Toothed Wrack, Fucus serratus Linnaeus, is a common and familiar British seaweed. It grows on rocks of the lower seashore in the inter-tidal zone. The pictures in this post were taken on the Worms Head Causeway in Gower when the seaweed was exposed at low tide.
This algal plant can reach a length of 2 – 3 feet – that is, up to about a metre. A seaweed is not differentiated into a stem, roots and leaves like the higher plants. However, it does have different parts. The plant body, or thallus, has a branched hapteron or holdfast – a root-like structure at its base – by which it is attached securely to the rocks. The thin ‘branches’ of the hapteron grow into the cracks and crevices to hold the seaweed into position and make it very difficult for the actions of the sea to dislodge it.
A short cylindrical stalk or stipe connects the hapteron to a flat broad branched blade known as the lamina. In Toothed Wrack the edges of the laminae have an irregular serrated (saw-like or tooth-like) edge from which the seaweed gets its name.
Reproduction in Toothed Wrack is mainly sexual rather than vegetative. At certain times of the year, the terminal few centimetres of the branched laminae become swollen; and these parts are then called the receptacles. Small cavities named conceptacles sit just below the outer layer of the receptacles and are connected to the surface by minute openings known as ostioles. [Actually, these tiny cavities are found in other places on the seaweed but in these locations they are sterile and only contain hairs. If you look closely at Picture 4 in this post, you should be able to see regular grouping of these small hairs on some of the laminae.]
However, in the receptacles, the conceptacles are fertile and produce the gametes used in sexual reproduction. Fucus serratus L. has separate male plants and female plants, each producing gametes that are shed through the ostiole into the sea water where fertilisation and development take place.
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