22 NATURE OF RESPIRATION 



pounds that are built up through its power. The energy is 

 locked up, so to speak, in the foods and finally it is lodged in the 

 compounds that are constructed from the foods, i. e., in the 

 living substance and in the tissues of the plant. If now any of 

 these compounds are decomposed into their constituent parts 

 then the energy will be set free unimpaired. Oxygen is the 

 principal agent in carrying on this decomposition. It has a 

 great attraction for nearly all the elements occurring in the plant 

 and so it is able to detach one element from another and so 

 effect decomposition and the liberation of the locked up energy. 

 You are not to think of the oxygen entering directly into combi- 

 nation with the foods or with the substances formed from the 

 foods but rather with the decomposition products that are 

 brought about by the living matter. It is probable that respira- 

 tion starts in the living substance. Some agent, other than 

 oxygen and possibly enzymic in nature, inaugurates these 

 decompositions and then oxygen comes in at some stage in this 

 breaking down process and makes possible a further reduction. 

 As evidence that the oxygen does not combine directly with the 

 more complex substances mention may be made of the fact that 

 the absorption of oxygen is quite independent of the amount of 

 oxygen present. The plant respires at the same rate in an 

 atmosphere of pure oxygen as in an atmosphere containing one 

 twentieth part of the oxygen in the air. Furthermore the 

 amount of oxygen absorbed is independent of the amount of 

 foods stored in the plant. So respiration is not a simple and 

 direct process like combustion where the percentage of oxygen 

 and the amount of material present determine the reaction. 



In the majority of cases oxygen causes a continuation of the 

 decomposition processes until such simple substances as CO2 

 and HO compounds, such as water, are formed — in a word 

 until the very substances utilized in photosynthesis are formed. 

 Some of these simple products, such as H2O and CO2, escape 

 from the stomata as gas and water vapor. Accordingly respira- 

 tion is usually indicated by the absorption of oxygen and the giv- 

 ing off of CO2 and H2O. Some plants, however, have sufficient 

 oxygen in their tissues to enable them to breathe for considerable 



