorate the 
atmosphere. Healthy plants, enclosed in vessels of carbonic acid; 
are speedily destroyed, whether kept in the light or not. 
10th. Experiments show that whenever carbonic acid is produced 
in excess, the solid substance of the plant is lessened ; and, on the 
contrary, when oxygen is evolved, its solid materials are increased. 
11th. Are we not justified in concluding, from these results, that 
the production of oxygen and its converse, the formation of carbone 
acid, are the unvarying results of two different functions: ViZ- thi 
of respiration, and that of digestion; and that both are vegetall’? 
actions, dependent on vitality ? 
12th. The formation of carbonic acid is constant, both by day and 
by night, during the life of the vegetable ; it is equally carried 0 
whether in sickness or in health; it is essential to its existence fot 
the sustentation of its irritability ; for, if deprived of oxyge® and col 
fined in carbonic acid gas, plants like animals quickly die. ‘This fue 
tion, which is performed chiefly by the leaves and petals, though also” 
a less degree by the stems and roots, like the respiration of animals; 
attended with and marked by the conversion of oxygen into carbow 
acid; it is the respiration of plants. 
