524 



RESPIRATION 



TABLE 53 RATE OF CARBON DIOXIDE PRODUCTION IN MILLIGRAMS BY lOO YOUNG WHEAT 



SEEDLINGS AT 25° C. (dATA OF MACK, I930) 



As shown in this table the rate of respiration increased with increase in 

 oxygen concentration up to a first maximum at 9.8 or 16 per cent. Further 

 increase in the partial pressure of oxygen resulted in a fall to a minimum 

 value which was attained at 20 per cent, while still further increase in 

 oxygen concentration resulted in a rise in the rate of respiration until a sec- 

 ond maximum was attained at 90 or 95 per cent. Similar results were ob- 

 tained at other temperatures. The cause of the first maximum of respiration 

 is not clear, and perhaps this would not be shown by all species, or under 

 all experimental conditions. Some anaerobic respiration may have occurred 

 at the lowest concentrations of oxygen employed. In general these data sup- 

 port the results of several other investigators which indicate that the oxygen 

 concentration can deviate considerably from that normally present in the 

 atmosphere without greatly influencing the rate of respiration. 



5. Carbon Dioxide Concetitration of the Atmosphere. — The higher the 

 carbon dioxide concentration, the lower the rate of respiration of some plant 

 tissues. In a study of white mustard seedlings it was shown that the rate 

 of respiration decreased with increase in carbon dioxide concentration (Table 

 54). This effect was shown whether respiration was measured in terms of 



TABLE 54 EFFECT OF CARBON DIOXIDE CONCENTRATION UPON THE RATE OF RESPIRATION OF 



GERMINATING WHITE MUSTARD SEEDS. INITIAL CONCENTRATION OF O2 IN EACH EXPERI- 

 MENT 20 PER CENT. DURATION OF EXPERIMENTS, I4 HOURS. (dATA OF KIDD, I9I5) 



COo evolved (cc.) 

 O2 absorbed (cc). 

 Respiratory ratio 



Percentage of CO2 initially present 



58 



71 

 0.82 



ID 



48 



57 

 0.84 



20 



38 



49 



0.77 



30 



22 



45 

 0.73 



40 



26 



38 

 0.69 



80 



17 

 32 

 0-53 



