STRUCTURE OF VEGETATIVE BODY AND REPRODUCTION iN VOLVOX
The genus Volvox comprises
about 20 species which is world-wide in distribution. The genus is found to
occur in both temporary and fresh water tanks, ponds, pools, etc. It appears as
a minute floating ball, floating and moving on the surface of water. During and
rainy season, the surface of water in which Volvox occurs look
green in colour because of the abundance of Volvox colony.
During early summer Volvox abruptly disappears and it remains
in a resting zygote condition during rest of the year.
B. STRUCTURE OF THE VEGETATIVE BODY:
Volvox occurs in colony i.e., it is coenobial
form. Each colony i.e., coenobium is free swimming and appears as a small
pinhead-like (about 0.5mm in diameter) spherical or ovoid, hollow sphere of
mucillagenous mass in which definite number of cells are arranged in its
periphery in a single layer – the number of which may vary from 500 to 60,000
or more in different species.
Each cell of the coenobium is biflagellate and
is provided with individual gelatinous sheath or mucilage
envelope. The cells remain interconnected with each other by cytoplasmic
connections. The cells of most species are oval in shape. Sometimes the
cells are pear-shaped (V. aureus).
Each cell has a central nucleus, one cup shaped or a
laminate chloroplast with one or more pyrenoids towards
the posterior pole of the cell, the reddish eye spot at the
anterior end, two flagella of equal length at the anterior end
of the cell and 2-5 contractile vacuoles near the base of the
flagella. The flagella of the peripheral cells penetrate through the pores of
the outer sheath of the coenobium.
C. REPRODUCTION: Volvox reproduces both by asexual and sexual
methods of reproduction. A coenobium has all its reproductive cells either
entirely asexually when the seaseon is favourable for growth but sexually
towards the end of growing season.
1. Asexual Reproduction:- A few cells (2 to about 50) in the posterior half of the
coenobium take part in asexual reproduction. These special reproductive cells
gradually push back into the colony, withdraw their flagella, increase ten or
more times the size of the vegetative cells and become more or less rounded in
shape. They are recognized by their well defined nucleus and dense granular
cytoplasm. Such reproductive cells are called parthenogonidia or gonidia.
The protoplast of each gonidium by successive longitudinal divisions forms
a daughter coenobium within a parent cell wall. This is a
tightly packed ball of cells.
2. Sexual Reproduction:- Sexual reproduction is of oogamous type.
Some of the species are monoecious (V. globater) while others are
dioecious (V. aureus). Certain cells in the posterior region of the
mature coenobium enlarge, withdraw their flagella and behave as gametangia.
The gametangia are large, round cells with many pyrenoids but no flagella. The
male gametangia are called antheridia or androgonia and the
female oogonia.
(a) Antheridium:- The biflagellate cell destined to form
the antheridium enlarges, withdraw its flagella and pushes back into the colony
but keeps its connection with the adjacent vegetative cells by cytoplasmic
strands. The protoplast of the antheridium undergoes successive mitotic divisions
and forms 64-128 small conical sperm cells. They are arranged in a
bowl shaped plate or hollow sphere phialopore. The sperm cells
develop their flagella in the anterior ends directed towards
the inside. As the bowl or sphere matures, the sperm cells undergo inversions
so that flagella remain in the exterior side.
Each sperm is biflagellate elongated, conical
or fusiform structure. It has a small, yellow, green or pale green chloroplast.
The flagella are inserted apically or sub-apically at the long, pointed
anterior end. The sperms are liberated by the rupture of the antheridial wall.
(b) Oogonium:- The cell destined to form oogonium enlarges considerably many
times the size of the ordinary vegetative cell. It becomes rounded or
flask-shaped and projects inwards into the colony and withdraws its flagella.
The entire protoplast of the oogonium gets metamorphosed into a single,
non-flagellated, green, spherical egg or oosphere. It has a large
central nucleus, and a parietal chloroplast containing
numerous pyrenoids. It also has a reserve food stored absorbed from
the surrounding vegetative cells through cytoplasmic connections. The oosphere
often develops beak like protrusion which marks the point of entrance of the
sperms.
(c) Fertilization:- The mass of antherozoids after
liberation from the antheridium swims about as a colonial unit, until it
reaches the vicinity of the egg. At the time of fertilization, the individual
antherozoids swim through the gelatinous sheath around an egg and enter through
the protrusion of the oogonium. Finally, one of the antherozoid fuses with the
egg to complete the fertilization. As a result of fertilization
a diploid zygote or oospore is produced.
With the return of the
conditions favourable to growth, the oospore germinates. Prior to germination
the zygote nucleus undergoes meiotic division to form four haploid
daughter nuclei. Of these three degenerates and only one remains functional
which later develop into new individual coenobium.
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