The steady-state phototropic bending of Phycomyces sporangiophores was studied using apparatus designed to keep the growing zone vertical and the angle of illumination constant over long periods of time. The bending speed is quite constant if the intensity and angle of illumination are fixed. A phototropic inversion occurs in response to a sudden change in intensity, either an increase or a decrease. A bending component lateral to the illumination direction is strongly evident at normal incidence. It is shown that this component is due to a rotation between the stimulus and response loci about the axis of the growing zone, which is probably related to the spiral growth of the cell. The steady-state bending speed is at a maximum value for illumination directions ranging from normal incidence to about 45°. From 45 to 14° the bending speed decreases linearly with angle, reaching zero at 14°. Angles less than 14° elicit a weak negative phototropic response. Using an optical model of the growing zone, the intracellular intensity distribution was determined as a function of the angle of illumination. Several hypotheses relating the intensity distribution to the phototropic response are discussed.

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