Among these operative factors in phototropic reaction may be mentioned:—
(1) The difference of effects induced by light at the proximal and distal sides of the organ.
(2) The modification of the latent period with the intensity of stimulus.
(3) The after-effect of stimulus.
(4) The modifying influence of tonic condition on response.
(5) The effect of direction of light.
(6) The effect of intensity of light.
(7) The effect of duration of stimulation.
(8) The transmitted effect of light.