The behaviour of the littoral diatoms of the River Eden estuary was examined both in the laboratory and on the shore at a position with a 7 hour exposure period. The water in the estuary was particularly clear, and owing to the adhesive nature of the soil lacked the customary turbidity of estuarine waters. The diatoms did not exhibit a tidal rhythm, but a diurnal one dependent only on the intensity of incident illumination. This finding is important, for it strikes at the roots of the `physiological clock' hypothesis as applied to littoral diatoms. The Eden diatoms did not slavishly respond to some inherent rhythm irrespective of environmental conditions, neither were they compelled to descend into the soil at the time of coverage by the flood tide. Instead, as long as the incident light exceeded a certain intensity, they remained on the surface.
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