76 RESPIRATION 



respiratory index of the entire plant continuously declines with 

 increasing age. For example, entire plants 2 days from 

 germination gave 3 mg. carbon dioxide per gram of dry weight 

 per hour, whilst plants 136 days from germination yielded 

 but -39 mg. carbon dioxide per gram of dry weight per hour. 

 A similar decrease in the respiratory index is exhibited by the 

 stem, leaves and flowers. In the stem the value fell from '8 

 mg. on the thirty-sixth day from germination to *o8 mg. on the 

 1 36th day from germination; during the same period the 

 measure for the leaves decreased from I "56 to -44 mg. The 

 fact that the initial respiratory index of successive leaves de- 

 creases with the age of the plant indicates a respiratory decre- 

 ment of the merismatic tissue with age and from this it follows 

 that the fall in the respiratory index of the whole plant is not 

 due to the proportionate increase with age of dead tissue, 

 sclerenchyma and tracheae for instance. 



Nicolas* also found that the presence of anthocyanins 

 was a factor, or rather the symptom of an internal factor, of 

 importance. Leaves containing a red pigment either as a 

 youth form or as a permanent character absorbed more oxygen 

 and exhibited an increased respiration as compared with green 

 leaves, whilst leaves turned red either by accident or by stress 

 of conditions! showed a decreased respiration as compared 

 with green leaves, the amount of carbon dioxide being smaller. 



The intensity of respiration commonly is measured by the 

 amount of an end product given off in unit time : in aerobic 

 respiration the final product measured is carbon dioxide or, in 

 special cases, temperature is measured. 



With regard to the measurement of carbon dioxide, it is 

 clear that the results obtained may not be a true expression of 

 the respiratory activity since the exhalation of the gas may be 

 greatly hindered by various circumstances. The surface of 

 the respiring organ is one such : Hoffmann J found that the 

 amount of carbon dioxide evolved in twenty-four hours per 

 kilogram of large, medium, and small-sized potatoes was 259, 

 314, and 326 mg. respectively; barley gave confirmatory 

 results. He also found that the nitrogen content was im- 



* Nicolas: "Compt. rend.," 1918, 167, 131. 



t See Vol. I., p. 251. 



J Hoffmann : " Journ. f. Landw.," 1916, 64, 289. 



