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The tall conidiophores of Aspergillus giganteus are produced by large foot cells formed by the mycelium. They only continue to grow when exposed to light of wavelength below 530 mp. The photochemical reaction involved in the inductive process is a 'low energy' reaction, i.e. light acts as a 'triggering' mechanism. The response to light is localised in the areas of the mycelium actually exposed to the radiation. The density of tall conidiophore production, but not their height, is influenced by the concentration of glucose in the medium. The tall conidiophores have the usual type of growth curve; they reach their maximum rate of elongation when they are c. 3 mm tall. Elongation is confined to the terminal portion of the conidiophore. At low light intensities the conidiophores show regular oscillations along their length. The rate of conidiophore growth is influenced by light intensity; they do not show 'light growth reaction' of the kind found in Phycomyces sporangiophores. The conidiophores only continue to elongate in light when the relative humidity is close to 100%. There is a rapid reduction in the rate of conidiophore growth during vesicle formation; no further growth takes place after the vesicle has been formed. The optimum temperature for mycelial growth is c. 30 G, while the optimum temperature for maximum conidiophore height is 20 - 23 G.The presence in the medium of the riboflavin inhibitors, 1-lyxoflavin and mepacrine, or the p-carotene inhibitor, diphenylamine, did not inhibit the photoinductive process. The presence of gaseous carbon dioxide is not essential either during the photoinductive process or for conidiophore growth. Free oxygen is not required during the actual period of photoinduction but is required for conidiophore growth. Photoinduction in the absence of gaseous oxygen is less effective in inducing carotenogenesis than photo induction in the presence of oxygen. The conidiophores are strongly positive phototropic but do appear to be sensitive to the stimulus of gravity. There is a reversal of the normal phototropic response in ultra-violet radiation of 280 mp. and when the conidiophores are submerged in liquid paraffin.