STUDIES IN PLANT RESPIRATION AND PHOTOSYNTHESIS. 



95 



show a slightly higher initial rate of respiration than the ones which 

 had been cut and placed in the apparatus in the preceding period. 

 Thus the first leaf was taken after an entire day of insolation, and 

 the succeeding ones after 26.56, 117.11, and 177.17 hours of darkness. 

 The results are given in table 55. 



From the graph (fig. 25) in which these determinations are plotted, 

 it is evident that the leaves which had been kept in the dark show a 

 continual decrease in respiratory activity. Moreover, the amount of 

 carbon dioxid fixed also shows a decline with decreasing respiration. 

 In the last two periods of the fourth leaf there are signs of internal 



Table 56.— Rates of respiration and photosynthesis on three successive days of an 

 excised leaf of Helianthus annuus kept in the dark. 



disturbance. The leaf had a sUghtly mottled appearance and was 

 curled in on the edges, so that the very high rate of carbon-dioxid 

 emission in the final period, No. 21, was probably due to protoplasmic 

 disturbances. In two subsequent 2-hour periods the rate of carbon- 

 dioxid emission gave even sUghtly higher values and the leaf showed 

 unmistakable signs of injury, so that these results are not included. 

 In the experiment described under 1, the longer periods of illumina- 

 tion are followed by an increase in respiratory activity. It is 

 apparent that in the present experiment, however, there apparently 

 has not been sufficient carbohydrate synthesis during the periods of 

 illumination to produce an increase in respiration when the leaf is 

 again put in the dark. 



3. In the two preceding experiments the concentration of carbon 

 dioxid was relatively low, that of atmospheric air. In the following 

 experiment this was increased ninefold. The leaf was cut from the 



