Polysiphonia

 

Polysiphonia

                                                                                                            Class: Rhodophyceae

                                                                                                            Sub-class: Florideae

                                                                                                            Order:       Ceramiales

                                                                                                            Family :Rhodomelaceae

                                                                                                            Genus  :Polysiphonia

Distribution and Habitat:

            It is a common red algae with about 200 species on sea coasts. It is abundant in Atlantic and Pacific oceans. Most of them are found in littoral zone in tidal marshes, brackish estuaries and tide pools frequently growing as epiphytes on rock weeds.

            The genus is represented in India by about 16 species, which occur in the southern and western coasts. P. platycarpa, P. ferulaceae, P. urceolata, P. variegata are common India species.

            P. variegate inhabits polluted water near estuaries and frequently found on roots of mangroves. P. urceolata is an epiphyte on Laminaria. P. fastigata is found on the fronds of a brown sea weed Ascophyllumnodurum and occasionally on Fucus.

Thallus Structure:

            Polysiphonia has a filamentous thallus which is generally brownish red to purplish red in colour. The filaments branch and re branch several times giving the plant body a beautiful, delicate, feathery appearance.The thallus is attached to the substratum in water by a holdfast.

The plant body is heterotrichous consisting of an erect or projecting system and a filamentous prostrate system.

Basal Prostrate System:

            This creeps over the substratum. It is anchored to the substratum by means of thick-walled, elongated, unicellular rhizoids arising from the  peripheral cells facing the substratum. The distal ends of rhizoids expand to form flat irregularly lobed attachment pads or discs. The creeping filaments function as a means of perennation.

Upright or Vertical Filaments:

            These arise from the creeping filaments. They may attain a height of 25-30 cm. These filaments remain free-floating in water. The branching is lateral and branches are of two kinds, long and short.

            The short branches are of limited growth and are known as trichoblasts. The trichoblasts are colourless, hair-like and forked. They are borne on the long, erect branches of unliminted growth. The trichoblasts usually bear male and female reproductive structures. Trichoblasts are usually deciduous and are shed annually in the perennial species before winter.

            The long branches arise from the basal cells of short branches. They show unlimited growth due to the activity of apical cell and do not bear reproductive structures.

            The main filament and long branches, each consists of a system of parallel filaments. These are called the siphons. There is one central filament termed the axial or central siphon. The central siphon is surrounded by peripheral filaments called the pericentral siphons. The number of pericentral siphons varies from 4-20 (P. elongate – 4, P. spiralis – 5, P. variegate – 6).

            Such a thallus having central siphon surrounded by many pericentral siphons is called as Polysiphonous. This algal genus thus derives its name from the polysiphonous nature of its thallus. The cells of the central and pericentral siphons are inter connected through pit connections, a feature characteristic of the red algae.

            The long branches are polysiphonous or multiaxial. The short branches or trichoblasts are made up of only central siphons, and the pericentral siphons are absent, so they are monosiphonous.

            Because of nearly the same length of the cells in the central and pericentral siphons and the regular manner in which the cells are arranged, the main axis appears to be differentiated into nodes and internodes.

Cell Structure:

            The central and pericentral siphons are made up of elongated, cylindrical cells consisting of the cell wall enclosing the protoplast. The cell wall is thick and is differentiated into two layers. The outer layer is made up of pectic materials and the inner of cellulose.

            The centre of the cell is occupied by a vacuole bounded by tonoplast. The cytoplasm is restricted to the periphery of the cell. The peripheral cytoplasm encloses a single nucleus and a number of discoid chromatophores. Chromatophores are devoid of pyrenoids.

           

The photosynthetic pigments located in chromatophores are chlorophyll – a, Chlorophyll – d, α,β carotene, biliproteins – r phycoerythrin and r- phycocyanin and a few xanthophylls.

The reserve food materials are floridean starch and floridoside.

Growth:

            The thallus grows by means of a dome shaped apical cell. It is situated at the  extreme tip of the naked part of the central siphon. The apical cell by transverse division forms a series of segments parallel to its posterior face. These segments elongate to form the axial siphon.

            Some of the sub-terminal axial cells divide periclinically and form a definite number of pericentral cell around the axial row of cells. Both central and pericentral cells elongate into their respective siphon with pit connections.

Reproduction:

            In life cycle of Polysiphonia, three types of plants are found. They are :Gametopyte, the Carposporophyte and the tetrasporophyte.

Gametophyte:

            It is a free living haploid plant. It is concerned with sexual reproduction and bears sex organs.

Carposporopohyte:

            It is diploid plant and develops from the zygote. It remains attached to the female gametophyte plant, on which it is parasitic. It reproduces asexually by producing diploid spores called the carpospores.

Tetrasporophyte:

            The diploid carpospores germinate to give rise to the tetrasporophyte plant. It is an independent diploid plant. It reproduces asexually by producing the haploid tetraspores.

            The haploid gametophytes and diploid sporophyte plants are similar in their morphological structure, but differ in producing the reproductive organs.

  

Gametophyte:

            The free-living Polysiphonia plant is a haploid gametophyte. It undergoes sexual reproduction which is an advanced oogamous type. The gametophyte plant is heterothallic or dioecious i.e., male sex organs and female sex organs are borne on different plants called the male gametophyte and female gametophyte resepectively. The male and female plants are morphologically similar.

Male gametophyte:

The Male sex organs, spermatangia or antheridia develop on fertile trichoblasts present on tip of male gametophytic plant. The male trichoblast  when only 2-3 celled, divides dichotomously. In most of the species one branch remain sterile and the other bears spermatangia. In some species both branches become fertile.

The cells of fertile uniaxial trichoblast  divide periclinally to form  pericentral cells. The  pericentral cells form the  spermatangial mother cells on outer side. Each spermatangial mother cell cuts off 2-4 sporangia on outer side. The complete structure makes cone shaped cluster of spermatangia.

The mature spermatangium is a globular or oblong, unicellular structure. Its cell wall is differentiated into three layers – inner refractive , middle gelatinous and outer thick layer. The uninucleate protoplast of spermatangium forms a male gamete or spermatium. The spermatium is non-motile and is released through an apical pore.

Female gametophyte:

The female sex organ is called carpogonium. The carpogonium develops on trichoblast on female gametophyte plant.  The trichoblast initial arises from a cell, 2-4 cells behind the apical cell. It develops into 5-7 celled female trichoblast.

The 3 lower most cells of the female trichoblast divide vertically to form an ensheathing layer of pericentral cells. One of the pericentral cell on the adaxial surface (facing the axis side) functions as supporting cell.

The supporting cell cuts of a small initial cell at its free end known as procarp initial. The initial divides to form a small, curved four-celled branch called the carpogonial filament or procarp.

The terminal cell of the carpogonial filament functions as carpogonial mother cell. The carpogonial mother cell gets modified into the carpogonium. The carpogonium has a basal swollen portion having female nuclei or egg and an upper elongated, slender portion called the trichogyne. The trichogyne functions as a receptive organ

Meanwhile, the supporting cell cuts off two sterile cells, one towards its base called the basal sterile filament initial and one on the lateral side called the lateral sterile filament initial.

At this stage the carpogonium is ready for fertilisation.

Fertilisation:

The liberated spermatia are carried passively by the currents of sea water. As they reach the carpogonium, one of them adheres to trichogyne. At the poing of contact the common walls between them dissolves. The male nucleus then enters the trichogyne and passes down and reach the eagg. The male nucleus and the egg nucleus fuse and a diploid zygote is formed.

Post- Fertilisaion Changes:

After fertilisation many changes takes place within and around carpogonium. The two-celled lateral sterile filament divides and becomes 4-10 celled. Then basal sterile filament initial divides to form a 2-celled basal sterile filament. The sterile filaments are nutritive in function.

Meanwhile, the supporting cell divides transversely to form an auxillary cell at its upper end. It lies between the supporting cell and the carpogoniium. It has a haploid nucleus. Soon, the auxillary cell establishes a tubular connection with the carpogonium.

The diploid zygotic nucleus of the carpogonium dividies mitotically into two daughter nuclei. One of the daughter nuclei migrates into the auxillary cell through the tubular conncetion. Other daughter nuclei remain within the carpogonium. The haploid nuclei of the auxillary cell degenerate at this stage.

After, the migration of the diploid nucleus into the auxillary cell, the carpogonial branch gradually begins to degenerate.

The migrated nucleus present in the auxillary cell divides mitotically into two daughter nuclei. One of these remains in the auxillary cell and the other migrates into a small lateral outgrowth arising from the upper side of the auxillary cell. This small outgrowth on the auxillary cell is called as gonimoblast initial.

The gonimoblast initial cuts off a number of cells. Every time a new cell is cut, the zygote nucleus divides and its derivative enters the newly formed cells. Thus, gonimoblast initial grows into a number of short threads, the gonimoblast filament.

The terminal cell of the gonimoblast filament becomes swollen and it develops into a pear-shaped carposporangium. The diploid protoplast of the carposporangium develops into a single, diploid carpospores.

With these development taking place, the supporting cell, the auxillary cell and cells of sterile filaments fuse resulting in a large, irregularly shaped structure called the placental cell. The placental cell provides nourishments to the growing carposporophyte.

Meanwhile, the pericentral cells of the female trichoblast adjacent to the supporting cell grow into outgrowths known as the enveloping threads.

The enveloping threads finally develop into an urn-shaped envelope or sheath around the developing frutification. This sheath is called the pericarp. It consists of two layers and has a wide aperature, the ostiole at its distal end.

The entire structure consisting of the placental cell, gonimoblast filaments bearing carposporangia and the surround pericarp is called the cystocarp. It is partly haploid and partly a diploid structure.

Carposporophyte or Cystocarp:

The post-fertilisation fructification respresenting the diploid phase and second individual in the life-cycle of Polysiphonia is called as carposporophyte. It remains attached to the female thallus of Polysiphonia and thus lives parasitic on it.

It is urn-shaped and protected by a two layered wall called the pericarp. The pericarp is a haploid structure. The cystocarp has an opening on its top called the Ostiole. The cystocarp contains a placental cell, gonimoblast filaments and carposporangia.

Each carposporangia produces a single, uninucleate, diploid carpospores. Carpospores are liberated through the ostiole and they are carried by the water currents.

Germination of Carpospore:

On coming in contact with substratum diploid carpospores secrets a wall around it and attaches itself to the substratum. It divides transversly and forms a smaller lower cell and a larger upper cell. Each of these again divide transversely forming a four-celled small filament.

The basal cell of the filament is called the rhizoidal cell. It is colourless, elongated and form the rhizoids. The end cell of the filament is dome shaped and functions as the apical cell. The lower axial cell divide vertically to cut off the pericentral cells. In this way a full-fledged tetrasporophyte is formed.

Tetrasporophyte:

It is an independent, diploid plant developed from the carpospores. It is morphologically similar to the haploid gametophyte. It consists of a central siphon encircled by the perincentral silphons. The thallus is laterally branched.

Asexual Reproduction:

The tetrasporophyte plant reproduces asexually by means of non-motile haploid spores called tetraspores. Tetraspores are produced within spherical, sac-like diploid structures called the tetrasporangia.

The tetrasporangia are developed from the pericentral cells. Only one of the pericentral cell in each transverse tier produces a tetrasporangium. The fertile branches bearing the tetrasporangia become swollen and twisted.

The fertile pericentral cell is smaller in size than the other cells in the same tier. This cell divides by a vertical wall into two cells, the outer and inner.

The outer cell again divide to form two cover cells. The inner cell functions as the sporangial mother cell. The sporangial mother cell divides by a transverse wall into a lower stalk cell and an upper tetrasporangium cell.

The tetrasporangium cell increases considerably in size. It has a diploid nucleus. The nucleus undergoes meiosis forming four haploid daughter nuclei.

This is followed by cleavage of the cytoplasm from the periphery towards the centre resulting in formation of four uninucleate meiospores. These four meiospores are arranged tetrahedrally in the tetrasporangium, hence called as tetraspores.

When tetraspores reach maturity, the sporangial wall ruptures and two cover cells move apart longitudinally. Thus, the tetraspores are liberated.

The liberated tetraspores germinates and give rise to the haploid gametophyte plants. Two of the tetraspores give rise to male and other two to the female plants.