Occurrence
Polysiphonia is a marine alga.
It is present along the coast of oceans.
It is attached to the rocks or other substratum.
Some members are epiphytes.
Vegetative structure
Plant body is composed of branched filamentous and basal attachment disc.
- Attachment discs: The basal attachment disc is formed of several unseptate, branched rhizoids.
- Vertical portion: The vertical portion is formed of laterally branched axis.
It is polysiphonous. Its axis is formed of a bundle of filaments or siphons.
The cells of each filament lie at the same level.
These cells are placed one above the other and form bundles of siphons.
The central siphon is primary axis. It is surrounded by variable number of pericentral filaments.
The number of pericentral cells in each tier is constant for a particular species.
Corticating filament:
The central axis in the basal portion is further surrounded by one or more layers of corticating filaments. The corticating filaments are present in the basal portion.
Trichoblasts:
The tips of branches are monosiphonous. So these are tapering. These branches are fine hair like. So these are called trichoblasts.
Trichoblasts are borne in spiral manner on the axis. The cells of the trichoblasts are uuinucleate and colourless.
Growth
- Plant grows by single apical cell. This cell divides transversely. It produces a row of cells which form the central siphon.
- The cells of this central siphon divide obliquely near the upper end. It produces a trichoblast initial. The trichoblast initial divides transversely to produce a dichotomously forked trichoblast filament.
- A trichoblast initial function as an apical cell. It gives rise to a literal branch. The lateral branches arise from the younger portion.
- After cutting off the trichoblast initials, the cells of the central siphon undergo several oblique vertical divisions. It produces the characteristic number of pericentral cells. The central cell and surrounding pericentral cells become longer than broad.
- The pericentral cells in the basal portion undergo longitudinal divisions. It produces corticating filaments or the cortex.
Cell structure
Young cells are always uninucleate. But the older cells are multinucleate.
Each cell contains several discoid chloroplasts.
Its pigments are chlorophyll a and carotene, xanthophyll, phycoerythrin and phycocyanin.
It lacks pyrenoid.
The reserve food material is floridean starch. Floridean starch is intermediate between the true starch and the dextrin.
Reproduction
Polysiphonia shows an isomorphic alternation of generation.
Both gametophytic (haploid) and sporophytic (diploid) generations are identical. Sexual reproduction occurs in gametophytic plants and the asexual reproduction occurs in sporophytic plants.
Sexual Reproduction
Majority of the species are heterothallic (dioecious).
Spermatangia:
The male sex organs are spermatangia. Spermatangia are produced in clusters on fertile branches.
Each fertile branch has a central row of cells. This row of cells is surrounded by pericentral cells. Each pericentral cell cuts off one or more spermatangial mother cells. Each spermatangial mother cell produces one to four spermatangia. The contents of each spermatangium are changed in to single non-motile spermatium.
The spermatangial wall ruptures and release spermatium. Water currents carry them to the carpogonia.
Carpogonia:
The female sex organs are carpogonia. They are produced singly on the female fertile branches. Fertile branch remains short. They are about five to seven cells in length. The two basal cells produce the pericentral cells.
The adaxial (that facing the main axis) pericentral cell of the upper tier (second from the base) produces a carpogonial initial.
Transverse divisions occur in carpogonial initial. It produces a curved four-celled filament carpogonial filament. The apical cell of this carpogonial filament is charged into carpogonium.
Carpogonium is flask shaped. its basal portion has single egg or oosphere.
Its upper elongated neck is called trithogyne.
The supporting cells produce a short sterile filament.
The pericentral cells adjacent to the supporting cell divide simultaneously:
They produce a single layered covering around the carpogonium.
This sterile covering develops an opening at the tip. This opening is called ostiole,
Fertilization:
Spermatium falls on the trichogyne.
The wall between the two dissolves.
The spermatial nucleus with its cytoplasm enters into the trichogyne.
It moves down to the basal portion of the carpogonium.
Fertilization occurs and diploid zygotic nucleus is produced.
It soon divides into two nuclei.
After fertilization, the supporting cell cuts off an auxiliary cell towards the upper side. This auxiliary cell develops a tubular connection with the basal portion of the carpogonium.
One daughter diploid nuclei move from the carpogonium to the auxiliary cell through this connection.
Auxiliary cell, supporting cell and cells of sterile filaments fuse together. They form a single large cell called the placental cell. The diploid nucleus in the auxiliary cell divides several times. It produces a large number of diploid nuclei. The original haploid nuclei in the placental cell disintegrate.
Several small protuberances are produced on the surface of the placental cell. One diploid nucleus migrates into each “protuberance. The tube is cut off by the formation of a septum. It functions as an initial of the gonimoblast filament. This initial undergoes few transverse divisions. It produce c short gonimoblast filament.
The terminal cell of each gonimoblast filament acts as a carposporangium. It produces a single non-motile diploid carpospore.
The mass of gonimoblast filaments and the placental cell form he carposporophyte. It is covered with sterile covering or pericarp. Pericarp forms the fruiting body or the cystocarp. The carposporangium rupture and release carpospores through ostiole.
The carpospores germinate to produce sporophyte. Sporophyte is also known as tetrasporophyte. It produces tetraspores.
Asexual reproduction
Asexual reproduction occurs in sporophyte. It produces tetraspores in terasporangium. Tetrasporangia are produced from the fertile pericentral cells. Only one pericentral dell in each tier is ferile. The fertile pericentral cell cut off a small peripheral cell and one or two cover cells. The pericentral cell itself diN. ides transversely. ft forms two unequal cells. The lower smaller cell becomes the stalk cell. The upp:.:r larger cell becomes tetrasporangium. The nucleus of tetrasporangium undergoes meiosis (reduction division). It produces four haploid tetraspores. The sporangium ruptures and releases these spores. The haploid tetraspores germinate to firm gametophyte.
Alternation of Generation
Polysiphonia shows isomorphic alternation of generation.
1. Gametophyte generation: The gametophyte plants produce
Gametes.
The male gametes (spermatia) and female gametes (oosphere) fuse to form diploid nucleus. It is the beginning of the sporophytic stage.
2. Sporophyte generation:
Diploid carpospores are produced on he carposporophyte.
Carpospores germinates to form sporophytic plants (tetrasporophyte).
This sporophyte produces the tetraspores.
Tetraspores are produced by reduction division. So they are haploid.
Tetraspore is the start of gametophytic stage.
They give rise to the male or female gametophytic plants.