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).
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