OXYGEN RESPIRATION 3 



Hence it is apparent that through respiration green plants 

 release again, in the form of chemical energy and heat, and use 

 for various \dtal requirements the radiant energy which was 

 stored up by the sunhght. Plants without chlorophyll are 

 likewise nourished by substances formed in green plants at the 

 expense of solar energy. Thus in this case also the solar energy 

 stored up by the chloroplasts is released. 



The above equation of oxygen respiration is of course only 

 important as a general scheme indicative of a complete vital 

 combustion of sugar to carbon dioxide and water. If we regard, 

 with Lavoisier, respiration as a slow combustion, it must be 

 borne in mind that respiration is not analogous to a pile of wood 

 burning in the middle of a field but is more like a combustion 

 of fuel in a steam engine. In the latter case alone is the heat 

 of combustion not completely dispersed but, by means of 

 ingeniously designed apparatus, transformed to a large extent 

 into mechanical energy. Vital combustion in a plant cell also 

 effects various transformations of energy and causes the mys- 

 terious apparatus of the li\dng protoplasm to function.*^ We 

 therefore regard respiration as a characteristic criterion of life. 

 Respiration becomes more energetic with increased activity. 

 On the contrary if the spark of life burns feebly, the respiratory 

 metabolism gradually fades out. 



2. THE GAS EXCHANGE OF RESPIRATION 



Although the respiratory energy amounts to but a small 

 fraction of the amount of photosynthetic energy of the green 

 plants,^ it is a matter of importance that the respiratory gas 

 exchange continues day and night. On this account the con- 

 sumption of respiratory material, calculated per unit of living 

 substance, is very important. The respiratory energy is 

 largely dependent upon the stage of development and the 

 amount of li\dng substance. In plant organs which are growing 

 rapidly and which are rich in protoplasm, the respiratory metab- 

 olism is scarcely less than that of animal organisms. 



' C/. e.g. Willstatter, R. und A. Stoll. Untersuchungen iiber Assimilation der Kohlensaure, 

 1918; Kostytschew, S. Ber. d. bot. Ges. 39: 319. 1921. 



*■ It is quite certain that the solar energy acts directly no further than the 

 synthesis of a hexose sugar (possibly as far as sucrose). All other energy avail- 

 able for the life and growth of the cell must come from respiration. — Ed. 



