Give an account of the habit, structure of thallus and mode of reproduction in Fucus.

Ans. Systematic Position :

Class : Phaeophyceae

Sub-class : Cyclosporae

Order : Fucales

Family : Fucaceae

Genus : Fucus

Occurrence: It is a cold water marine alga in Northern latitudes. It is abundant as a lithophyte where the sea-shore is rocky. It is also found in salt marshes either in free-floating condition or embedded in the mud. F. vesiculosus and F. serratus are British species and are resistant to cold. F. furcatus and F. edentatus are American species. F. ceranoides is a brackish water species. F. evanescens is resistant to desiccation.

Structure: The species which are attached to the rocky surfaces have a discoid holdfast. The plant body is a flat ribbon-like dichotomously branched structure. The ribbon-like fronds are costate i.e., they have a clear mid rib. The flattened segments do remain in the lower region of the thallus. Here they are destroyed by the swift currents except the mid rib which forms a stalk-like structure. Therefore, the so-called stalk of the Fucus plants is nothing but the mid rib minus flattened thallus.

The ribbon-like thallus may be smooth as in F. vesiculosus or serrate as in F. serratus. In a cross section of the thallus, the following structures are visible under the microscope :

1. Meristoderm: The first layer is made up of epidermis like cells which divide only anticlinally and possess a mucilaginous `cuticle’. As these cells are meristematic, the outermost layer is called meristoderm.

2. Cortex: It is made up of ordinary parenchymatous cells, containing fucosan and their function is storage. According to Fritsch, their function may be mechanical as is evident by the presence of frequent thickenings in the cells.

3. Medulla: Some thick cell of irregular shape from middle region called the medulla. These are associated with hyphae like structures which are quite prominent as in Fig.

The air bladders are also found on the thallus of some species which are present near the apex and formed as a result of surface growth. In F. vesiculosus the inner side of the bladder is occupied by short unbranched threads containing chromatophores. Carbon dioxide was not supposed to be present in the bladders but the work of Collz has shown that the bladders contain the gases which are present in the atmosphere and carbon dioxide is maximum while oxygen is minimum during night. While and Collz refers to the function of the bladders as respiratory and are supposed to give buoyancy to the thallus.

Reproduction: It is highly oogamous taking place with the help of well differentiated male and female organs of reproduction called antheridia and oogonia respectively. The sex organs develop within the conceptacles which are restricted on the receptacular surfaces of the thallus. The Thalli of Fucus may be dioecious as in F. vesiculosus or monoecious as in F. fastigatus, both the sex organs occurring either within the same conceptacle (F. spiralis) or in the different conceptacles though on the same receptacle. The receptacular surfaces have different shapes e.g. flattened in F. serratus, swollen in F. vesiculosus, fusiform in F. ceranoides and elongated in F. edentatus. The receptacles are confined to particular places like the tips of the thallus but sometimes the cryptoblasts (sterile conceptacles) may be scattered all over the thallus (cryptoblasts are marginal in F. vesiculosus)

Development of conceptacle: The development of a conceptacle starts with a superficial cell which lies in the vicinity of an apical cell of the receptacle. It soon becomes deep-seated owing to the growth and divisions of the surrounding cells. Now the conceptacle initial divides by a transverse wall into two-a lower fertile cell and an upper tongue. According to Nienburg, the latter does not divide and degenerates. According to Fritsch, in case of F. spiralis it forms a hair. However, it is the fertile cell which divides and redivides and finally forms the floor of the conceptacle. The fertile layer in the conceptacle of Fucus may be two or three cells in thickness. By this time, the conceptacle becomes flash-shaped or pitcher-shaped with a small opening, the ostiole.

Hair-like structures are also present in the cavity and are called paraphyses which may be branched or unbranched. Hairs in the upper region which project through the ostiole are unbranched and small and are called paraphyses. The sex organs develop within these conceptacles.

Development of antheridium : Antheridia develop from the cells of the fertile layer or at the base of branched paraphyses. According to Yamanouch the first division of the nucleus of an antheridium is reductional, followed by mitotic divisions. These divisions continue till thirty-two or sixty-four nuclei are formed. Finally each nucleus gets metamorphosed into a uninucleate biflagellate antherozoid. According to Kylin the posterior flagellum is longer and an inner gelatinous endochite. The antherozoids go out by the rupture of the exochite owing to the absorption of water by it, but they are still surrounded by endochite which soon gelatinizes and ultimately they are set free to the outside through the ostiole.

Development of oogonium : An oogonium initial is a superficial cell of the fertile layer. Soon it divides into two-a basal cell which forms the stalk of the oogonium and an upper cell which gives rise to the oogonium prop erly. The former remains as such while the latter enlarges and becomes more or less spherical. The first two divisions of the nucleus of the oogonium are meiotic. The four nuclei so formed once again divide and thus eight nuclei are formed. These eight nuclei form eight eggs or ova. When young, oogonium has three walls an outer exochite, a middle mesochite and an inner endochite. These layers are lacking where the stalk cell and the oogonium meet.

When mature, the oogonium absorbs water with the result that the exochite ruptures and eggs slip out of it, still surrounded by mesochite and endochite. The mesochite also ruptures and endochite gets dissolved and thus ultimately all the eight eggs are set free in the water. As stated earlier, all the eight eggs are functioning in case of Fucus.

Fertilization: Many antherozoids surround an egg and finally attach to it by their anterior flagellum. Attachment of antherozoids sets the egg in motion. Usually, one antherozoid enters the egg though, according to Farmer and Williams and Yamanouchi there are cases of polyspermy also. Soon the male nucleus unites with that of the female resulting in the formation of a zygote. According to Whiteker soon after fertilization the zygote fixes itself to any substratum with the help of gelatinous wall.

In Fucus interspecific crosses have been observed by many workers. Thuret recorded the ova of F. vesiculosus being fertilized by the antherozoids of F. spiralis. Gard mentioned a hybrid formed as a result of fertilization between F. spiralis and F. ceranoides. Williams was successful in getting a plant intermediate between Ascophyllum and Fucus when he crossed the two.

Post-fertilization stages and formation of a new thallus: The zygote soon after its formation sends out a protuberance called the rhizoidal protuberance. Thus, polarity is established at quite an early stage in the zygote of Fucus and according to du Buyer and Olson, it is due to the presence of some auxin which is present in the eggs.

The divisions in the zygote are rapid though its growth is slow. First division in the zygote is transverse forming an upper cell and a lower rhizoidal initial. According to Nienburg and Oltmanns, the lower cell then divides transversely and the upper cell divides vertically. Thus, a four celled embryo is formed which is three cells in height. The upper cell further divides, this division being at right angles to the first. Thus, a quadrant is formed and each cell of this quadrant divides transversely. Periclinal divisions now follow differentiating the outer cortex and inner medulla. The median cell also divides more or less in the same way and adds to the growth of the young embryo. The lower cell gives rise to the primary rhizoid many cells long and two cells broad in the upper region. Secondary rhizoids develop from upper cells of the primary rhizoid.

As development proceeds, superficial cell at the apex of the young embryo gives rise to an unbranched multicellular hair. According to Nienburg and Oltmanns, the superficial cells in the vicinity also give rise to such hairs and thus a tuft of hairs is formed. This tuft of hairs lies in a depression. Finally, all the cells of the first hair disappear except the basal one. According to Nienburg and Oltmann, this cell functions as an apical cell and further growth is initiated by this cell or derivatives of this cell.

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