122 VEGETABLE PHYSIOLOGY 



and decomposition of carbon dioxide, is proceeding, accom- 

 panied by the evolution of oxygen. It is frequently said 

 that during daylight the process of the respiration of a 

 green plant is masked by that of carbon dioxide decom- 

 position. To put this statement into somewhat different 

 terms, the carbon dioxide which is liberated in the course 

 of respiration by the green plant, and which is in com- 

 paratively small amount, is reabsorbed by the green parts 

 of the cells, and undergoes the same decomposition as that 

 which is brought to the plant by the surrounding air. It 

 thus escapes observation unless special means, such as 

 those detailed, are adopted to bring it into evidence. 



The respiratory processes are easily observed in the 

 case of all plants, and parts of plants, that are not green, 

 as there are in such cases no gaseous interchanges that 

 would interfere with their manifestation. 



If a plant be carefully weighed at the commencement 

 and at the end of such an experiment as has been described, 

 it will be found to have lost weight during its stay in the 

 receiver, so that respiration is associated with a loss of 

 weight to the plant. This may readily be inferred from 

 the fact that the oxygen absorbed and the carbon dioxide 

 exhaled are approximately equal in volume, carbon dioxide 

 being perceptibly heavier than oxygen. Besides the 

 carbon dioxide, however, there is always also a certain 

 exhalation of watery vapour which takes place quite 

 independently of any supply from the root or the cut end 

 of the stem. The nature of the metabolism, or the vital 

 processes, is such that the living substance gives off both 

 water and carbon dioxide, while it coincidently absorbs 

 oxygen. This is quite independent of any constructive 

 processes, for it can be observed when no nutritive material 

 of any kind is supplied to the plant. 



Though respiration is constantly proceeding wherever 

 living substance is found, the activity of the process is 

 by no means uniform. With care it can be detected in 

 such quiescent parts of plants as resting seeds, or buds 



