The Maximum Efnciency of Photosynthesis : A Rediscovery 93 



After several days, when the cells had multiplied several fold, and the pH had risen 

 0.5 to 1 unit, the cultures were used for yield experiments. Such cells gave stable 

 respiration values and high yields without exception. This culture method, which 

 had been employed by one of us (D.B.) for many years in Washington, was found 

 to be an important improvement over the method of 1923 l followed by most later 

 investigators but which we have now discarded. In the earlier method, slowly 

 aerated cells settled down in Erlenmeyer flasks in Sediments that were inadequately 

 illuminated and inadequately supplied with carbon dioxide and oxygen until 

 reshaken up. 



Manometry. Approximately 300 mm 3 of cells, resuspended in 7 cm 3 of a fresh 

 culture medium of pH 4.9, were put into rectangular vessels of 14 to 20 cm 3 

 volume and saturated with 5 percent CO-2 in air. Two vessels were used to deter- 

 mine, as described in 1924 3 , the oxygen as well as the carbon dioxide exchange 

 The gas volumes in the two vessels were different, but the volumes of the Solutions 

 and therefore the concentrations of the cells were equal. The vessels were shaken 

 at 20° C. by horizontal motion, with an amplitude of 2 cm. and a frequency of 

 150 per min. In spite of this rapid motion, which moved the vessels 600 cm. per 

 minute, no splashing or foaming occurred, even in experiments of more than 20 hr. 

 duration. It was a further improvement owing to this motion that physical tran- 

 sition effects were not observed upon change from dark to light and vice versa, that 

 is, the gas equilibration was virtually perfect for our purposes. 



The well known requirement of the two-vessel method, that the metabolism in 

 the two vessels must be identical (jco 2 = x'o* and xcoo = x'coo) was complied 

 with by eliminating a dangerous differential time factor as nearly as possible.The 

 light beam of measured intensity (630 to 660 m//) was shifted by a mirror alternately 

 from one vessel to the other at intervals of 10 — 60 min. or these vessels were shifted 

 alternately into and out of the fixed light beam. Thus in every case the one vessel 

 was illuminated when the other vessel was dark and vice versa, and when many 

 light and dark periods followed each other and the pressure changes of all dark 

 periods and of all light periods were summed, the metabolism in the two vessels 

 was virtually identical for the same periods of elapsed time. The total pressure 

 changes effected by light were usually of the order of magnitude of + 20 to + 50 

 mm. These figures were differences between two rates of oxygen consumption 

 (negative pressure changes) in the experiments with noncompensated respiration. 

 But when the respiration was compensated by white light (see following section) 

 the figures + 20 to -) 50 mm. were the directly observed positive pressure changes, 

 produced by the added red light. To measure such great pressure changes simple 

 Haldane-Barcroft blood-gas manometers could be used instead of the special 

 differential manometer heretofore employed. 



Measurement of the quantum intensity. The bolometer used in 1923 was replaced by 

 the chemical actinometer described a few months ago (i . This resulted not only 

 in a simplification but in an improvement in accuracy. When the oxygen produced 

 by the red light been measured, the vessel containing the cells was replaced by a 

 similar vessel containing 2 mg. of ethyl chlorophyllide, 200 mg. of thiourea and 



