PROBLEMLight is essential for chlorophyll formation in higher plants, and it is likely that this process involves the building up of a light-sensitive precursor in the dark, and the subsequent formation of chlorophyll upon radiation. That such a precursor exists has been demonstrated many times since Pringsheim first recognized it in 1874. He observed an absorption band at 620 to 640 mg in an extract of etiolated leaves, corresponding to a substance which he named etiolin. Monteverde, in 1893, obtained essentially the same result from a spectroscopic examination of extracts of wheat, maize, and sunflower. He called this red-absorbing pigment protochlorophyU, the name by which it is most commonly known today. Lubimenko ~1928) believed protochlorophyll to be a breakdown product of the true chlorophyll precursor, chlorophyllogen, because he was unable to identify protochlorophyll in living plants. Subsequently the protochlorophyll band was observed in living, etiolated leaves of Zea plants (Scharfnagel, 1931), establishing it as a natural component of etiolated plants. However, it is not possible to observe directly the conversion of protochlorophyll into chlorophyll in the living plant (Noack and Kiessling, 1929) because the appearance of the strong absorption band of chlorophyll at 620 to 640 m/~ would mask the protochlorophyll band in the same place.Protochlorophyll has not been isolated in pure state, but chemical studies have been conducted on impure extracts of the inner seed coats of the pumpkin seed, which contain a greenish substance with the protochlorophyU absorption bands (Noack andKiessling, 1929, 1930). This work reveals the presence of a magnesium-rich molecule with porphyrin-like 15roperties. It remained for Fischer, Mittenzwei, and Oestreicher (1939) working on the same experimental material, to identify the molecule as the phyUin of vinylpheoporphyrin ai phytyl ester, in other words differing from chlorophyll a only in lacking two hydrogens in the 7, 8 position.Nevertheless, the main question conceming the pumpkin seed protochlorophyll still remains unanswered: is this substance the true precursor of chlorophyU, or is it a breakdown product of chlorophyll? Both lines of thought can be found in the literature.The basic fact remains that in etiolated plants there is a substance which

, I Nature of WorkIt is the commonest of observations both in actual flying and in pressure chamber experiments that fights become dim at high altitudes and then brighten on return to low altitudes. These events are due to the absence and presence of an adequate oxygen supply, as can be shown by their easy elimination when sufficient oxygen is available.The phenomenon itself has been demonstrated quantitatively as a rise in threshold of the dark-adapted eye during anoxia by NicFarland and Evans (1939), by McFarland and Forbes (1940), and by Wald, Harper, Goodman, and Krieger (1942). (C]. also Bunge, 1936, andFischer andJongbloed, 1936.) In addition MCFarland and Halperin (1940) found that visual acuity is affected by decreased oxygen tensions.Brightness discrimination and visual acuity are closely allied visual functions and it is not surprising that Schubert (1932-33) and Gellhorn (1936) detected an influence on brightness discrimination, though they recorded sharply conffieting results. However, McFarland, Halperin, and Niven (1944) have definitely shown that brightness discrimination is impaired by anoxia, and have explored the effect over the significant range of cone vision.Our own work--completed in 1942 but only now available because of its origins---confirms the findings of NicFarland, Halperin, and Niven. Our experiments were made somewhat differently from theirs; theirs cover a greater brightness range, but in most of their work a single low oxygen concentration was compared with room air. We compared seven different oxygen concentrations with room air, at three moderate brightnesses involving cone vision only. II Apparatus and ProcedureIf I is the light intensity of a uniform field of vision, and M is the fight intensity which needs to be added to a part of this field so that the particular *

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