Aspects of the reproductive biology ofFucus vesiculosusfrom the coast of SW Finland

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Abstract
The reproductive biology of permanently submerged Baltic Fucus vesiculosus L. (Phaeophyceae) has been investigated at two sites (exposed and sheltered) in S. W. Finland. Phenological observations included the time of receptacle initiation, the periods of receptacle growth, oogonial maturation and receptacle abscission. The earliest stages of receptacle initiation were detected in the second week of October, when plants had been exposed to short-day ( < 12 h) stimulus for only 1-2 weeks. These receptacle initials were only a few mm in length and sexually immature. They remained dormant over winter until the April ice melt, when rapid increase in length occurred at water temperatures below 5°C. By early June, receptacles had reached maximum length but dry weight increased as gametangia (oogo-nia) matured. Spent receptacles were shed at the end of July. The effect of photoperiod on receptacle initiation was studied and initiation was found to occur under short-day conditions. The populations did not differ significantly in reproductive phenology or in fecundity indices (F.I. exposed = 0.61; sheltered - 0.53 p > 0.05). Differences were observed in plant size (exposed: short), receptacle morphology (exposed: long, narrow) and differences were highly significant in reproductive allocation (R.A. exposed - 0.23; sheltered - 0.11 p 蠄 0.001). The phenological similarities are thought to reflect the short Baltic growing season while the differences between populations are likely to be due to local variation in wave action.