Abstract
Sporangiophores of Phycomyces do not grow directly towards a horizontal beam of light, but equilibrate at an angle of about 30° above the horizontal. After describing several related observations, this paper suggests that the dioptric properties of an obliquely illuminated cylindrical lens, illustrated by a dummy cell, as well as a negative geotropic response, play major roles in determining the direction of growth. The shift of the equilibrium direction of growth towards the vertical, or a purely geotropic response, over a tenfold range of very low intensities (around 106 quanta/cm2 sec., or 10-13watt/cm2) has been studied, and an action spectrum made, measuring the quantum fluxes producing a standard intermediate equilibrium direction of growth at different wavelengths. This may differ from the action spectra at higher intensities in lacking conspicuous maxima from 370 to 490 mµ. However, in the ultraviolet it parallels the other spectra, although without showing the much higher quantum efficiency of ultraviolet relative to visible light previously noted. Possible interpretations are discussed.
