94 



STUDIES IN PLANT RESPIRATION AND PHOTOSYNTHESIS. 



had been exposed to bright sunHght in the greenhouse during the 

 two previous days. The leaf was taken in the afternoon and placed 

 immediately in the apparatus for determination of the respiration 

 and photosynthesis rates. The entire plant was covered with a 

 large hood of black paper, so as to exclude all light without altering 

 other conditions. On succeeding days leaves were cut from this 



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ffi 



u 



Figure 25. 

 Rates of respiration and photosynthesis of leaves of Helianfhus annuua at 24°, after increas- 

 ing periods of darkness. The values above the zero line indicate rates of photosynthesis, those 

 below, rates of respiration, expressed on the ordinates in mg. CO2 fixed, or emitted, per sq. cm. 

 per hour X 10. Data taken from table 65. 



plant and their respiration and photosynthesis rates determined. 

 In this manner leaves were used which had been attached to the 

 plant during the period of starvation and their store of carbohydrates 

 was not depleted as rapidly as when they were cut from the plant. 

 Moreover, the temperature of the greenhouse in which the covered 

 plant stood did not rise above 22°, while the determinations were 

 carried out at 24°. For these reasons some of the leaves at first 



