Caulerpa plants were grown under a 12-hr light and 12-hr dark (12L-12D) regime for 8 days followed by 8 days of continuous light (24L-0D). Under both light regimes the elongation of the rhizome was by means of“tip growth“. However, the rate of rhizome elongation in 24L-0D regime (10.9 mm/day) was higher than that of 12L-12D regime (7.9 mm/day). Jn the region of 1.5 mm from the tip, the RERE (relative elemental rate of elongation) under 24L-0D and 12L-12D regimes were respectively 4and 3elongation per hr. Under 12L-12D photoperiod the subapical part of the rhizome exhibited distinctive oscillation: bending upward during the light period or at the time of rhizoid cluster initiation, and in the dark becoming relatively straight except at the time of cluster initiation. No such distinctive oscillation was observed in continuous li
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