RESPIRATION OF PLANTS. 13 



The experiments of Priestley, in 1771, showed that plants when put 

 into an atmosphere containing a considerable proportion of carbonic 

 acid, and exposed to light, purified the air by removing carbon and 

 producing oxygen. Air in which animals had died, was thus rendered 

 again fit for breathing. Scheele made a series of experiments with 

 nitrogen in place of carbonic acid, and he found that plants did not 

 purify an atmosphere composed of nitrogen alone. The foul air, then, 

 in his experiments, differed completely from that in Priestley's experi- 

 ments, and hence the difference of results. Ingenhouz and Senebier 

 performed numerous experiments, which proved that during the day, 

 plants gave out oxygen gas, while during darkness, this process was 

 suspended. Saussure stated, that during the night, oxygen gas was 

 absorbed in different quantities by plants. Fleshy plants absorbed 

 least; next came evergreen trees, and then deciduous trees and 

 shrubs. This absorption of oxygen is attended with the formation of 

 carbonic and other acids. It has been said that some leaves, on 

 account of this process of oxidation, are acid in the morning, and 

 become tasteless during the day. Decandolle, Ellis, Daubeny, and 

 numerous other observers, have confirmed the conclusions drawn by 

 the early experimenters. The results of all these observations are, 

 that plants, more especially their leaves and green parts, have the 

 power of decomposing carbonic acid under the influence of solar 

 light, and of evolving oxygen. While in darkness, no such decom- 

 position takes place, oxygen is absorbed in moderate quantity, and 

 some carbonic acid is given off". The former process caused by the 

 deoxidizing power of plants, much exceeds the latter in amount. 



283. Burnett endeavoured to show that there are two processes 

 constantly going on in plants, one being what he calls digestion, con- 

 sisting in the fixation of carbon and the evolution of oxygen, and 

 only carried on during the day; the other being what he calls proper 

 respiration, consisting in the evolution of carbonic acid gas, and 

 carried on at all periods of a plant's growth. He thinks that his 

 experiments prove the disengagement of carbonic acid from the leaves 

 of plants, both during night and during day. These opinions are not 

 confirmed by other experimenters. What is generally called vege- 

 table respiration, may be regarded as equivalent to digestion, con- 

 sisting, as it does, of the decomposition of certain matters, and the 

 fixation of others by a process of assimilation; but there is no evidence 

 of the constant elimination of carbonic acid, in the same way as occurs 

 in animal respiration. It would appear to be more correct to con- 

 sider the processes in animals and vegetables as opposed. Eespiration 

 in the former being the elimination of carbon, while in the latter it is 

 the elimination of oxygen. 



284. The changes produced in the atmosphere, are caused chiefly 

 by the superficial green parts of plants. It was ^long ago supposed 



