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Normally, the dioptrics in air of the cylindrical sporangiophore of Phycomyces blakesleeanus confer on the distal side a focusing advantage of about 30 per cent for unilateral stimuli of parallel light. This advantage can be nullified or reversed to produce negative curvatures by means of diverging light stimuli. A thin cylindrical glass lens was positioned 0.15 mm from the light-adapted growing zone with its long axis parallel to the long axis of the sporangiophore. A 3 minute blue stimulus was given and the lens removed. Reproducible negative curvatures were observed with a maximum of 13 degrees occurring within 8 minutes after the beginning of the stimulus. Experiments in air were done in a water-saturated atmosphere to minimize avoidance responses due to the proximity of the lens. The data support Buder's conclusion that the focusing advantage is the principal mechanism which produces the response differential necessary for phototropism. When the lens advantage is small, the attenuation becomes important in determining the direction of the response. Data obtained from sporangiophores immersed in inert liquids indicate that the attenuation is about 14 per cent. Therefore, whenever the focusing advantage is less than 14 per cent, negative curvatures are produced by unilateral stimuli.

The Lens Effect and Phototropism of Phycomyces