Amongst the commonest seaweeds found round the British coasts are the brown
Algas belonging to the genus Fucus. Fucus serratus, the common wrack, is characterized by the serrated edge of the thallus, while Fucus vesiculosus, the bladder wrack, is so called because of the air bladders which occur along the thallus and enable the plant to float.
The thallus of Fucus is attached to a rock by an irregular flat disc called the holdfast. A more or less cylindrical stalk-like portion grows out from the holdfast and gives rise to the branches of the thallus. Each branch has a central midrib, on either side of which there is a flattened wing. Growth in length takes place by the division of apical cells which occur at the tip of a branch in a slight groove. When branching takes place the thallus forks regularly and the branches formed are of equal size. Such regular branching into two is said to be dichotomous. Sometimes there is a tendency for one limb of the fork to become longer than the other.
The reproductive organs are borne inside small narrow-mouthed cavities called conceptacles, which occur crowded towards the tips of the branches. When the male and female reproductive bodies which occur on separate plants in the common wrack are ripe, they are carried out of the conceptacles by the mucilage which exudes from the mouth of these cavities at low tide. The anthcridia and oogonia burst in the rising tide to set free the male and female gametes, and fertilization , D takes place in the water. From the fertilized egg or zygote a new plant develops.
Practical Work on Fucus
Examine fronds of Fucus serratus with either male or female conceptacles. The male plants produce an orange-coloured slime containing anthcridia which oozes from the conceptacles, while the female plants produce a greenish slime in which the oogonia are clearly visible.
Examine prepared slides of transverse sections of the conceptacle regions of the fronds.
Compare fronds of the bladder-wrack with those of the toothed wrack.