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 function. 



7th. Plants do not at one time respire carbonic acid, and convert 

 it into oxygen, and at another respire oxygen, and convert it into car- 

 bonic 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. 



8th. Light, assisted only by the filamentary forms of lifeless matter, 

 is unable to effect those changes which the living plant so quickly and 

 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. 



9th. 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 carbonic 

 acid, are the unvarying results of two different functions : viz. this 

 of respiradon, and that of digestion ; and that both are vegetative 

 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 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 con- 

 fined in carbonic 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. 



