1098 THE LIGHT FACTOR. II. QUANTUM YIELD CHAP. 29 



absorbing 67% of red light (a band 10 mju wide at 650 ran, isolated by a 

 Christiansen dispersion filter) and 39% of green light (567.5 m^u), or with a 

 "thin" suspension absorbing 22% of red light; the absorption was deter- 

 mined by means of the "ellipsoid photometer" (c/. chapter 25, page 844). 

 Each run included (a) a "dark adaptation" period, (6) a period in which 

 respiration was measured, (c) a "light adaptation" period, (d) an illumina- 

 tion period, (e) a second "dark adaptation" period and (/) a final period of 

 respiration measurement — each period lasting from 15 to 30 min. The 

 illumination intensity was high enough for photosynthesis to exceed respi- 

 ration (from 500 to 5000 erg/cm. ^ sec). The quantum yields obtained 

 varied between 0.25 and 0.19, for both dense and thin suspension, with the 

 lower value (0.19 to 0.22) observed only at the higher light intensities and 

 interpreted as indications of an incipient light saturation. Similar y 

 values were found at 567.5 m/x. The quantum yield was found to be con- 

 stant over a wide spectral region, including the near infrared (where other 

 observers found no photosynthesis at all; cf. chapter 30, page 1155). 



According to a review by Franck and Gaffron (1941), Emerson and 

 Lewds, as well as Rieke, have tried to imitate Eichhoff's experiments (par- 

 ticularly with respect to the method of cultivation of the algae), but were 

 unable to obtain the high yields claimed by him. 



One peculiar feature of Eichhoff's light curves {cf. fig. 30.7) is the early saturation 

 in red light. 70% of maximum photosynthesis is reached, according to these curves, 

 with an incident intensity of only 3 kerg/cm.^ sec. Eichhoff's figures suggest that as 

 little as 5 "energetic meter candles" of red light (he calls a monochromatic energy flux 

 equal to the total "white" flux from a Heffner candle an "energetic meter candle") are 

 equivalent, as far as photosynthesis is concerned, to 15 klux of white light from a 500 

 watt incandescent lamp! The latter, according to page 838, corresponds to a flux of at 

 least 60 kerg/cm.^ sec, counting only the photosynthetically active region (400-700 m/i), 

 while 5 "energetic meter candles" are equivalent (using Gerlach's value for the radiation 

 of a Heffner candle) to only 4.7 kerg/cm.^ sec. Even though the suspension may absorb 

 red light three or four times more efficiently than the (infrared- free) white light, the dif- 

 ference between the amounts of red light and white light required to bring about the 

 same rate of photosynthesis remains striking. It suggests that the absolute intensity 

 of the red light might have been underestimated by Eichhoff by as much as a factor of 

 three or five. If this was the case, all quantum yields calculated by Eichhoff must have 

 been in error by the same factor (since no quantum yield determinations were made in 

 white light). 



More recently, Warburg (1946, 1948) undertook, with Kubowitz, to 

 repeat the original experiments of Warburg and Negelein, taking into con- 

 sideration Emerson's criticism. Warburg calculated that, in order to ac- 

 count for his 1923 results in the way suggested by Emerson, the average 

 photosynthetic quotient during the 10 min. exposure must have been Qp = 

 AO2/— ACO2 = —0.26. (In other words, four volumes of carbon dioxide 

 must have been produced in light for each volume of oxygen liberated.) 



