Systematic position
Division-Rhodophyta
Class-Florideophyceae
Order-Ceremiales
Family-Rhodomelaceae
Genus-Polysiphonia
Morphology.
The name itself indicate, the thallus of Polysiphonia is polysiphonous. The plant body often reaches a length of 10cm. The thallus is branched which appear as a delicate, beautiful and feathery structure. The color of the thallus varies from brownish red to dark purple red. The plant body is heterotrichous consisting of basal prostrate and erect, upright or vertical filament.
Life cycle
The life cycle of Polysiphonia includes three kinds of morphologically similar plants fallowing one after other.
1. Gametophyte plant
The plant reproduce sexually. Sexual reproduction is oogamous type and the plant are heterothallic and dioecious. Both male and female plants are morphologically similar.
i) Male reproductive organs
The male reproductive organs are called spermatangia
Development of spermatangia
Except two lower most cells, all cells of a male trichoblast cut off variable number of encircling pericentral cells, each of which cuts off one or more spermatangial mother cells on its outer face. Spermatangial mother cells are arranged in the form of compact layer. Now, each of them divides to produce two to four spermatangia, depending upon the species. Thus, a compact cone-shaped cluster of new spermatangia are formed subsequent new spermatangia develop as soon as the old ones are cast away. Spermatangia may also be multiplied within the empty wall of the discharged older spermatangia.
ii) Female reproductive organ
The female reproductive organs are called carpogonia. They are formed on a separate female plant.
Development of carpogonium
On the adaxial side of the trichoblast, one fertile pericentral cell divides and forms a basal supporting cell cuts off a small initial cell called carpogonial branch initial. The carpogonial branch initial divides and redivides and forms a 3-4 celled carpogonial filament. The carpogonial filament may be 3 celled or 4 celled. The terminal cell of the carpogonial filament functions like carpogonial mother cell which becomes modified into carpogonium. Each carpogonium consists of a lower basal portion and a distal long slender portion, the trichogyne.
Mean while, the supporting cell cuts off two sterile cells, one towards its base and other towards lateral side. These are called basal sterile filament initial and lateral sterile filament initial, respectively. The lateral sterile filament initial again divides and forms a two celled filament.
Just before fertilization, the adjacent pericentral cells grow out into outgrowths which after fertilization develops into an envelope around the fruiting body. It is a time for fertilization and the whole structure is called procarp.
Fertilization
The liberated spermatia are carried to the trichogyne passively by the water currents. When they come close to carpogonium one of them adheres to the trichogyne. The wall dissolves at the place of contact and content of the spermatium along with male nucleus migrates down into the carpogonial base. The male and female nuclei fuse and forms a diploid zygote nucleus.
2. Carposporophyte plant
Post fertilization changes and development of carposporophyte
After the act of fertilization the sexual apparatus undergoes profound changes. The lateral sterile filament initial divides and becomes two celled. The lateral sterile filament is 4-10 celled. The sterile filaments are nutritive in function.
At the same time, the supporting cell produces an auxiliary cell at its upper side. It contains a haploid nucleus. Soon, auxiliary cell makes its connection with the base of carpogonium through a tubular connection. The zygote nucleus divides mitotically and one of its daughter diploid nucleus passes into the auxiliary cell through tubular connections. Later, carpogonial filaments gradually shrivel, begine to degenerate and finally disappear. The haploid nucleus of the auxiliary cell degenerates.
Now, the auxiliary cell contains a migrated diploid nucleus. It divides mitotically into two daughter nuclei of which one goes into the protuberance arising from the auxiliary cell. Soon, this protuberance becomes constricted, develops septum and is known as gonimoblast initial. The process is repeated and thus several gonimoblast initials are produced. Every time when a new gonimoblast cell is cut, the zygote nucleus divides and the daughter nucleus enters the newly formed cell. Each gonimoblast filament elongates into an elongated pear shaped structure called carposporangium. The carposporangial content ultimately metamorphoses into a single diploid carpospore.
The sterile filaments, auxiliary cell and supporting cell fuse together to form placenta of irregular shape, Their nuclei are used up by the growing carpogonial filaments. The carpogonial filament does not take part in the formation of placental cell but withers. Meanwhile, with the above changes, the pericentral cells, adjacent to these supporting cells, divide and form a protective flask shaped covering, the pericarp. Actually, it starts its developmet as an outgrowth just before the fertilization.
The entire structure consisting of placental cell, gonimoblast filaments with terminal carposporangia and surrounding pericarp is termed as cystocarp. A cystocarp is partially haploid and partially diploid.
Germination of carpospore
Each carpospore is diploid and germinates to produce a sporophytic plant called tetrasporophyte.
3. Tetrasporophytic plant
i) Development of tetrasporophytic plant
On reaching a suitable substratum, the carpospore germinates. The first division is asymmetrical and transverse results in the formation of a lower small and upper large cell. Both cell again divide transversely forming a linear tetrad of four cells. The lower most cell of the tetrad grows into a long unicellular rhizoid. The distal end of the rhizoid expand into an irregularly lobed attachment disc. The uppermost cell function as an apical cell and cuts of series of cells in a linear row. A ring of pericentral cells is produced by the vertical tangential divisions of the cells of the linear filament. Ultimately, a mature plant, morphologically similar to the parent gametophytic plant is formed. The newly formed plant is known as tetrasporophyte. However, this plant is morphologically different from gametophytic plant because of diploid nature. When mature, a tetrasporophyte plant produces tetrasporangia and tetraspores.
Asexual reproduction
Development of tetrasporangia
Asexual reproduction is brought about by tetraspores formed in tetrasporangia as soon as tetrasporophyte plant mature. Tetrasporangia are produced in successive tiers from the pericentral cells of the main axis. Usually, a single tetrasporangium develops from one pericentral cell of transverse tier. The fertile pericentral cell is smaller in size than the other pericentral cells of the same tier. It divide vertically into an outer and an inner half. The outer half may cut off two cover cells or two cover cells and a small peripheral cell. In both cases and in other forms, the inner half divides by transverse wall into lower daughter cell and an upper cell. The lower cell forms the stalk cell while the upper cell is called tetrasporangial cell. It enlarges several times to its original size. Its diploid nucleus undergoes meiosis and produces four haploid nuclei. Each part of the cytoplasm becomes tetraspore enclosing a single haploid nucleus. Thus, four tetraspores are formed within a single tetrasporangium which are arranged tetrashedrally. The wall of the tetrasporangial cell acts as a wall of tetrasporangium. Each tetraspore is a haploid structure with no firm and definite wall. They are liberated by the rupture of tetrasporangial wall. Each tetraspore germinate to form gametophytic plant of Polysiphonia. Their germination is similar to that of a carpospore. It is believed that of the four tetraspores, two give rise to male gametophytic plants and the remaining two to female gametophytic plants.
