STUDIES IN PLANT RESPIRATION AND PHOTOSYNTHESIS. 



71 



the carbon dioxid of respiration was fixed through photosynthesis. 

 The carbohydrate-content of the leaves, at the end of the periods 

 of illumination, was therefore, in all probability, higher than at the 

 beginning of illumination. It must also be borne in mind that the 

 petioles of the leaves were in a 7 per cent d-glucose solution during 

 the entire experiment. 



Table 52. — Rate of emission of CO2 by 6 leaves of Helianthus annuus at 25°; petioles in 

 nitrogen-free nutrient solviion containing 7 per cent d-glucose. 

 The leaves weie illuminated during periods Nos. 5, 10, and 15. CO2 absorbed in Ba(0H)2 

 solution 0.12170 normal, 125 c. c. of which has the equivalent of 0.3346 gram CO2. 



The experiment shows clearly that following a period of illumi- 

 nation there is a decrease in the rate of respiration; with continued 

 darkness the rate rises slowly again until the next period of illumi- 

 nation. It must be remembered that under the conditions of the 

 experiment the leaves had a large supply of available sugar in the 

 form of d-glucose. From the experiments which have been described 

 it becomes evident that there are at least two factors affecting the 

 rate of the respiratory process. One of these is the supply of 

 carbohydrates and the other is the amount of amino-acids. A change 

 in either of these factors alters the rate of respiration. Under most 

 circumstances light seems to affect these two factors in an opposite 

 manner: the carbohydrates are increased by the photosynthetic 

 activity of light, while under these conditions the amino-acids 

 decrease. It is conceivable that in this manner the rate of respiration 

 tends to be equalized under varying external conditions. 



