Phototropic "Indifference" and the Light-Sensitive System of Phycomyces

Abstract

1. Sporangiophores of Phycomyces of high sensitivity to light, stimulated by exposure to unilateral illumination of sufficiently high intensity, fail to give the usual phototropic response. This condition is known as phototropic "indifference." 2. Sporangiophores which exhibit such indifference are nevertheless shown to give a distinct direct growth response as a consequence of the same unilateral illumination. The state of indifference is therefore not characterized by the absence of photic excitation, but by the failure of the light to evoke a differential acceleration of growth on the two sides of the sporangiophore. 3. Stimulation of sensitive "indifferent" sporangiophores by flashes of light of progressively reduced duration of exposure leads to the discovery of a critical duration below which indifference is abolished and phototropic bending occurs. 4. The critical duration of exposure to light for the appearance of the phototropic response corresponds to the critical duration of exposure for minimum reaction time in the direct growth response. 5. Phototropic bending therefore appears when the action of light on one side of the sporangiophore is submaximal. Conversely, indifference is due to equal and maximal photochemical action on both sides.