PHOTORESPONSES OF PHYCOMYCES BLAKESLEEANUS: INITIATION AND DEVELOPMENT OF SPORANGIOPHORE PRIMORDIA

Phycomyces blakesleeanus (Burgeff) produces both giant and dwarf sporangiophores from superficial and submerged hyphae. The morphogenesis of submerged primordia has been studied in cultures grown in petri dishes on a defined nitrogen‐poor medium at low temperature under varied conditions of illumination. The primordia of dwarf and giant sporangiophores differed markedly in size, morphology, tropistic behavior, developmental plasticity, photosensitivity, and conditions required for initiation. Dwarf primordia, abundant only in dark‐grown cultures, began as hyphal thickenings that later developed characteristic ramifications not seen in giant primordia. The formation of dwarf primordia was correlated spatially and temporally with termination of mycelial expansion near the rim of the dish and in local regions elsewhere. Illumination strongly suppressed the ramification process in existing primordia but did not prevent the maturation of dwarf sporangiophores that had already emerged from the nutrient surface. Giant primordia were prominent only in illuminated cultures. With continuous light from the time of inoculation, giant sporangiophores and giant primordia were found chiefly in midregions of the dish. If illumination was delayed until dwarfs had begun to form near the rim of the dish, the giant primordia were also concentrated near the rim. In that case about half of the giants were formed by conversion of a small fraction of the existing dwarf primordia and the rest were formed de novo from vegetative hyphae.