POLYSIPHONIA - STRUCTURE OF VEGETATIVE BODY AND REPRODUCTION


A. CLASSIFICATION:
Division – Rhodophyta
      Class – Rhodophyceae
             Order – Ceramiales
                     Family – Polysiphonieae
                           Genus – Polysiphonia
The genus Polysiphonia comprises about 150 species and is entirely marine and grows in shallow and quiet water along the coasts of Atlantic and Pacific oceans. The genus also occurs in littoral and sub-littoral zones as well as in tidal marshes, brackish estuaries, etc. Many species of Polysiphonia also occurs as epiphytes on other plants. P. elongate is a common lithophyte occurring attached on the rocks of sea shores.
Some common Indian species of Polysiphonia are – P. platycarpa, P. urecolata, P. ferulacea, P. variegate, etc.
B. STRUCTURE OF THE VEGETATIVE BODY:
The name Polysiphonia has been give because of the presence of a number of siphon-like cells arranged in definite tiers. Plant body i.e., thalli are generally small, being a few to several centimeters in height. The thallus is well branched and reddish or dark-brown or bluish-red in colour. Some species exhibit heterotrichous habit with – erect system of branches and prostrate creeping system.
The erect filaments arise from the prostrate creeping system. This erect system can consists of a central axis from which arises many branches. The branches arise in a radial or spiral symmetry. All the branches including central axis are feathery in appearance and siphonaceous in structure. Many short uni-siphonaceous branches called trichoblasts are present in the apical portion. Trichoblasts are either simple or dichotomously branched and tapering like hairs.
The prostrate creeping portion functions as an organ of attachment and also for perennation. Unicellular rhizoids develop from the lower surface of the prostrate system, which helps the plant to get attached on the substratum.

C. REPRODUCTION: In the life cycle of Polysiphonia occurs three distinct stages – gametophyte, carposporophyte and tetrasporophyte.
1. Gametophytes:- The gametophyte plants of Polysiphonia are concerned with the production of gametes which involves oogamous type of sexual reproduction. Polysiphonia is doiecious i.e., male and female sex organs are borne on different gametophyte plants called the male and female gametophytes respectively.
(a) Male gametophyte – It bears the male sex organs called spermatangia. The spermatangia are borne in dense clusters and are closely packed forming a compact cone shaped structure on short monoecious branches near the apices of the male plants. The branches are called the male trichoblasts.
The male trichoblast usually consists of two basal cells constituting the stalk. The stalks usually forks into two branches. In some species both the branches may become fertile. In others, one branch may become fertile and the other one sterile. The sterile branch may develop into a repeatedly forked sterile axis.
The entire contents of the spermatangium is metamorphosed into a single male nucleus i.e., spermatium.

(b) Female gametophyte – It bears the female sex organs called the carpogonia. The carpogonium is somewhat a flask-shaped structure. It consists of a swollen basal portion and a long tubular portion also known as trichogyne. The female gamete is the uni-nucleate protoplast. The trichogyne simply functions as a receptive organ.
The carpogonium is situated at the tip of a short lateral curved branch consisting of 4 cells, known as carpogonial branch or filament. The basal cell of the carpogonial filament is known as supporting cell. It cuts off two sterile filament initials – one towards the base and the second laterally. The basal sterile cell remains undivided while the lateral sterile filament immediately divides to form a 2-celled lateral sterile filament. This is the structure of the carpogonium at the time of fertilization. The two pericentral cells now divide and re-divide to form the envelope of the carpogonium which is known as pericarp.

         (c) Fertilization – The liberated spermatia are carried by the currents of sea water. As they come in vicinity of the carpogonium, one of them adheres to the trichogyne. At the point of contact, the wall between the two dissolves. The male nucleus then enters the trichogyne and moves down and finally reaches the female nucleus, and finally fuses with it to accomplish the fertilization.
       After fertilization, a series of development takes place which results in the formation of cystocarp. The cystocarp consists of the placental elements, gonioblast filaments bearing carposporangia and the surrounding sheath known as pericarp.
            2. Carposporophyte:- The diploid portion of the cystocarp constitutes the carposporophyte. It consists of the zygote and the gonioblasts bearing the carposporangia. It remains attached to the female plant of Polysiphonia and is responsible for the production of carpospores.
            The carposporophyte is surrounded and protected by a two-layered pericarp, opened at the tip. At maturity the pericarp contains a diploid carpospores which float out through the opening or ostiole and are carried away by water currents.
 On coming in contact with the solid object, the carpospores germinates to produce a diploid tetrasporophytic generation.

           3. Tetrasporophyte:- It is a free living individual. The thallus in its vegetative structure exactly resembles the gametophytic plants. It consists of a central siphon encircled with the placental siphons. The branching of the thallus is lateral. The branches arise from the apical region of the central filament.
            The tertasporophyte produces tetrasporangia from the pericentral cells in the apical region of the branch. The diploid nucleus of tetrasporangium by meiotic division produces four haploid tetraspores (n).
 When the tetraspores attain maturity, the sporangial wall ruptures and the tetraspores escape. On attachment with the substratum, the tetraspores germinate to produce a haploid gametophytic plant again.
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