In this article we will discuss about Sargassum:- 1. Occurrence of Sargassum 2. Thallus Structure of Sargassum 3. Growth.
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.
(A) Main Axis:
The main axis is circular in outline and internally it is differentiated into three regions:
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.
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).