68 



STUDIES IN PLANT RESPIRATION AND PHOTOSYNTHESIS. 



effect of this increased amino-acid content becomes noticeable at 

 once. Thus excised leaves which had been kept in the dark for about 

 40 hours and then given d-glucose exhibited an immediate increase 

 in the rate of carbon-dioxid emission. This is not the case when 

 leaves are given d-glucose immediately after cutting from the plant; 

 there is then a primary drop in the CO2 curve and a rise after about 40 

 hours (see figs. 10 and 11). The combination, then, of the accumu- 

 lated amino-acids after 40 hours of darkness and an abundant sugar 

 supply results in an accelerated rate of respiration. 



Table 50. — Rate of CO2 emission, at 24°, of 8 leaves of Helianthus annuus. 

 The excised leaves were placed in a nutrient solution free from organic naaterial and nitrogen 



This fact becomes evident from the following experiment, in which 

 8 Helianthus leaves were cut from the plant, placed immediately 

 in nitrogen-free mineral nutrient solution, and kept in the dark. 

 After 43.5 hours the leaves were put in the respiration chamber 

 with fresh nutrient solution containing 7 per cent of d-glucose. 

 From the results of the determinations of the rates of carbon- 

 dioxid emission in table 50 and figure 18 it is apparent that the rate 

 of respiration rises immediately and, with some irregularities, then 

 attains its maximum. 



It is, moreover, a noteworthy fact that when the foregoing experi- 

 ment is repeated in such a manner that the nutrient solution con- 

 tains, besides d-glucose, 0.11 per cent of glycocoll, there is practically 

 no difference in the rate of carbon-dioxid emission. In other words, 

 it appears that the natural accumulation of amino-acids is just as 

 effective in stimulating respiratory activity as when amino-acids 

 are fed to the leaves. It was shown in the previous experiments 

 that the influence of amino-acids when fed to the leaves was espe- 

 cially noticeable in the first 40 hours of the experiment, that is, before 

 the natural accumulation of amino-acids in the dark becomes eft"ective. 



