The Maximum Efficiency of Photosynthesis 105 



the second 5-minute periods of illumination were actually more positive (less 

 negative) than for the first 5-minute periods (-- 1.86 for the second period com- 

 pared with — 2.17 for the first; + 0.11 for the second period compared with 

 — 2.13 for the first). This result, offered as a typical experimental example, was 

 a direct contradiction of an outburst and of 4(Fig - 2 ) provided to illustrate an initial 

 pressure burst graphically. In a later paper 5 , in which identification of the pressure 

 burst with a burst of C0 2 was attempted, the only data again available 5(Figs2and3 

 were obtained by the two-vessel method in which the two vessels were studied not 

 simultaneously but 5 hours apart, and the vessel-constant data necessary to calcu- 

 late cmm O2 and CO2 from pressure changes were unfortunately not supplied 

 so that it is not possible for the reader to check the calculations independently or to 

 arrive at an estimate of the error involved; the latter ordinarily may be shown to 

 become large as values of y approach — 0.5 to 0,and extremely large for positive 

 values (0 to + 4). 



2. Later Experiments 



In 1945 ,! the assimilatory quotient y == CO2/O2 was redetermined for Chlorella 

 suspended in acid culture medium, but this time the manometric instead of the 

 gas analysis method was employed, and the illumination periods were of the order 

 of 10 min., not hours. Again a y value of about minus one (— 0.93) was obtained. 



A new series of efficiency determinations was then carried out with the method 

 of 1923, but this time with different volumes of fluid and gas phases in the vessels. 

 While the relation, zjf/*>g = = volume of fluid/volume of gas, had been 2.24 in 1923, 

 this time it was 1.24. Again an average quantum requirement of 4 was obtained, 

 when again the experiments were computed with a y value of - - 0.9. According to 

 the principle of the two-vessel method 7 , the same quantum requirement could not 

 have been obtained in 1923 and 1945, if the true y value had differed markedly 

 from minus one. 



At the end of 1948, a few two-vessel efficiency experiments were carried out by 

 us at the Botany Laboratory of the University of Illinois, Urbana, through the 

 courtesy of Dr. Robert Emerson, with cultures of Chlorella provided by him. 

 The cells, when washed in fresh acid culture medium and strongly illuminated 

 with an incandescent lamp, produced 2 like normal cells, i.e., the Blackman 

 reaction was normal. But at low-light intensities of red light, where the photochemi- 

 cal reaction was the limiting reaction, the O2 production was at first often small or 

 zero, or even negative (O2 being absorbed in light instead of being produced). 

 Evidently the photochemical reaction was abnormally ineffective, or some photo- 

 öxidation process was superimposed. After a time, when the cells had been shaken 

 for many hours in the fresh acid culture medium, the photochemical reactivity 

 often recovered. On 3 of 10 days when experiments were carried out, quantum 

 requirements of 4 — 6/molecule of O2 produced or CO2 consumed were obtained 

 after long "latent" periods. 



Such experiments left little doubt that quantum requirements as low as 4 — 6/mo- 

 lecule of O2 produced or CO2 consumed were experimentally observable by the 



