Thallus structure:
1) External structure:
The thallus of sargassum is diploid and sporophytic. It is erect, branched and is differentiated into holdfast and the main axis; the latter bears many leaf like lateral branches and air bladders. The holdfast is a diacoid structure with the help of which the thallus remains attached to the substratum. In some species the holdfast is stolon like, the main axis or stem is erect, cylinderical or flattened, varying in height from 15-25cm, but sometimes it attains a height of more than a meter. The main axis bears large number of primary laterals in a spiral phyllotaxy of 2/5. The primary laterals have unlimited growth, hence they are also called long shoots. The main axis as well as the primary laterals bear flat leaf-like branches, known as secondary laterals or leaves. The leaves are flat and simple with distinct mid ribs. The margins of the leaves are serrate, dentate or entire. On both surfaces of the leaves small pores, known as oatioles are present. These pores open in small flask shaped sterile cavities, called cryptostomata or sterile conceptacles. The leaves present in the lower part of the primary lateral bear rudimentary axillary branches and those present in the upper portion have axillary branch systems. One or more basal members of the axillary branch are modified into globular or spherical air bladders. The air bladders are filled with air and help the plant in floating by increasing its buoyancy. In some species the air bladder terminates into a leaf. The subsequent members of the axillary branch system develop into long repeatedly branched cylenderical or flattened structure called receptacles.
Internal structure:
Internal structure of the main axis:
In transverse section the main axis is almost circular in outline and is differentiated into three distinct region i) Meristoderm ii) Cortex and iii) Medulla
The meristoderm is the outermost layer and function as epidermis and Iit is made up of compactly arranged columnar cells, these cells are denaly packed with chromatophores and fucosan. In the nature part of the axis meristoderm is covered with a thin layer of mucilagenous cuticle. The cortex forms the most part of the axis and it has narrow and elongated cells. These cells are loosely arranged with well developed intercellular spaces. They are filled with reserve food material, and as such the cortex forms the storage region of the axis. The central part of the main axis forms medulla, it is made up of thick walled narrow and elongated cells. The function of the medulla is to transport water and metabolites to different parts of the thallus.
Internal structure of the leaf:
Just like the main axis, the leaf is also differentiated into meristoderm, cortex and medulla. The meristoderm consist of small radially elongated, meristematic cells. These cells have plenty of reserve food and chromatophores. The meristoderm is followed by cortex which consist of thin walled cells, rich in reserve food material. The cortex is the thickest in the midrib region and gradually narrows towards the blades. The central part of the leaf is made up of thick walled cells which make up medulla. On both the surfaces of the leaf are present many sterile conceptacles, known as cryptoblasts. The cryptoblast is flask shaped structure and it opens on the surface of the leaf by small pore, known as ostiole. The wall of the cryptoblast is made up of sterile cells which have chromatophores. Many unbranched multicellular filaments arise from the wall of the conceptacles protrude through the ostiole these filaments are called paraphysis.
Internal structure of air bladder:
The air bladder is differentiated into meristoderm and cortex only, medulla is absent. The meristoderm is made up of radially elongated narrow cells, envoloped with a mucilagenous layer. It is fallowed by a 4-8 layered patenchymatous cortex. The central part of the air bladder is occupied by a large air cavity.
Reproduction
Sargassum reproduces by vegetative and sexual methods asexual reproduction is absent.
1) Vegetative reproduction:
Fragmentation is the cheif method of vegetative multiplication. Thallus may break into segments due to death and decay of the older parts or due to injury. These segments grow into new thalli.
2) Sexual reproduction:
Sexual reproduction is oogamous. The sex organs are produced in special flask shaped cavities, known as conceptacles. Conceptacles are borne on the receptacilar branches of the sporophytic thallus. The male sex organs are called antheridia and the female oogonia, male and famele sex organs are produced on separate conceptacles i, e male conceptacles and female conceptacles respectively. In homothallic species antheridia and oogonia are present in the same conceptacles or in different conceptacles on the same plant. In diocious species male and female sex organs develop in different conceptacles on separate plants. The development of fertile and sterile conceptacles is similar. The only difference is that the basal cells in sterile conceptacles form paraphysis, where as fertile conceptacles they give rise to antheridia and oogonia.
a) Development of conceptacles:
The conceptacles develop from a single superficial cell on the receptacular branch. This flask shaped conceptacle initial can be easily distinguished from the adjoining cell by its larger size and prominent nucleus. It devides at a slower rate than the adjoining cells. Consequently, the initial cell, which was at the surface of the thallus in the beginning, is gradually pushed at the base of the flask shaped cavity. The conceptacle initial devides by a curved septum, forming an outer tongue cell and inner basal cell. The tongue cell elongates and devides transverly to form a small filament which soon disappears. The basal cell however, undergoes repeated verticle divisions and the resulting cells form the basal fertile layer of the conceptacle and reproductive organs develop from the cells of this layer.
Development of antheridium:
The antheridial initial is densly cytoplasmic and grows into papilla like outgrowth. It decide by a transverse wall into a lower stalk cell and an upper antheridial cell. Each of these cell have a diploid nucleus. The antheridial cell develops into an antheridium. It is pushed on one side due to rapid growth of the stalk cell. The growing stalk cell again devides into a badal stalk cell and upper antheridial cell this process is repeated several times and consequently a paraphysis with many antheridia is formed. The terminal cell of the paraphysis is usually sterile, it does not develop into an atheridium. The antheridial initial has only one diploid nucleus which undergoes a meotic and several mitotic divisions consequently, 32-64 haploid nuclei are formed in the developing antheridium. The multinucleate proplast of yhe young antheridium decide into many uninucleate segments and each uninucleate segment metamorphosis into a biflagellate antherozoids. The mature antheridium is an oval structure surrounded by a two layered wall, the outer layer exochite where as the inner layer, endochite is gelatinous. The antheridium remains attached to the base of the conceptacle with the help of a stalk cell. At maturity the antheridium cell comes out of the conceptacle through the ostiole. The anantheridial wall is gelatinized when in contact with water and the flagellated antherozoids are liberated.
Development of oogonium:
Oogonium also develop from the basal fertile layer of the conceptacle. The cells of this layer function as oogonial initials. Each initial cell devide by a transverse wall into two unequal cells. The smaller basal cell forms the stalk cell and the upper larger cell, the oogonial cell latter gradually enlarges and becomes almost spherical, it is densly cytoplasmic and has oil droplets. The diploid nucleus of the oogonial cell undergoes a meiotic division and the four nuclei thus formed devide mitotically, consequently 8 haploid nuclei are formed in the oogonium. Out of 8 nucleus 7 degenerate the remaining nucleus function as the egg nucleus. This functional nucleus is located in the centre of the oogonium. The oogonial wall is diffrentiated into three layers, the outer exochite, the middle mesochite and the inner endochite. The mature oogonia are discharged from the conceptacle through the ostiole but they remain attached with the conceptacle base by a long gelatinous stalk formed by exochite. According to some algologist, the exochite of the oogonium ruptures at the time of it's discharge from the conceptacles and the gelatinous stalk is formed by the mesochite. The discharged egg is this surrounded by the endochite only.
Fertilization:
At the time of fertilization although the oogonium is outside the conceptacle base by a gelatinous stalk. A large number of antherozoids surrounded the extraded oogonium. They attach to the oogonial wall with the help of the anterior flagellum and swim in water by the posterior flagellum. Only one antherozoid penetrate the oogonial wall and fertilze the egg. After fertilization both nuclei lie side by side for sometime and then they fuse to form zygote.