METHODS OF REPRODUCTION IN ALGAE
Algae reproduce by three different methods of
reproduction – Vegetative, asexual and sexual reproduction.
1. VEGETATIVE REPRODUCTION:- In this method the new individuals are
formed directly from the vegetative parts without any change in the protoplast.
It may be of the following types –
(a) Simple cell division and Fission:- In unicellular forms of algae, the
individual cell divides by simple cell division and forms two daughter cells (Synechococcus).
In fission the cell divides by formation of deep constriction on the sides of
the cell as seen in Diatoms.
(b) Fragmentation:- In filamentous algae, the plant body
i.e., the filament breaks up into one or more fragments, each of which develops
into a new individual plant. Fragmentation may occur by – accidental,
by formation of special structures called separation discs or
by some mechanical forces e.g., Spirogyra.
(c) Splitting of Colonial form:- In this type, the mature colonies split up into
one or more small parts, each of which gives rise to new daughter colonies,
e.g., Dictyosphaerium, Aphanothece, etc.
2. ASEXUAL REPRODUCTION:- Asexual reproduction takes place by the
formation of various kinds of motile and non-motile spores. These are as
follows –
(a) Zoospores:- These are flagellate, motile and naked
spores which may be formed either within a specialized structure called zoosporangium or
from the vegetative cell directly. The zoospores may be bi-flagellate (two
flagella), quadric-flagellate (four flagella) or multi-flagellate (many
flagella). Each zoospore swims in water, comes to rest, drops its flagella and
germinates into new individual.
(b)
Aplanospores:- These
are non-flagellate, non-motile spores formed inside a special spore producing
structure called aplanosporangium. Each aplanospore is surrounded
by a definite wall which is distinct from a parent cell wall. Each aplanospore
germinates and gives rise to new individual plant in response to suitable
environmental condition.
(c) Hypnospores:- Very thick-walled aplanospores with
abundant food reserves are called hypnospores. A hypnospore germinates after a
resting period to produce a new individual plant.
(d) Akinetes:- These are thick-walled and elongated
spores produced during dry condition from the vegetative cells of some
filamentous algae, e.g., Oedogonium, Nostoc, etc. Each akinete
under favourable condition germinates into new individual plant.
(e) Autospores:- These are non-motile resting spores
resembling the parent cell in shape and structure. Autospore formation occurs
in many members of Chlorococcales, e.g., Chlorella.
(f) Hormospores:- These are actually fragments i.e., short
portion of trichomes called hormogonia (hormogones). Such hormogones when
develop at the tips of trichome become different shaped cells with much
thicker walls and become multicellular spore like structure called hormospores.
Hormospores tide over unfavourable conditions, and when favourable condition
exists they germinate to produce new individual plant, e.g., Westiella,
Nostoc, etc.
(g) Endospores:- These are non-flagellate, thin-walled
spores formed internally i.e., inside the cell as a result of division of the
entire protoplast of the vegetative cell of some unicellular algae, e.g., Dermocarpa.
(h) Exospores:- These are also thin-walled, non-flagellate
spores which are cut off externally from the protoplast of the vegetative cell
of some unicellular algae, e.g., Chamaesiphon.
(i) Heterocysts:- These are specialized, larger and
thickened, double walled vegetative cells of some filamentous algae. In some
algae heterocysts germinate producing new individual.
3. SEXUAL REPRODUCTION:- Sexual reproduction takes place by the
union of sexual reproductive units called gametes. Three types of
sexual union can be observed in algae. These are –
1. Isogamy:- In this type, two morphologically
identical gametes, i.e., gametes of equal size and form, fuse to form a zygote.
The fusing gametes may be flagellate (motile) or non-flagellate (non-motile)
and are called isogametes, e.g. Ulothrix.
2. Anisogamy:- In this type, two morphologically non
identical gametes, i.e., gametes of unequal size and form, called anisogametes or heterogametes fuse
together to form a zygote, e.g., Chlamydomonas. Of the two non-identical
gametes, the smaller gametes are called micro-gametes and the larger gametes
are called macrogametes.
3. Oogamy:- In this type fusion takes place between
two heterogametes. Of the two mating type, one is smaller, active and
flagellate (motile), and is known as antherozoidor sperm or male
gamete. The other one is larger, passive and non-flagellate (non-motile)
and is known as ovum or oosphere or egg or
female gamete. As a result of fusion, zygote is formed
which directly develops into new individual plant.
0 comments:
Post a Comment