1374 INDUCTION PHENOMENA CHAP. 33 



However, Franck and French pointed out that the inactivation of "catalyst A" 

 does not offer sufficient explanation for aftereffects lasting for 20 minutes and more, 

 because from fluorescence experiments of Franck, French and Puck (1941) they con- 

 cluded that Ea can be regenerated in less than 1 second. They therefore suggested that, 

 in addition to the injury to Ea, photoxidation affects the capacity for photosynthesis 

 also by burning the stocks of intermediate products of photosynthesis and respiration. 



In section 4 above, we described the experiments of Steemann-Nielsen 

 which made him beheve that the induction which follows transition from 

 saturating to hmiting Hght intensity is caused by partial inactivation (per- 

 haps photoxidation) of a part of the photochemical apparatus — either of 

 chlorophyll itself, or of an enzymatic factor associated with chlorophyll so 

 closely that its inhibition makes the corresponding part of chlorophyll in 

 the cell incapable of contributing to photosynthesis. Steemann-Nielsen 

 also concluded that inhibition by excessively strong light (> 100 klux) may 

 have, at least partially, the same origin. Longer exposures to excessive 

 light may cause both deactivation of the photochemical apparatus (as sug- 

 gested by Steemann-Nielsen), and the photoxidation of an enzyme (or 

 enzymes) kinetically independent of chlorophyll, as suggested by Franck 

 and French (as well as photochemical burning-up of intermediates of photo- 

 synthesis and respiration). 



8. Induction in the Photoreduction by Algae 



The algae (Scenedesmiis, Raphidium) that will liberate hydrogen after 

 anaerobic incubation, if illuminated in an atmosphere of nitrogen, will 

 absorb hydrogen if illuminated in an atmosphere of hydrogen (slowly in pure 

 hydrogen, and much more rapidly in a mixture of hydrogen and carbon 

 dioxide, in which the absorbed hydrogen can be utilized for the reduction 

 of carbon dioxide). In chapter 6, this "photoreduction" of carbon dioxide 

 was discussed as a significant variation of normal photosynthesis. In 

 strong light, hydrogen liberation or photoreduction rapidly gives place to 

 normal photosynthesis. If only the net pressure change is measured, very 

 complex "induction curves" can be obtained (cf. Vol. I, fig. 14); the 

 "positive induction" that characterizes many of these curves is caused by 

 an initial liberation of hydrogen. 



Hydrogen absorption by anaerobically adapted algae also has an induc- 

 tion period. But, contrary to ordinary photosynthesis, this induction 

 period's duration increases with decreasing light intensity (cf. Vol. I, fig. 14). 

 Franck and Gaffron (1941) suggested that "hydrogen induction" is due to 

 the fact that carbon dioxide is first reduced at the cost of accumulated 

 organic hydrogen donors (fermentation products), before the reduction at 

 the cost of molecular hydrogen can get under way. If the disposal of ac- 



