REPORT OF THE SECRETAHY 91 



graphic method of carbon dioxide measurements previously used has 

 been adapted to a constant-flow technique with a rapid time re- 

 sponse. The intensity of fluorescence was measured with a Alter- 

 photocell combination. 



Experiments so far carried out under a wide range of conditions 

 may be described in terms of two processes. In one, an inverse rela- 

 tionship appears to exist between the rate of carbon dioxide uptake 

 and the intensity of fluorescence. In the other, there is a direct 

 relationship seems to be superimposed. 



of wheat seedlings in low oxygen concentration when suddenly ex- 

 posed to high light intensity. In this case the fluorescence curve 

 shows an abrupt initial rise, a slower secondary rise, and a decay 

 toward the steady state. The simultaneously observed rate of carbon 

 dioxide uptake follows a course inversely related to fluorescence. 

 Thus, when the intensity of fluorescence or rate of carbon dioxide 

 uptake are plotted against time, the two curves are almost perfect 

 mirror images (as to time). For wheat in normal oxygen concen- 

 tration, the mirror-image relationship is less perfect, and a direct 

 relationship seems to be superimposed. 



The dependence of the direct relationship on oxygen and the 

 observation of a greater rate of carbon dioxide uptake in low oxy- 

 gen suggests that this process involves a photooxidation. In the 

 alga Ghlorella 'pyrenoidosa the induction behavior is greatly influ- 

 enced by the previous conditions of culture. Cells grown in 4 per- 

 cent carbon dioxide show a response comparable to that of wheat. 

 When the cells are acclimated to air of 0.03 percent carbon dioxide 

 the photooxidation type of response predominates. 



Further and more quantitative work is being undertaken along 

 this line, for it is felt that fluorescence in these experiments is a 

 useful tool in the study of the mechanism of photosj^nthesis. 



Preparatory to other experiments on photosynthesis, respiration 

 and chlorophyll studies have been continued with the recording spec- 

 trographic carbon dioxide apparatus. Attention was especially di- 

 rected to detecting any difference in respiration of etiolated barley 

 seedlings that might occur in a change from darkness to light of low 

 intensities. As has been pointed out in other reports, this information 

 is essential in the measurement of photosynthesis as determined by 

 gaseous exchange. Repetition of these experiments indicated a slight 

 increase in the rate of respiration when the plants were illuminated. 

 However, the rates of respiration were different on successive periods 

 so that it was necessary to look for possible sources of error. It was 

 found that etiolated seedlings placed in the growth chamber connected 

 with the carbon dioxide measuring apparatus did not become green 

 in a normal manner. The amount of chlorophyll formed v.as 20 to 30 

 percent lower than in a control chamber not connected with the ap- 



