36 



STUDIES IN PLANT RESPIRATION AND PHOTOSYNTHESIS. 



and roots a large reserve of carbohydrates on which it can draw for 

 the maintenance of its respiratory activity. Any changes in respira- 

 tion due to carbohydrate exhaustion do not, therefore, in the case 

 of the attached leaves, appear for a considerable time, while in the 

 case of excised leaves such changes become apparent after relatively 

 short periods of darkness. For this experiment an entire Helianthus 

 annuus plant was used. The plant was growing in soil in a glass jar. 

 Before the respiration rates were determined, the soil was covered 

 with tinfoil and the edges and portions around the stem of the plant 

 were sealed with cocoa-butter. The experiment was allowed to run 

 for 94.25 hours, after which the plant was still in perfectly healthy 

 and vigorous condition. The results of the determination are given 

 in table 8. 



Table 8. — Rate of emission of CO2 by a small potted plant of Helianthus annuus. 

 Soil covered with tinfoil and sealed with cocoa-butter. CO2 absorbed in Ba(0H)2 solution, 

 0.11835 normal, 125 c. c. of which haa the equivalent of 0.3254 gram CO2. 



No. 



Time. 



ll''45'°a.m. to 1 p.m. . 

 1 p. m. to 4 p.m 



4 p. m. to lOi'SO" p.m 

 10''30°> p.m. to 5 a.m . . 



5 a.m. to 10 a.m .... 

 10 a.m. to 4 p.m 



4 p.m. to lO'^SO'" p.m 

 10''30™p.m. to 5 a.m. . 



5 a.m. to 10 a.m 



10 a.m. to 4 p.m 



4 p.m. to 10''30'° p.m 

 itf'SO'^p.m. to 5 a.m. . 



5 a.m. to 10 a.m 



10 a.m. to 4 p.m 



4 p.m. to 10*^30" p.m 

 10'>30'" p.m. to 5 a.m . . 



5 a.m. to 10 a.m 



These results are graphically represented by the curve in figure 7. 

 An initial drop in the respiratory rate is noticeable here; it is very 

 much more prominent in the experiments with excised leaves. 

 The striking feature of this curve is that during about the first 48 

 hours there is a gradual increase in the rate of respiration. During 

 all this time the available supply of fuel material is diminishing and 

 there are no alterations in temperature or other external conditions. 

 It therefore seems justifiable to conclude that some internal factor 

 comes into play which acts in a stimulatory manner. From the 

 experiments hereinafter recorded it would seem that the amino-acids 



