COMPARATIVE REPRODUCTIVE-BIOLOGY OF THE GULF OF FINLAND AND THE IRISHSEA FUCUS-VESICULOSUS L

Variation in reproductive characters of two geographically distinct po pulations of Fucus vesiculosus L. (Phaeophyta, Fucales) from both open and sheltered shores of Baltic, the Gulf of Finland and Atlantic, the Irish Sea coast has been quantified. The period during which the popu lation contains fertile plants is longer on Atlantic than on Baltic co asts, where all receptacles are shed by the end of July. There are dif ferences in reproductive allocation, the value of Atlantic plants bein g double that of the Baltic population. The mean oogonial length is gr eater in the Atlantic populations and the size frequency is different, with Atlantic plants bearing some immature oogonia at peak fertility time. Baltic plants have oogonia of more uniform size. Receptacles of Baltic plants have a looser web-like organisation of hyphae in the med ulla, which is also characterized by presence of enlarged air cavities formed by schizogeny. The hyphal cells are wider in Atlantic plants t han in Baltic plants. Atlantic receptacles are greater in size, they c ontain more mucilage and lack air-spaces. Baltic Fucus, which is perma nently submerged, has receptacles modified for buoyancy while those of intertidal Atlantic coasts have a structure more related to mechanica l strength. The reproductive phenology of Baltic Fucus reflects the re latively short season when the growth is possible. Unlike floral struc ture, which is relatively conservative, fucoid reproductive characters are subject to considerable environmental modification.