THE CARBON DIOXIDE FACTOR 



1899 



light compensation point." Since then, several papers have appeared 

 deahng with this subject. Gabrielsen (1948) and Gabrielsen and Schou 

 (1949) found the compensation point of excised leaves at 0.009% CO2, in 

 artificial light of 10 klux as well as in full sunlight. (The latter was a 

 confirmation of an early observation of Garreau, 1849, 1851, not noted in 

 Chapter 27.) Gabrielsen, as well as Audus (1947), believed to have ob- 

 served a [CO2] "threshold" of photosynthesis (at 0.0009%, or 0.006% 



Vol-% CO2 

 0.05 



om 



0.03 



0.02 



0.01- 



1 — T^ 

 150 m\n 180 



Fig. 37D.5. Carbon dioxide compensation (after Egle 1951). Fegatella conica, 

 natural growth, 8 X 10 cm., with partially overlapping thalli: (a) 13 klux, 20° C, 

 initial [CO2] = 0.05%; (6) same, initially no CO2; (c) same as {h), but in darkness. 



CO2, respectively), below which the net gas exchange was the same in light 

 and in the dark (as if the photosynthqtic mechanism were "shut off"). 

 Similar conclusions by Chesnokov and Bazyrina were reported in chapter 

 21 (p. 907); we noted there their contradiction with the most rehably 

 determined [CO2] curves. The renewal of the "threshold" concept by 

 Audus, and Gabrielsen, brought a rejoinder by Egle (1951). He gave 

 figs. 37D.5 and 37D.6 for the gas exchange of Fegatella conica (observed 

 with a heat conduction CO2 meter, cj. chapter 25, p. 853), in a closed cir- 

 culation system. The first figure shows the carbon dioxide concentration 

 in the gas space to approach 0.009% after about 1.5 hours of exposure to 

 light (13 klux), independently of whether the initial pressure was higher 

 or lower. The deviation of curve h from the straight line c (which rep- 

 resents CO2 liberation in darkness) proves that photosynthesis begins 



