In this article we will discuss about Occurrence of Sargassum. After reading this article we will discuss about:- 1. Occurrence of Sargassum 2. Thallus Structure of Sargassum 3. Growth of Sargassum 4. Reproduction 5. Life Cycle.
Contents
Occurrence of Sargassum:
The genus Sargassum is represented by about 150 species. The genus is widely distribute; specially in warmer regions mainly in tropical and subtropical seas of the southern hemisphere. The plants form large floating masses in the Atlantic ocean of the African continent between 20° and 35° north latitude. This part of Atlantic ocean is called the Sargasso Sea.
The alga grows abundantly both in east and west coasts of India, Australia and Ceylon. In India Sargassum is represented by about 16 species Some common Indian species are: S. carpophyllu S. christifolium, S. cinereum, S. duplicatum, S. ilicifolium, S. myriocystwn, S. plagiophyllum and S. wightii. The alga grows attached to the rocks in little bushes in the intertidal zone or in the shallow puddles of the zone.
Thallus Structure of Sargassum:
The thallus of Sargassum is diploid and sporophytic. The thallus is differentiated into holdfast and the main axis (Fig. 9). The attaching disc or holdfast is discoid or warty structure, it helps in attachment of thallus to substratum. In some species the hold fast is stolon like and in some free floating forms the holdfast is absent.
The main axis or stipe or ‘stem’ is erect, elongated, cylindrical or flat up to cm in length. In some species e.g., S. filipendula the thallus can be more than a meter in length. The main axis bears large number of primary laterals or branches in spiral phyllotaxy of 2/5 or the primary laterals are arranged on two sides of the main axis. The branching is always monopodial.
Sargassum plants are highly differentiated algae in the organization of the thallus. The main axis and primary laterals bear flat leaf-like branches known as secondary laterals or “leaves” (Fig. 2 A, B). The leaf-like laterals are flat and simple with blade, veins and petiole like structure.
The leaf is a short sterile lateral organ provided with mid rib. The mid rib is absent in some species like S. enerve. The margins of the leaves are entire, serrate or dentate. On the surface and margins of the ‘leaves’ are small pores known as ostioles. These pores are openings of small flask shaped sterile cavities called crypto-stomata or sterile conceptacles. These cavities bear hairs and paraphyses inside.
The branch system arises from the base of a ‘leaf’ like lateral. The little branched laterals which arise from the base of ‘leaves’ are variously modified.
The laterals specially those of the lower branchlets modify into air bladders (Fig. 2 A-C). These are globular or spherical, air filled structures. They help in floating of plants by increasing buoyancy. According to some algologists the air bladders also help in respiration. In some species the air bladders terminate into leaf-like structures.
Another modification of these laterals is in the form of highly branched or swollen structures bearing reproductive bodies called receptacles. The receptacles bear reproductive structure in special flask shaped cavities called as conceptacles.
Growth of Sargassum:
The growth in Sargassum is apical. The growth of plant in length is initiated by a single apical cell at the apex of each branch. Increase in diameter of the axis is initiated by the activity of a lateral meristem zone or meristoderm.
Internal Structures:
(A) Main Axis:
The main axis is circular in outline and internally it is differentiated into three regions:
(i) Meristoderm
(ii) Cortex
(iii) Medulla.
The meristoderm is single cell thick outer-most layer (Fig. 3). It is made of compactly arranged columnar cells. The meristoderm functions as protective layer epidermis and as assimilatory layer due to presence of chromatophores in cells. The meristoderm is covered with thin layer of mucilaginous cuticle.
The cortex zone is present between meristoderm and the medulla, this makes the largest part of the main axis. It is made up of narrow, elongated parenchymatous cells. The cells are loosely arranged with intercellular spaces between them. The cortex cells contain reserve food material and form the storage region of the main axis.
The medulla is present in the central part of the main axis.
The medulla is made of thick walled, narrow and elongated cells.
Sometimes the cells may have scalariform thickenings. The function of medulla is transport of water- and metabolites.
(B) Leaf:
The internal structure of leaf is like that of main axis. It is differentiated into meristoderm cortex and medulla (Fig. 4A).
The meristoderm is the outermost layer and functions as epidermis. It is made of radially elongated meristematic cells. The cells contain chromatophores and reserve food. The cortex is present between meristoderm and medulla. It is made of thin walled parenchymatous cells. The cortex is thickest in midrib region, it gradually becomes narrow towards the margins. The function of cortex is mostly storage.
The mid rib region or medulla is made of thick walled cells like those in main axis. The function of medulla is conduction. On the margins of leaves and on surface are present many sterile cavities called sterile conceptacles, crypto stomata or crypto blasts.
These are flask shaped structures which open on surface of “leaf” as small pore called ostiole. The wall of crypto stomata is made of sterile thin walled cells. Many un-branched filaments arise from the wall of conceptacles; these filaments are called paraphysis (Fig. 12 B).
(C) Air Bladder:
The structure of air bladder is also like main axis and leaf. It is differentiated into meristoderm and cortex but medulla is absent. The meristoderm is made of radially elongated narrow cells. Inner to meristoderm is 4-8 layered parenchymatous cortex. The central part of the bladder is made of large hollow cavity. The air bladder helps in buoyancy and gaseous exchange (Fig. 5).
Reproduction in Sargassum:
The reproduction takes place by vegetative and sexual methods. The asexual reproduction is absent.
(A) Vegetative Reproduction in Sargassum:
Sargassum multiplies profusely by vegetative fragmentation. The thallus breaks into fragments due to mechanical injury or death and decay of older parts. The species like S. hystrix and S. natans growing in Sargasso sea are completely sterile as they do not form any reproductive structures. In these species the fragmentation is the only method of multiplication.
(B) Sexual Reproduction in Sargassum:
Sexual reproduction in Sargassum is oogamous. The male sex organs are called antheridia and the female oogonia. The sex organs develop in special flask shaped cavity called conceptacle. These conceptacles are present is specially modified laterals called receptacles (Fig. 6 A-C). The male and female sex organs develop in separate conceptacles.
The conceptacles bearing antheridia are called male conceptacles and those bearing oogonia are called female conceptacles.
In homothallic or monoecious species the male conceptacle and female conceptacles are produced on same receptacle, but antheridia and oogonia are not produced in same conceptacles. In dioecious plants the male and female conceptacles are produced on separate male and female plants. Sargassum species are mostly monoecious.
Development of Conceptacles:
The conceptacle develops from a single superficial cell on the receptacular branch. This cell called conceptacle initial is flask shaped and differs from the adjacent cells due to its larger size and prominent nucleus (Fig. 6A). The initial cell divides slower than other cells. As a result it gets lower in position than adjacent cells.
The initial cell divides by transverse division; the two cells formed are separated by a curved septum. The lower cell is called basal cell and the upper is called tongue cell (Fig. 6 B, C).
The tongue cell divides transversely to make small filament which later disintegrates. The basal cell undergoes many vertical divisions to make fertile layer of the conceptacles. The cells of fertile layer later form antheridia and oogonia (Fig. 6 D-G).
In fertile conceptacles the cells of basal layers do not spread in upper part, this forms narrow opening called ostiole.
Development of Antheridium:
Any cell of the fertile layer can function as antheridial initial. This cell is dense cytoplasmic and develops a papilla like outgrowth. It divides by transverse division to make lower stalk cell and upper antheridial cell (Fig. 7 A-B). The antheridial cell rounds off to make antheridium.
The stalk cell elongates and pushes the antheridium to one side. The growing stalk cell divides again to make basal cell and the antheridial cell. This process repeated many times and results in formation of many antheridia and a sterile paraphysis (Fig. 71).
The antheridia are oval structures with two layered cell walls. The outer wall is called exochite and the inner is called endochite (Fig. 7 G). At young stage the antheridia are inside conceptacles and on maturity the antheridia are detached from stalk and come out of ostiole.
The antheridium has one diploid nucleus which divides first by meiotic division and later by mitotic divisions. This results in formation of 32-64 haploid nuclei. The protoplast of antheridium also divides in equal number of segments. Each protoplast segment with haploid nucleus develops into an antherozoid (Fig. 7 H). The antherozoid is pear shaped structure with two lateral flagella.
The flagella are heterokontic, one being acronematic and the other pantonematic.
The antherozoids are liberated in water after gelatinization of the antheridial wall.
Development of Oogonium:
Any cell of the fertile layer of the female conceptacle can function as oogonial initial (Fig. 8 A). The oogonial initial divides by transverse division to make small, lower stalk cell and the large, upper oogonial cell (Fig. 8 B). The stalk cell further does not divide or elongate, so the oogonial cells are almost sessile.
The oogonial cell enlarges and makes spherical oogonium. The oogonia wall has three layers—the outer exochite, middle mesochite and the inner endochite. On maturity of the oogonium the exochite ruptures, the mesochite forms the gelatinous stalk and the oogonial nuclei- and protoplast remains surrounded by endochite.
The diploid oogonial nucleus undergoes meiotic and mitotic divisions to form 8 nuclei. The seven of these eight nuclei degenerate and only one remains functional. This nucleus with protoplasm forms single ovum or oosphere (Fig. 8 C-H).
The cells of female conceptacle which do not form oogonia develop into long hair like paraphyses.
Fertilization:
The antherozoids are released in water and the oogonia remain attached to the conceptacle base by mucilaginous stalk. The oogonia protrude out of the ostiole (Fig. 8 J). A large number of antherozoids surround the oogonium and attach to oogonial wall with the help of anterior flagellum (Fig. 9 A). Only one antherozoid penetrates the oogonial wall. The male and female nuclei fuse to form a diploid zygote (Fig. 9 B).
Germination of Zygote:
The zygote germinates immediately after fertilization when the oogonium still remains attached to the wall of conceptacle by a mucilaginous stalk. After some time the zygote is liberated by gelatinization of the oogonial wall. After liberation the zygote gets attached to any substratum in sea water. The zygote first divides by transverse division to make a lower cell and upper cell (Fig. 9 C-F).
The lower cell forms the rhizoids. The upper cell first divides by transverse division and later by anticlinal and periclinal divisions. It results in the differentiation of three layers—the meristoderm, cortex and medulla. The divisions of upper cell result in formation of a diploid, sporophytic Sargassum plant.
Life Cycle of Sargassum:
The life cycle of Sargassum is diplontic type and there is no alternation of generation. The thallus is diploid sporophytic. It forms diploid antheridia and oogonia. The reduction division in antheridia and oogonia forms haploid antherozoid and oognial nuclecus. The gametes only are haploid structure in the life cycle. After fertilization a diploid zygote is formed which divides to make a diploid sporophytic thallus (Fig. 10, 11).
