Absorption and screening in Phycomyces.

In vivo absorption measurements were made through the photosensitive zones of Phycomyces sporangiophores and absorption spectra are presented for various growth media and for wavelengths between 400 and 580 mmicro. As in mycelia, beta-carotene was the major pigment ordinarily found. The addition of diphenylamine to the growth media caused a decrease in beta-carotene and an increase in certain other carotenoids. Growth in the dark substantially reduced the amount of beta-carotene in the photosensitive zone; however, growth on a lactate medium failed to suppress beta-carotene in the growing zone although the mycelia appeared almost colorless. Also when diphenylamine was added to the medium the absorption in the growing zone at 460 mmicro was not diminished although the colored carotenoids in the bulk of the sporangiophore were drastically reduced. Absorption which is characteristic of the action spectra was not found. Sporangiophores immersed in fluids with a critical refractive index show neither positive nor negative tropism. Measurements were made of the critical refractive indices for light at 495 and 510 mmicro. The critical indices differed only slightly. Assuming primary photoreceptors at the cell wall, the change in screening due to absorption appears too large to be counterbalanced solely by a simple effect of the focusing change. The possibility is therefore advanced that the receptors are internal to most of the cytoplasm; i.e., near the vacuole.