90 RESPIRATION. 



which was supposed to consist in an absorption of oxy- 

 gen and evolution of carbonic acid by night, and an 

 evolution of oxygen along with absorption of carbonic 

 acid by day; this last, however, is to be more pro- 

 perly looked upon as dependent upon digestion, and has 

 been before explained. 



80. The process of respiration essentially consists in 

 the plant at all times inhaling oxygen from the air by 

 means of^^Teaves, and other foliaceous parts, etc., 

 which combines with a portion of the carbon left free 

 during digestion, and is now given off in the form of 

 carbonic acid. It is probable, however, that the whole 

 amount of oxygen taken in by the leaves is not at once 

 employed in removing the superfluous carbon un- 

 required for elaborative purposes, but that some portion 

 of it is absorbed into the nourishing juices of the plant 

 to serve particular purposes in its economy. 



81. In looking at the processes which take place du- 

 ring respiration and digestion, it will be seen that du- 

 ring 'the former a plant is continually depriving the air 

 in which it grows of a portion of its oxygen, and load- 

 ing it with carbonic acid, whilst in the other again oxy- 

 gen is restored to it, and some of its carbonic acid is 

 removed for the purposes of nutrition. 



82. As the evolution of oxygen from the leaves only 

 takes place when the plant is under the influence of 

 light, and as the absorption of it and disengagement of 

 carbonic acid, as is fully proved, ensues at all times, it 

 will follow that during the night, or in darkness, the air 

 in which a plant vegetates will be vitiated and rendered 

 detrimental to the life of another being, but that during 

 the day-time it will either be not materially altered, or 

 it will be rich in oxygen, according to the relative 

 amount of oxygen evolved during digestion in relation 

 to that absorbed during respiration. That the quantity 

 of oxygen given off by the leaves, and the absorption of 

 carbonic acid by them not only restores the balance 

 arising from respiration, but even tends on the whole to 

 materially enrich the air so deteriorated, seems evident 



