Photomotion in Euglena gracilis* I. Photokinesis II. Phototaxis

SYNOPSIS. Photokinesis or the non‐directed rate of swimming of Euglena gracilis for various light intensities was studied. The rate of swimming in white light was found to increase with light intensity until it reaches 0.16 mm./sec. at a saturation of 40 ft. candles, and then decreased with increasing light intensities. Polarized light appeared to be more effective (0.18 mm./sec. at 13 ft. candles) than non‐polarized light. The action spectrum for photokinesis showed two peaks, a major one near 465 mμ, and another in the neighborhood of 630 mμ, having the values of 0.18 mm./sec. and 0.16 mm./sec. respectively at an intensity of 4 ft. candles. The action spectrum for phototaxis, or directed orientation, had a peak near 420 mμ and showed a maximum near 490 mμ. Both dark‐grown and dark‐adapted Euglena showed spectral peaks similar to those of the light‐grown cells. For polarized light, the action spectrum had two additional peaks at 468 mμ and 508 mμ. The polarized light effect may indicate more than one light‐absorbing pigment within the “eye spot” or that other pigments within Euglena were absorbing.