CHAPTER XVI 



PHOTOSYNTHETIC EVOLUTION OF OXYGEN IN THE COMPLETE 

 ABSENCE OF CARBON DIOXIDE 



Photosynthetic evolution of oxygen by Hydrilla in complete absence of 

 supply of carbon dioxide — Photosynthesis of specimens in acid con- 

 dition under increasing supply of carbon dioxide — Translocation of 

 the characteristic points in photosynthetic curve — Electric response 

 of acid plants — Unreliability of method of C0 2 -absorption — Assimi- 

 latory and respiratory quotients — Assimilation of organic acids — Effect 

 of increasing strengths of malic acid on photosynthesis — Possible use of 

 product of internal respiration in photosynthesis. 



The experiments on the effect of different C0 2 -concentrations 

 carried out during winter months showed that the minimum 

 concentration necessary for the initiation of photosynthesis 

 was about 2-5 mg. of C0 2 per 100 c.c. of water ; that the 

 turning-point occurred at a concentration of about 9 mg., 

 after which the photosynthetic curve became rounded off, 

 exhibiting a slow rise. In spring-specimens the minimum 

 C0 2 -concentration for photosynthesis was 1 mg. per 100 c.c. 

 of water, and a reversal of curve indicative of depression 

 occurred above the concentration of 12 mg. per 100 c.c. The 

 experiment was repeated in April, which may be regarded 

 as the beginning of summer. Three specimens from the 

 same pond were examined, and determinations made of 

 the minimum C0 2 -concentration necessary for initiation 

 of photosynthesis. One of these gave the normal result — 

 that is to say, the minimum C0 2 -concentration was 1 mg. 

 per 100 c.c. The results given by the second and third 

 specimens were, however, found to be extraordinarily 

 different, for they continued to evolve oxygen in the complete 

 absence of any supply of carbon dioxide. The specimens 

 had in fact been placed in distilled water, yet the rate of 



