Polysiphonia: Salient Features, Occurrence, Thallus Structure, Reproduction

Polysiphonia is a filamentous marine red alga of the family Rhodomelaceae. It is mainly found along the coasts of the Atlantic and Pacific Oceans.

The plant body of Polysiphonia remains attached to a rocky surface or other algae by rhizoids. It shows polysiphonous organisation.

Salient Features of Polysiphonia

The salient features of Polysiphonia are as follows:

  • The plant body is red, profusely branched, filamemtous, and polysiphonous.
  • The thallus is heterotrichous in nature and consists of two types of filaments: prostrate filaments and erect filaments.
  • The prostrate filament creeps over the substratum by means of rhizoids or holdfasts.
  • Branches are dimorphic, i.e., two types of branches can be seen in Polysiphonia: small branches and long branches.
  • Small branches, or trichoblasts, bear sex organs.
  • All cells are connected to each other by pit connections (cytoplasmic connections).
  • Cells contain many pigments, such as chlorophyll-a, chlorophyll-d, β-carotenes, xanthophylls, phycocyanin-R, and phycoerythrin-R, in the chromatophores.
  • Floridean starch is the reserve food material.
  • Apical growth takes place by means of a single dome-shaped apical cell.
  • Sexual reproduction is of the advanced oogamous type.
  • The male sex organs are spermatangia, produced in dense clusters upon the fertile trichoblast of the male gametophyte.
  • The female sex organs are called carpogonia or procarps. They are produced terminally upon a greatly reduced fertile trichoblast in the female gametophyte.
  • The zygote shows an elaborate post-fertilization stage, forming cystocarp.
  • Cystocarp contains carposporangia, which produce diploid carpospores.
  • Presence of an isomorphic alternation of generations with a triphasic life cycle (i.e., a haploid gametophyte, a diploid carposporophyte, and a diploid tetrasporophyte).

Occurrence of Polysiphonia

Polysiphonia is a genus of about 200 species. It is found exclusively in marine habitats and is widely distributed all over the world.

Most of the species (e.g., P. elongata) prefer to grow on rocks as lithophytes. Some species are epiphytic on other plants and algae; e.g., P. ferulacea occurs on Galidium pusillum, P. urceolata grows on the thallus of Laminaria, and P. variegata is found on the roots of mangroves.

Polysiphonia fastigiata occurs as a semiparasite on some brown algae, such as Fucus and Ascophyllum nodosum. A few species (e.g., P. variegata) grow in polluted brackish waters near the estuaries of the sea.

Diagram of Polysiphonia
Figure: Plant body of Polysiphonia

Species of Polysiphonia also occur abundantly in shallow, stagnant water along the coasts of the Atlantic and Pacific Oceans.

Thallus Structure of Polysiphonia

The plant body of Polysiphonia is uniaxial (i.e., one main axis and all the others are side branches), branched, filamentous, and generally red or purple-red in colour. The filaments are profusely branched and consist of numerous siphon-like cells arranged in definite tiers. Hence, the name given to this genus is Polysiphonia.

Due to continuous branching and re-branching, the plant appears as a bush. The bush is generally small but may reach up to 30 cm in height.

In most species, the thallus exhibits a heterotrichous habit. It is differentiated into a prostrate system and an erect system.

Prostrate System

The prostrate system is basal and creeps over the substratum. In many Polysiphonia species (e.g., P. nigrescens, P. urceolata), it may be multiaxial and well developed.

The prostrate system is attached to some suitable substratum by many rhizoids. The rhizoids are well developed, thick-walled, and unicellular. They are much lobed at the apex and form definite attachment discs.

Polysiphonia showing prostrate system with erect filaments
Figure: Prostrate and erect systems of Polysiphonia

In a few species, such as P. violacea and P. elongata, the multiaxial prostrate system is absent, and the rhizoids develop from the lower­most cells of the erect system.

Erect System

The erect filaments of the thallus develop from the prostrate system. The erect system is feathery in appearance and is made up of a main axis from which many branches are developed.

The branches are of two types: long branches (branches of unlimited growth) and short branches (branches of limited growth).

The main axis and long branches consist of a central siphon (large central filament) of many elongated cylindrical cells situated in a vertical row. The central siphon is surrounded by 4–20 smaller pericentral cells, or pericentral siphons.

Arrangement of cells showing siphonaceous in structure
Figure: L.S. of a portion of branch

The cells of the central siphons and pericentral siphons remain connected with one another through pit connections (cytoplasmic connections). Hence the complete plant body is polysiphonous in structure.

In most of the species, the peri­central syphon is covered by 3 layers of cortical cells. They formed due to periclinal and anticlinal divisions of the cells of the pericentral siphon in the older branches.

The cortical cells are also called cortical siphons and are parenchymatous in nature. They may be present in the lower part of the thallus (e.g., P. mollis) or throughout the thallus (e.g., P. crassiuscula).

The branches of Polysiphonia are lateral and monopodial. The long branches (branches of unlimited growth) are called long lateral branches, while the short branches (branches of limited growth) are called trichoblasts.

Short Branches or Trichoblasts

The trichoblasts are uniaxial (monosiphonous) in structure and lack pericentral siphons. They are spirally arranged, dichotomously branched, and fine hair-like. Due to the absence of chromatophores, trichoblasts are colourless.

A portion of branch with trichoblast
Figure: Trichoblast in Polysiphonia

The trichoblasts arise on both the main axis and long branches. The short branches are deciduous. Hence, perennial species shed these branches before winter and develop again in the spring.

Some trichoblasts bear sex organs and are known as fertile trichoblasts.

Development of Trichoblasts

The trichoblast develops from a single trichoblast initial. The trichoblast initial is differentiated by the oblique division of the apical cell. It is a small cell and lies 2–3 cells below the apical cell.

The small trichoblast initial divides repeatedly to form a dichotomously branched, multicellular, uniseriate, hair-like trichoblast.

Long Lateral Branches

The long lateral branches are similar to the main axis. They are polysiphonous (multiaxial) at the base and monosiphonous in the distal parts.

The long branches develop in radial or spiral symmetry.

Development of Long Branches

The long branches arise from the basal cells of the short branches. In species like P. violacea, they occur as outgrowth from trichoblast initials.

The long branches develop along with trichoblasts, and after a few divisions, the trichoblasts are pushed aside, so the branches appear to arise from trichoblasts dichotomously.

Cell Structure of Polysiphonia

The cells of the central and pericentral siphons are elongated and cylindrical. Each cell is surrounded by a thick cell wall. The cell wall is differentiated into two layers: an outer pectic layer and an inner cellulosic layer.

A thin plasma membrane encloses the cytoplasm. The cytoplasm contains a single nucleus and many discoid chromatophores.

The chromatophores are devoid of pyrenoids and consist of pigments like chlorophyll-a, chlorophyll-d, β-carotenes, xanthophylls, phycocyanin-R, and phycoerythrin-R.

Each cell possesses a large central vacuole in the cytoplasm. The vacuole is delimited by the membrane tonoplast.

Floridean strach and floridoside are the reserve food materials found in the cytoplasm.

Growth of the Thallus

Growth of Polysiphonia takes place by the dome-shaped apical cell, situated at the tip of the central siphon. The apical cell cuts off segments at its posterior surface by transverse divisions and forms the central siphon.

Some of the lower cells then divide periclinally to form pericentral cells around the central siphon.

Leave a Reply

Your email address will not be published. Required fields are marked *