48 



STUDIES IN PLANT RESPIRATION AND PHOTOSYNTHESIS. 



spite of the slightly lower carbohydrate-content, the leaves showed 

 a higher respiration rate than at the beginning of the experiment. 

 This indicates that after about 35 hours there are produced within 

 the leaf conditions which favor a more active catabolism of carbo- 

 hydrates. From the experiments already described and others 

 which are to follow, it appears certain that, when leaves are kept in 

 the dark, synchronous with the reduction of the carbohydrates 



Table 25. — Rate of emission of CO2 by 6 leaves of Helianthus annuus at 25°. 



Petioles in nitrogen-free nutrient solution containing 7 per cent d-glucose. CO2 absorbed 

 in Ba(0H)2 solution 0.1169 normal, 125 c.c. of which has the equivalent of 0.3214 gram CO2. 



there takes place an increase in the amino-acids. Moreover, an 

 increase in the amino-acids stimulates the rate of carbohydrate 

 catabolism. Under ordinary circumstances, when the stores of 

 carbohydrates are being depleted rapidly, the effect of the natural 

 increase in amino-acids is but slightly noticeable, as was seen in 

 figure 8. When, however, the leaf is fed sugar and the amino-acids 

 increase appreciably, the result is a decided stimulation of CO2 

 emission. These relations are, however, of a more complex nature 

 than would appear at first glance. They of course involve the 

 metabolism of the proteins as a source of the amino-acids, and the 

 converse question arises as to what extent the formation of amino 

 acids from proteins is affected by a decreasing supply of carbohy- 

 drates. These questions will be referred to later in this paper. 



Before taking up the effect of amino-acids on respiration in more 

 detail, there remains to be described the course of respiration under 



