Reproduction in Sargassum The plant body of Sagassum is a diploid sporophyte. It does not multiply asexually by means of spores. Instead it reproduce by vegetative method and the formation of sex organs. Vegetative reproduction : FRagmentation is the only known method of reproduction in the free floating species of sargassum. Sexual reproduction : The reproduction is oogamous and involves the fusion of motile sperm or male gamete with a nonmoile ovum or female gamete. They are respectively borne inside antheridia and oogonia. They are also termed as microsprangia and megasporangia. The spores germinate inside the sporangia to produce gametes. The sex organs are produced inside special cavity called conceptacles. the conceptacles are found inside small finger-like branchlets called receptacles. The conceptacles usually bears only one type of sex organs. Antheridia : A large number of antheridia develop in each male conceptacle. They are borne on the lower branches of paraphyses. The first antheridium is borne at the tip of a 3-celled filament which arises from a cell of the fertile layer. The basal cell remains at the level of the fertile layer. The middle one is known as stalk cell. The uppeer or antheridial cell develops into an antheridium. The antheridium is ovoid in outline. It has a thick wall which becomes mucilaginous towards maturity. Internally the antheridium contains initially a single diploid nucleus and a few chromatophores. The diploid nucleus divides meiotically and then mitotically to form 64 haploid nuclei. The chromatophores also divide. They ultimately get plced near the nuclei in one to one ratio. The ctoplasm cleaves and the contents of the antheidium get divided into 64 haploid protoplasts. Each such protoplast transforms itself into a single sperm or antherozoid. Thus 64 sperm develop inside an antheridium. The wall of the mature antheridium becomes mucilaginous. It breaks its connection from the paraphysis and comes out. Here the wall dissolves and the sperms are released in sea water. A sperm is a pear shaped biflagellate structure of pale brown colour. The flagella are inserted laterally. They are unequal in size. The anterior shorter flagellum is of tinsel type while the posterior longer flagellum if of whiplash type. The nucleus is larger. Chromatophore is reduced and posses an eye spot. Oogonia : Only a few oogonia are borne in a conceptacle. They are formed directly from the cells of the fertile layer, very early in the development of the conceptacle. Any cell of the fertile layer can function as oogonial initial. It divides transveresly into a lower stalk cell and an upper oogonial cell. The stalk cell gets pressed between the growing oogonial cell or oogonium and the cells of the fertile layer. The young oogonium contains a conspicuous diploid nucleus, dense cytoplasma and a number of oil droplets. The diploid nucleus undergoes meiosis and mitosis to produce 8 haploid nuclei. 7 of them usually degenerate while the functional haploid nucleus enlarges and becomes centrally placed. The protoplast of the oogonium then functions as a single ovum or oosphere. The mature oogonium is globular or ellipsoidal in outline. It has thick 3 layered wall and a single ovum or oosphere. The three wall layers are outer exchite, middle mesochite and inner endochite. The middle wall layer or mesochite imbibes water and becomes gelatinized. It swelling breaks the exochite. The oogonium is slowely extruded to the outside of theostiole. It however remains attached to its original position by means of a mucilage stalk formed by the gelatinized mesochite. Fertlization : The ova of sargassum are not shed in sea. They remain covered over by the gelatinized wall of the oogonia and are held in position just outside the ostiole by means of mucilaginous stalk. They attract a large number of sperm freely swimming in water. The number of sperm attach themselves to the gelatinous sheath of the ovm by mean of their anterior flagella. The posterior flagella continue to lash. One of the sperm penentrate the mucilaginous covering and fuses with the ovum. The other sperm swim away. The fertilized ovum has a diploid nucleus and is called zygote. The fertilized egg begins its germination while surrounded by the gelatinous sheath derived from the oogonial wall and still attached to the interior of the conceptace by means of mucilage stalk. After some time the gelatinous sheath dissolves and the fertilised eg in its early stage of germination falls down on some solid substratum. The fertilized egg elongates and divides into two cells. The lower rhizoids which later become esptate. The upper cell divide transversely at first. Vertical or longitudinal walls appear in the terminal region which then shows differntiation of outer and inner cells, followed by disiction of meristoderm cortex and medulla. Simultaneously a three side apical cell appears at the tip of the germling. It produces the typical thallus.