RESPIRATION 235 



manner in which they are utilised within the tissues of plants 

 during the respiration process is not understood. The oxidation 

 is controlled and is dependent upon the protoplasm, for respira- 

 tion ceases when life becomes extinct, and the amount and nature 

 of the chemical changes carried on are not altered either by con- 

 siderably reducing or increasing the amount of oxygen in the 

 surrounding atmosphere. 



The absorption of oxygen and the subsequent emission of 

 carbon dioxide are the beginning and end respectively of a 

 long series of chemical changes, the intermediate stages of which 

 are at present unknown. 



The disappearance of starch, sugars, fats and other carbon 

 compounds during respiration is not due to simple direct oxida- 

 tion ; probably the protoplasm itself is directly attacked by the 

 absorbed oxygen after which it uses up the carbon compounds 

 to repair its waste. 



The proportion of oxygen absorbed to the carbon dioxide gas 

 given off is dependent on the energy of growth and on the 

 materials consumed during respiration. In certain plants the 



volume of carbon dioxide produced , , /• j 1 t_ 1 



ratio i c — 3 — has been found to be as low 



volume of oxygen consumed 



as '3, while in others it has been observed as high as i"2. 



In germinating seeds, tubers and bulbs containing starch and 

 sugars, and in most flowering plants, the volume of oxygen 

 taken from the air during active normal respiration, is equal to 

 that of the carbon dioxide exhaled ; but in the respiration carried 

 on during the germination of seeds containing fats and oils, the 

 volume of oxygen consumed is greater than that of the carbon 

 dioxide exhaled, some of the oxygen absorbed by such seeds 

 being apparently used up in oxidising the fats into some form 

 of carbohydrate. 



It is by means of the energy set free by the oxidation of 

 various compounds in the respiration-process that the plant is 

 enabled to maintain its vital activity, and the vital energy of 



