196 LECTURE XI. 



It is comparatively easy to detect the evolution of carbon 

 dioxide by plants which are not green, and even of those 

 which are green when they are not exposed to light. When 

 a green plant is exposed to light the gaseous interchange 

 which is the expression of its constructive metabolism is so 

 much more considerable than that which accompanies its 

 destructive metabolism, that (as pointed out in Lecture v., 

 p. 79) the former obscures the latter. Still, as the researches 

 of Garreau shew, the respiration of green plants can be de- 

 tected even in the presence of light. In order to avoid the 

 disturbing influence of the action of light experiments upon 

 respiration are usually conducted in the dark. It may be 

 assumed that all the experiments which will be cited below as 

 illustrations were performed in the dark, unless the contrary 

 is expressly stated. 



The respiration of young organs is, in general, more con- 

 siderable than that of old ones, for it is in young organs 

 (germinating seeds, leaf-buds, etc.) that the metabolic pro- 

 cesses of which respiration is an expression are being most 

 actively carried on. In the case of flowers, in which respira- 

 tion is especially energetic (as compared with leaves, for 

 example), de Saussure states that they respire more actively 

 when fully expanded than when in the bud; this increased 

 respiratory activity is doubtless connected with the process of 

 fertilisation or with the preparation for it 



We will begin our study of respiration by endeavouring to 

 ascertain in how far the exhalation of carbon dioxide and of 

 watery vapour is related to the absorption of oxygen. 



In considering the constructive metabolism of a green 

 plant under the influence of light we found that the volumes 

 of carbon dioxide absorbed and of oxygen exhaled are ap- 

 proximately equal, and we could account for this equality on 

 ascertaining the nature of the chemical processes in operation 

 in the chlorophyll-corpuscles. But there is no such constant 

 relation between the volumes of carbon dioxide exhaled and 

 of oxygen absorbed in respiration, and the processes of de- 

 structive metabolism are so complex that we cannot account 

 for the relation, whatever it may be, between the volumes of 



