Oedogonium
Classification
Sub-division — Algae
Class — Chlorophyceae
Order — Oedogoniales
Family — Oedogoniaceae
Genus — Oedogonium
Life Cycle & Practical Study
Oedogonium can reproduce asexually by fragmentation of
the filaments, germination of aplanospores and akinetes, and through zoospores.
In fragmentation, the filament splits apart and each fragment reproduces to
form a fully functioning thallus.
Sexual reproduction in Oedogonium is oogamous, and can
be monoecious or dioecious. Species may either be macrandrous (lacking dwarf
males) or nannandrous (possessing dwarf males). Dwarf males are small, short,
antheridium-producing filaments attached near the oogonia (female sex organ).
[I] Vegetative Reproduction
(A) Fragmentation
Oedogonium filament breaks into many small fragments
which have capability to grow into complete filaments under favourable
conditions.
Fragmentation takes place due to any of the following
reasons:
➤Accidental breaking of the filaments.
➤ Dying or dehydration of intercalary cells.
➤ Disintegration of intercalary cells due to
conversion in sporangia.
➤ Mechanical injury to the filament.
➤ Change in the environmental conditions.
(B) Akinete formation
The akinetes are formed under unfavorable conditions.
Akinetes are modified vegetative cells which become swollen, round or oval,
reddish brown and thick walled. These are rich in reserve starch and orange-red
coloured oil. Akinetes are formed in chains of 10 to 40 Akinetes germinate
directly under favourable conditions.
[II] Asexual Reproduction
Asexual reproduction takes place by means of
multi-flagellate zoospores produced singly in intercalary cap cell. Mostly the
newly formed cap cell functions as the zoosporangium.
The mature zoospore is oval, spherical or pear shaped
structure. The zoospore is uninucleate and contains a ring shaped chloroplast.
The zoospore is dark green in colour except at the hyaline pointed apical end.
A sub apical ring of flagella is present and such flagellation is called
stephanokontic type.
After liberation, the zoospore swims for about an
hour. Then it settles and attaches itself to a solid substratum with its
anterior end downwards. After attachment flagella are withdrawn and it starts
elongation. The lower hyaline part elongates to make holdfast and the upper
part divides repeatedly to make new filament.
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[III] Sexual Reproduction
Sexual reproduction is more frequent in still waters
than in running water. The factors influencing sexual reproduction are alkaline
medium, deficiency of nutrition, light and dark periods and increased
temperature. Depending upon the nature of antheridia producing plants,
Oedogonium species are of two types- Macrandrous and Nannandrous.
Thallus
Thallus is multicellular, filamentous and unbranched.
A filament is differentiated into three types of to
their position:- (i) basal, (ii) intercalary and (iii) apical.
The basal cells of filament functions as a holdfast.
The lower part of the holdfast is either disk-like or finger shaped. The upper
part is mostly broad and rounded. The basal part of cell generally lacks green
pigment and, therefore, is none-green unlike other cells of the filament.
➤A
cell at the tip of the filament is known as apical cell. It is rounded at its
free surface.
➤The
cells present between basal and the apical cells are intercalary cells. These
show typical cell structure.
➤The
typical cell is cylindrical.
➤Cell
wall is thick and three layered.
➤Internal
to cell wall is a reticulate chloroplast that runs parallel to the long axis of
the cell. Many pyrenoids are present in the chloroplast.
➤The
cell is uninucleate. The nucleus is situated near the cell wall and is held by
thin and delicate cytoplasmic strands.
➤Mature
and old cells show ‘cap cells’ at their upper end. These are characteristic of
the members of Oedogoniales.
Read This 👇
Oogonium
In Oedogonium the female sex organ oogonia are highly
differentiated female gametangia. These are mostly intercalary but sometimes
can be terminal.
The structure and development of oogonium is identical
in macrandrous and nannandrous species. Like antheridia any freely divided or
actively growing cap cell functions as the oogonial mother cell. The oogonial
mother cell divides by transverse division into two unequal cells, the upper
cell and the lower cell.
The upper larger cell forms oogonium and the lower
smaller cell function as supporting cell or suffultory cell. In some species
the oogonial mother cells directly forms the oogonium. If any of the two
divided cells again functions as oogonial mother cell many oogonia are formed
in chain.
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Study
of Oogonium
Place a few filaments and observe a filament with oogonia.
Stain such filaments with safranin, wash in water and mount in glycerine. Study
the oogonia.
➤Oogonia
occur in intercalary or terminal positions.
➤Oogonium
may be solitary or occur in a row of 2-3 or even more.
➤Oogonium
generally shows one or more cap cells at its upper end, «indicating its development
from a comparatively older cell.
➤It
is mostly spherical or oval in shape and larger than a vegetative cell.
➤At
the base of each oogonium lies a small and flat daughter cell, «known as
supporting or suffultory cell.
➤Oogonium
encloses a single large ovum.
➤The
wall of the oogonium has a small pore on one side, known as «receptive pore.
➤Just
opposite the receptive pore, protoplast of the oogonium has a hyaline
area-receptive spot.
➤Uninucleate
protoplast is rich in reserve food.
Antheridium
The antheridia develop on normal filaments, terminal
or intercalary in position. The initial cell which gives rise to antheridia is
called antheridial mother cell. It is normally a cap cell. The antheridial
mother cell divides by transverse division to form an upper smaller cell called
antheridium and a lower larger cell called sister cell.
The sister cell divides repeatedly to form a row of
2-40 antheridia. Rarely the antheridia are produced singly. The antheridia are
broad, flat, short cylindrical, uninucleate cells. The contents of an
antheridial cells divide either longitudinally or transversely into two.
The two antherozoids are positioned side-by-side or
one above the other if divisions are longitudinal and transverse respectively.
The antherozoids are liberated in the same fashion as zoospores (Fig. 6 B). The
liberated antherozoids or spermatozoids or sperms are pale green or yellow
green, oval or pear shaped.
Study
of Antheridium
Place a few filaments on a slide in a drop of water, observe
under the microscope for the presence of chain of antheridia. Isolate the
filaments, stain in safranin, wash with water and mount in glycerine.
➤Study
the antheridia of macrandrous species.
➤Antheridia
are mostly intercalary in position.
➤Numerous
antheridia form a long chain being arranged in a series.
➤An
antheridium is a small and flat cell.
➤Each
antheridium has two nuclei lying side by side, surrounded by dense cytoplasmic contents.
Each of such protoplasmic groups later on metamorphoses into a multiflagellate
antherozoid.
Dwarf male or nannandrium
The antheridia are formed on short or dwarf male
plants called dwarf males or nannandria. The dwarf male filament is produced by
the germination of a special type of spore known as androspore.
The androspore is produced singly within an
androsporangium. Androsporangia are more or less similar looking to the
antheridia of macrandrous forms and are produced in a similar manner from a
mother cell.
The Androsporangia are flat, discoid cells slightly
larger than antheridia. Each androsporangium produces a single androspore just
as in the case of zoospore. Liberation of androspore is similar to that of a
zoospore. The androspore look similar to zoospore except for the smaller size.
The Androspores are motile and have a subpolar ring of flagella.
After swimming about for some time, the androspore
settles on oogonial wall. The androspore germinates into a dwarf male or
nannandrium. Germlings at one celled stage may divide and produce two antherozoids.
Study
of Dwarf male or nannandrium
Stain a few filaments in safranin, wash in water, mount
in glycerine and study.
➤The
dwarf male (or nannandrium) is characteristic of nannandrous species.
➤The
dwarf male is produced by the germination of androspore.
➤Androspores
are formed inside the Androsporangia.
➤Androsporangia
form a long chain of small and flat cells in intercalary position in the filament.
➤Each
androsporangium develops a single multiflagellate androspore (in contrast, per antheridium
two antherozoids are produced).
➤Androspore
germinates to produce a dwarf male or nannandrium which remains attached either
to the wall of the oogonium or to the suffultory cell.
➤A
dwarf male is made of a stalk cell and a terminal row of 2-3 cells.
➤Stalk
cell is at the base by which the dwarf male is attached to the filament. It has
a-disc-like or finger-like structure at its base.
➤The
terminal Tow has 2-3 small, flat and narrow antheridia.
➤Each antheridium has two multiflagellate antherozoids.
Zygote
In Oedogonium the thallus is haploid and the life
cycle is haplontic type. The diploid stage in life cycle is only zygote. It
occurs for a short period. The zygote or oospore undergoes meiosis to make four
meiozoospores which again form haploid Oedogonium thalli. The variations in
life cycles of Oedogonium are due to macrandrous and nannandrous nature of
Oedogonium species.
Study
of zygote
➤Zygote
is a thick-walled structure developed as a result of fertilization.
The wall is generally three layered.
➤The
layer outside the innermost may be smooth, ornamented or verrucose.
➤Zygote
develops red colour due to the accumulation of reserve food
in the form of reddish oil drops.
Identification
➤Sub-division-
Algae. (1) Thallus filamentous, (2) Chlorophyll present, (3) Cell wall of cellulose.
➤Class-
Chlorophyceae. (1) Chloroplasts grass- green, (2) Photosynthetic reserve-starch,
(3) Motile structures flagellated, (4)
Flagella equal.in length.
➤Order-
Oedogoniales (1) Cells uninucleate, filaments branched or unbranched, (2) Cell
division forming ‘caps’, (3) Chloroplast reticulate, (4) Zoospores and
antherozoids bear a whorl of flagella, (5) Production of dwarf males.
➤Family-
Oedogoniaceae (A single family).
➤Genus-
Oedogonium, (1) Filaments unbranched, (2) Cells cylindrical, (3) Holdfast well
developed.
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