96 



STUDIES IN PLANT RESPIRATION AND PHOTOSYNTHESIS. 



plant after the latter had been exposed to the sunlight in the green- 

 house for 7 hours. The respiration and photosynthesis rates were 

 then determined. The leaf remained in the frame in the dark with 

 a constant air-stream passing over it. After about 24 hours the 

 respiration and photosynthesis rates were again determined, and 

 likewise after about 48 hours. The results of this experiment are 

 given in table 56 and in figure 26. 



Figure 26. 

 Rates of photosynthesis and 

 respiration, on three successive 

 days, of a leaf of Helianthus 

 annuus kept in the dark. The 

 values above the zero line indi- 

 cate rates of photosynthesis, 

 those below, rates of respira- 

 tion, expressed on the ordinates 

 in mg. CO 2 fixed, or emitted, 

 per sq. cm. per hour. Data and 

 conditions as per table 56. 





In this experiment, in which the carbon-dioxid content of the air 

 was greatly increased, the rate of photosynthesis shows, as in the pre- 

 vious experiments, a course which in general parallels that of the res- 

 piratory activity of the leaf. In all of these experiments the same 

 source and intensity of illumination was employed. 



4. The carbon-dioxid content of the air-stream was increased 

 further, so that it was 15.5 times that of atmospheric air. In this 

 experiment a sunflower leaf was also used. It is doubtful whether 

 with the intensity of Ught employed the carbon dioxid was the limiting 

 factor, because the photosynthesis per square centimeter per hour 

 in this experiment is slightly less than in the previous one, in which 

 the carbon-dioxid concentration was almost half of that used in the 

 present experiment. In table 57 and figure 27 the results of this 

 experiment are given. The general decline of the rate of photosyn- 

 thesis, with decreasing respiratory activity, also holds for these 

 conditions. The photosynthetic rates are calculated from the last 



