FUCUS - STRUCTURE OF VEGETATIVE BODY AND REPRODUCTION

A. CLASSIFICATION:

Division – Phaeophyta (Phaeophycophyta)

       Class – Cyclosporae

               Order – Fucuales

                        Family – Fucaceae

                                Genus – Fucus

The genus Fucus comprises more than 100 species and is an extremely marine alga, widely distributed along the sea coasts of temperate and arctic regions. They are attached on the rocks under cold water between the high and low tide marks by their flat, discoid holdfasts. They are commonly called as “rock weeds”.

B. STRUCTURE OF THE VEGETATIVE BODY:

The plant body i.e., thallus is more or less 30 cms in height. The vegetative body of the Fucus plant i.e., sporophyte is complex in organization. It consists of flat, dichotomously branched ribbon-like leathery and dark brown coloured thallus.

The thallus is composed of three parts –

(a) Holdfast – It is a basal flat and broad disc shaped structure which anchors the thallus to the substratum under water.

(b) Stipe – It is relatively short and stem-like portion. It is the lowermost part of the thallus.

(c) Frond or Blade – It is the expanded, ribbon-like dichotomously branched, leathery, parenchymatous and somewhat foliose structure.

Usually, the thallus is dark brown and slimy to touch. In many species of Fucus, the stipe is continued to form a mid-rib in the frond, but the mid-rib never reaches the apex of the frond. The margin of the frond may be entire, serrated or smooth.

*Oranization of the Frond:

The frond is a foliose-like and consists of different structures –

(a) Air Bladders – The thallus of some species is provided with numerous hollow, bladder-like, air-filled expansions in pairs here and there along the dichotomy. These are called air bladders or air vesicles or pneumatocysts.

These air bladders give buoyancy to the submerged thallus.

        (b) Receptacles – The swollen tips of the mature fertile branches of the thallus are called receptacles. These receptacles lack mid-rib and are covered with small scattered pimple-like outgrowths with minute openings called ostiole. These are flask-shaped structure and are called conceptacles. Conceptacles contain antheridia and oogonia.

*Internal Structure of the Frond:

The transverse section of the frond shows the following structures –

(a) Epidermis – It is the outermost single layered peripheral structure, consisting of thin walled, small, palisade-like cells containing numerous chromatophores. Its function is photosynthetic.

(b) Cortex – It is many layered and is present just beneath the epidermis. The cortex is composed of thin-walled, parenchymatous cells. Cortex is often differentiated into outer cortex and inner cortex. Its function is both photosynthetic and storage of food.

(c) Pith or Medulla – It is situated in the centre and consists of several-layered hypha-like elongated, thin walled colourless, inter-wooven cells. The walls of medulla cells have sieve plates.

C. REPRODUCTION: Fucus  reproduces both by vegetative and sexual methods of reproduction.

1. Vegetative Reproduction:- Vegetative reproduction takes place by fragmentation of the vegetative body into many parts, each of which grows into a new thallus. Sometimes adventitious branch develops on the stipe, which on separation forms a new plant.

2. Sexual Reproduction:- Sexual reproduction is of oogamous type. The species of Fucus are homothallic (F. spiralis) or heterothallic (F. serratus). The sex organs i.e., antheridia and oogonia in both homothallic and heterothallic species are produced in a special type of flask-shaped chamber called conceptacles.

(a) Antheridia – The male conceptacle bears numerous antheridia and paraphyses. Antheridia are small, club-shaped, unicellular and provided with a small stalk-like structure. Each antheridium has a delicate wall which is composed of 2 layers – outer firm layer called exochite and an inner gelatinous layer called endochite. It contains a single diploid nucleus (2n).

The diploid nucleus of each antheridium divides first by meiotic division to form haploid nuclei (n), which again divides and redivides forming 64 or more haploid nuclei. Nuclear division is followed by cleavage of the protoplasts. Each uninucleate protoplast directly metamorphoses into a single pear-shaped, biflagellate antherozoid or sperm.

At maturity the tip of the antheridium breaks which allows the mass of antherozoids to escape within the conceptacle.

(b) Oogonia – The female conceptacle bears numerous oogonia and paraphyses. Oogonia are oval or somewhat ellipsoidal in shape and are provided with one-celled short stalk. It contains a single diploid nucleus (2n).

The oogonium divides first by meiotic division to form four haploid nuclei (n), followed by mitotic division to form 8 haploid nuclei. At this stage the cleavage of the oogonial protoplast takes place and 8-uninucleate bits of cytoplasm are formed, which metamorphosed into eight eggs (n). The eggs are surrounded by three layered oogonial wall – exochite, mesochite and endochite.

Eggs come out within the conceptacle by rupturing of the exochite.

      (c) Fertilization:- At the time of fertilization, both sperms and eggs come out from the conceptacle through the ostiole and remain free in the surrounding water. Numerous free swimming antherozoids gather around the egg, but only one sperm penetrates the egg and fuses with it. As a result of fusion, a diploid zygote (2n) is formed.

         After fertilization, the zygote takes a rest of few hours and starts germinating, producing diploid sporophytic Fucus plant.

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