Fluorescence action spectra were determined, both at room temperature and at liquid nitrogen temperature, with various blue-green, red and green algae, and greening bean leaves.\ud \ud The action spectra of algae were established with samples of low light absorption as well as dense samples.\ud \ud Fluorescence at room temperature, with a maximum at about 685 mμ, was for the greater part emitted by a short wave “form” of chlorophyll a, possibly Ca670. To this form energy was transferred from β-carotene, chlorophyll b and phycobilins with an efficiency approaching 100%. The action spectra of blue-green algae suggested the presence of a 650-mμ phycocyanin “form”, which seemed bound more firmly to the photosynthetic apparatus than the bulk of phycocyanin absorbing around 620 mμ.\ud \ud The absence of action in the carotenoid region in blue-green and red algae and in bean leaves shortly after transformation of protochlorophyll to chlorophyll is ascribed to the presence of a high percentage of xanthophyll, presumably inactive in transferring energy to chlorophyll.\ud \ud All pigments active in producing the 685-mμ room temperature fluorescence band were found to be active in producing the 720-mμ low temperature fluorescence band. Additionally, the 720-mμ fluorescence band was excited by absorption in long wavelength chlorophyll a forms (e.g. Ca680 and Ca695) and, in blue-green and red algae, by one or more unidentified pigments with absorption maximum at 475 and 520 mμ.\ud \ud Marked differences between “high density” action spectra of blue-green and green algae are described
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