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.
