870 LIGHT AND LIFE 



became no longer measurable when absorption was about 95%. Never- 

 theless, the residual light could be combined with light in the orange 

 range to produce photosynthesis. In the range where the quantum 

 yield declines linearly with the decreasing amount of light absorbed 

 by the accessory pigments in the cell, the relationship may depend on 

 the intensity ratio of the light absorbed by the accessory pigments 

 to the light absorbed by chlorophyll a. In other words, in this range 

 light absorbed by chlorophyll a is able to produce photosynthesis 

 only in proportion to the amount of light absorbed by the accessory 

 pigments. 



The recent results of B. Kok and G. Hoch, and of Mary Belle Allen, 

 show that even more forms of pigments, absorbing in the far-red at 

 or beyond 700 m/x, will have to be considered in the interpretation 

 of photosynthesis. Allen, studying the chrysomonad Ochromonas 

 danica found strong absorption at 710 m^ and fluorescence at 725 

 mfx, and also in "apple green" mutants of Chlorella pyrenoidosa at 

 710 m/x absorption peak was found. 



Kok and Hoch used difference spectra derived from the absorbance 

 of samples just before and after a flash of bright white light. The 

 changes, negative in direction, represent a bleaching of pigments, 

 and the strongest such change occurs at a wavelength slightly longer 

 than 700 m^. It is prominent in Nostoc and Anacystis (blue-green 

 algae) and in Porphyra, a red alga. The pigment involved is simply 

 called "P 700" by these workers. The shortest flashes used (3 X lO--* 

 sec) were effective and gave evidence of a direct photochemical event, 

 leading to bleaching of the pigment. Restoration of the pigment in 

 the dark is spontaneous and has a half-time of about 7 milliseconds. 

 The quantum yield of the process was about unity. The activation 

 spectrum was mainly that of chlorophyll a. The rapid increases 

 and decreases in amount of P 700 which occur when the pigment is 

 thus exposed to bright flashes of light are superimposed on a back- 

 ground level of the pigment which is also modified by light. This 

 level slowly diminishes if many successive flashes of light absorbed 

 by chlorophyll a act upon it (a "negative light effect"). Contrariwise, 

 the general level of P 700 is increased when light is absorbed by the 

 accessory photosynthetic pigments (a 'positive light effect"). This 

 effect, attributable to absroption by phycocyanin, is observable only 

 when the detecting beam (of 700 m,x) is itself strong enough to 

 bleach a sufficient quantity of P 700 for the restorative action of the 

 phycocyanin-absorbed energy to become evident. Bleaching of the 

 pigment represents its oxidation; it is colored when reduced. Thus 



