IS THE DICHROIC PHOTORECEPTOR FOR Phycomyces PHOTOTROPISM LOCATED AT THE PLASMA MEMBRANE OR AT THE TONOPLAST?*

A rigorous quantitative description of the spatially distributed light stimulus and the spatial pattern of photo‐excitation shows that the dichroic photoreceptor for Phycomyces phototropism can not be located at the tonoplast. A photoreceptor located here could not be responsible for the transmission from the positive to negative phototropic bending as observed in sporangiophores immersed in media of varying refractive indices. The calculations support the alternative hypothesis that the photoreceptor is situated at the plasma membrane. Some evidence in favour of a transverse tangential orientation of the transition dipole at 495 tun is also present.