156 RESPIRATION OF PLANTS. 



during the night oxygen gas was ahsorbed in different quantities 

 by plants. Fleshy plants absorbed least ; next came evergreens, 

 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. De- 

 candolle, Ellis, Daubeny, and numerous other observers, have con- 

 firmed the conclusions drawn by the early experimenters. The results, 

 of aU 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 dark- 

 ness no such decomposition takes place, oxygen is absorbed in moderate 

 quantity, and some carbonic acid is given ofi. The former process, 

 caused by the deoxidising or rather decarbonising power of plants, 

 much exceeds the latter in amount. And thus the respiratory process 

 in plants and in animals is antagonistic, consisting in the former of 

 the elimination of oxygen, while in the latter it is the elimination of 

 carbon. 



Burnett endeavoured to show that there are two processes con- 

 stantly going on in plants, one being what he calls digestion, consisting 

 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 ia 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 day. Carpenter entertains similar opinions, believing that 

 under all circumstances vegetable respiration is a process continued 

 throughout, and essential for vegetable life ; that it consists of the 

 elimination from the system of the superfluous carbon, either by its 

 entering into combination with the oxygen of the air, or by giving off 

 carbonic acid to replace the oxygen absorbed. Mr. Pepys is of opiaion 

 that the evolution of carbonic acid indicates an abnormal condition of 

 the leaf, which, in the process of healthy active vegetation, absorbs 

 carbonic acid and disengages oxygen. He believes that the action of 

 light leads to the greater perfection of this function, which is less 

 energetically performed if not wholly suspended during the night. 

 The changes produced in the atmosphere are mainly caused by the 

 superficial green parts of plants. The oxygen evolved by plants 

 appears to be derived from the carbonic acid of the atmosphere, the 

 carbon of which is appropriated, and probably partly from the water, 

 the hydrogen of which is assimilated. Light is necessary for these 

 decompositions, and it is probable that the alkalies taken up by the 

 roots aid the process. 



If the leaves of a plant are bent under an inverted tumbler of 

 water, in a pneumatic trough, and exposed to the sun, bubbles of gas 



