70 STUDIES IN PLANT RESPIRATION AND PHOTOSYNTHESIS. 



increased. In table 27 and figure 12 is also given a case of decreased 

 respiration rate accompanied by increased sugar-content and de- 

 creased amino-acids. However, in table 12 were given the results 

 of the carbohydrate and amino-acid determinations of leaves from 

 plants which were kept in the dark, showing the gradual decrease 

 in carbohydrates and increase in amino-acids. When these plants 

 were again placed in the light the reverse process occurred; there 

 was an increase in sugars and a decrease in amino-acids. 



Figure 19. 

 Rate of respiration subsequent to periods of illumination. Six leaves of Helianthus 

 at 25°; petioles in a nitrogen-free nutrient solution containing 7 per cent d-glucose. The 

 solid lines indicate the CO2 emission in the dark, the dotted lines indicate the periods 

 during which the leaves are illuminated. The ordinate represents mg. CO2 per hour 

 per gram dry material, the abscissa the time in hours. 



There remains, therefore, to show the effect on the rate of respira- 

 tion of temporarily decreasing the amino-acids while the carbohy- 

 drate-content remains high. It has been shown that the effect of 

 Ught is to decrease the amino-acids. The following experiment was 

 carried out so that the leaves which from the beginning of the experi- 

 ment were fed d-glucose were illuminated for 6 to 7 hours after 

 having been in the dark for about 25 hours. Thus during the course 

 of the experiment there was a period of illumination of 7 hours 

 after the twenty-fifth hour, another period of illumination of 6 hours 

 after the fiftieth hour, and a third period of illumination after the 

 seventy-fourth hour. As a source of illumination there was used a 

 750-watt tungsten filament lamp at 40 cm. distance. The glass 

 cover of the respiration chamber was below the level of the water 

 in the thermostat. The results of this experiment are given in 

 table 52 and figure 19. 



During the periods of illumination the amount of carbon dioxid 

 absorbed was very low. This would indicate that a large part of 



