The geotropic responses of Phycomyces sporangiophores were studied under varying intensities of illumination, using a low speed centrifuge and a fixed beam of blue light. This light has a strongly inhibitory effect on the transient geotropic response, reducing it to 36 per cent of its magnitude in darkness. The inhibition does not vary systematically with light intensity over a range of 400-fold. The light sensitivity of the transient geotropic response thus differs from the light-growth response system, which shows the same growth rate in light and darkness. By contrast, the slower long term geotropic response is enhanced by light of moderate intensities, but is strongly inhibited by high intensities. At and above a mean intensity of about 1 μw/cm2, the long term response is completely removed. If the intensity is lowered from an inhibitory level, either to darkness or to a low level, the geotropic response appears after a time lag of 20 minutes. Furthermore an increase in intensity from one level to another, both levels normally enhancing, results in a transient reversal in the long term geotropic response, also after a time lag of 20 minutes. Thus it is suggested that light is acting at some intermediate step in the long term geotropic sensory system, a step that normally requires 20 minutes for completion.
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