Abstract
The sporangiophore (spph) of Phycomyces blakesleeanus shows a pronounced positive phototropism when illuminated with unilateral visible (blue) light. The spph is cylindrical and nearly transparent and acts as a converging lens. This results in the focusing of intense light on its distal side, and the maximal energy fluence rate of light on the distal side becomes much higher than that on the proximal side. We propose that the orientation of the phototropic response, positive or negative, is determined by the ratio of the maximal light fluence rate on the proximal side to that on the distal side of the spph cell. This ratio is influenced by intracellular light scattering and attenuation. We suggest that the length of the light path (cell diameter) is another important factor influencing this ratio. We find that the ratio can be estimated from the values for cell diameter, the light scattering constant of the spph cell, and the transmission of light after passage through the spph. The scattering constant is obtained by measuring the distance between the central axis and the focal point on the distal spph wall. The phototropic response of spphs can be predicted by measuring these factors.
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