46 



STUDIES IN PLANT RESPIRATION AND PHOTOSYNTHESIS. 



this is taken up in the leaves and stimulates the rate of respiration 

 and carbohydrate consumption. However, the excised leaves have 

 as their only supply of carbohydrates the material stored in the 

 leaves, which is depleted in a relatively short time. It has been 

 shown, though, that sugars are easily taken into the leaf through 

 the petioles from a nutrient solution. Under these circumstances 

 the respiratory rate exhibits some interesting variations. In table 23 

 are given the results on an experiment in which the excised Helianthus 

 leaves were fed d-glucose, so that the carbohydrate-content was 

 practically the same at the beginning and end of the experiment. 

 During the course of the experiment, 80 hours at 24°, the amino- 

 acid content increased about threefold. The analytical data are 

 given in table 22. 



Table 22. 



The determinations of the rate of respiration are given in table 23 

 and figure 10. These show that although the leaves were constantly 

 receiving sugar, the rate of CO2 emission decreased regularly during 

 the first 30 hours. Thereafter the respiration rate increased so 

 that after 80 hours this was slightly above the initial rate. It should 

 be noted that at this time the carbohydrate-content was slightly 

 below the initial condition, while the amino-acids had increased 

 considerably. 



Table 23. — Rate of CO2 emission of 9 leaves of Helianthus annuus at 24°- 

 Petioles in nitrogen-free nutrient solution containing 7 per cent of d-glucose. 



