Structure and Function of the Euglenoid Eyespot

Abstract
With Euglena gracilis neither the action spectrum for photokinesis nor that for phototaxis corresponds to the absorption spectrum of the stigma, as has been previously claimed. The phototaxis action spectrum may be interpreted instead as a combination of the photokinesis action spectrum, stigma absorption spectrum, and chloroplast absorption spectrum. This supports the contention that the paraflagellar body rather than the stigma is the probable photoreceptor of the eyespot apparatus, and indicates that both the stigma and chloroplasts provide the directional ‘light screen’. It also suggests that the absorption spectrum of this photoreceptor is the photokinesis action spectrum.

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