Coleochaete
Classification:
Sub-disision- Algae
Class- Chlorophyceae
Order- Chaetophoriles
Family- Coleochaetaceae
Genus- Coleochaete
★Thallus. Prepare a slide of discoid or cushionoid
thallus and study, its structure.
★Reproductive structures. Prepare a slide of discoid
thallus — - showing spermocarp. Study the structure.
Thallus:
Prepare a slide by first isolating a discoid or
cushionoid thallus from aquatic weed by scratching out the thallus with needle.
Stain it with safranin, wash with water and then mount in glycerine. Study the
structure of thallus and also a cell.
1. -Thallus is multicellular and heterotrichous.
2. It is either a disc-like structure in majority of
the species or cushionoid or filamentous in others.
3. If thallus is disc-like, the disc represents only
the prostrate system while a few setae or hair, represent erect system.
4. Filamentous thallus exhibits _ typical
heterotrichous habit with a branched prostrate system and a branched projecting
(erect) system.
5. In both the cases a few cells possess a
cytoplasmic outgrowth—setae. Setae are surrounded partly or wholly by a
gelatinous sheath at the base.
6. The thallus is distinctly enveloped by a gelatinous sheath or mucilage.
7. In discoid species cells of the thallus are joined end to end form branches.
These branches are laterally apposed to one another to form a pseudoparenchymatous disc.
8. Each cell uninucleate. it has a single, large, laminate and parietal chloroplast with a single pyrenoid. Rest part of the cell is occupied by the cytoplasm.
Reproductive structures:
Scrape out a few thalli from surface of aquatic weed. Observe under the microscope to see if Sspermocarps are present. Select such a thallus, stain in safranin, wash in water and mount in glycerine.
1. Thalli may be homothallic or heterothallic.
2. Sexual reproduction is oogamous.
3. Antheridia are generally borne at the tips in filamentous
species and in the middle or peripheral region in the discoid species.
4. Antheridia appear as a group of small cells.
5. Oogonia are also borne terminally in filamentous
species and towards periphery in the discoid species.
6. Oogonium is a flask-shaped structure with long
tubular trichogyne.
7. The fertilization results in the formation of a
Zygote which remains embedded inside the wall of the oogonium. It is a thick
walled structure.
8. Zygote known as_ spermocarp remains enveloped in
a parenchymatous tissue formed by the development of neighbouring cells. It is
conspicuously reddish-brown in colour.
9. Spermocarp remains dormant for a long period.
👉Life cycle of volvox, Sexual Reproduction in Volvox,
👉Protista Kingdom प्रोटिस्टा जगत
Reproduction in Coleochaete:
Coleochaete reproduces both asexually and sexually.
Asexual Reproduction:
It takes place by zoospore and aplanospore
formation.
Zoospore:
During spring or early summer, asexual reproduction
takes place by the formation of zoospores. Any cell of the plant body may
function as zoosporangium. Each zoosporangium produces single zoospore.
Zoospores are ovoid, unicellular, uninucleate and
biflagellate structure with a large parietal chloroplast. Eye-spot is absent.
The zoospores are liberated from the zoosporangium through a pore on the
sporangial wall. It loses flagella within a short time and secretes a wall on
the periphery.
Germination:
During germination the zoospore divides either
transversely or vertically and with subsequent divisions it forms new plant
body. In C. scutata, the first division is transverse .and forms two cells.
The upper cell forms seta and the lower one divides
repeatedly to form discoid thallus. But in species like C. pulvinata, the first
division is vertical and both the cells undergo repeated division to form a
prostrate system. The erect system develops later from the prostrate system.
Aplanospore:
Aplanospores are formed during unfavourable
condition. At that condition, the zoosporangium, instead of forming zoospore,
forms aplanospore. The aplanospores are non- flagellate, thick-walled and round
structure. During favourable condition the aplanospores germinate and form new
thalli.
Sexual Reproduction:
The sexual reproduction is highly specialised and
oogamous type. The plant body may be homothallic i.e., monoecious (C.
pulvinata) or heterothallic i.e., dioecious (C. scutata). The male sex organ is
called antheridium and female as oogonium or carpogonium.
Antheridium:
In well-developed heterotrichous form like C.
pulvinata, the antheridia are borne in groups at the apex of erect branches.
But in discoid form like C. scutata the antheridia are developed in the half
way between the centre and periphery of the thallus.
The antheridium develops as a conical outgrowth of
apical cell of the erect system which is cut off from the mother cell by wall.
A number of antheridia may develop at the apex of a lateral branch.
Antheridia may form on the same filament bearing
oogonia in homothallic species. Each antheridium produces single colourless,
unicellular, uninucleate, biflagellate antherozoid. They are liberated by
breaking the apical side of the antheridial wall.
Oogonium:
The oogonium develops at the apex of an erect
branch. Due to further growth of the lower cell, the oogonium becomes lateral
in position. The oogonium is a flask-shaped structure with an elongated,
colourless receptive trichogyne. In discoid species instead of trichogyne, the
neck is represented by small papilla. The basal enlarged portion contains a
single oosphere or egg.
Fertilization:
Before fertilization the apex of the trichogyne
breaks down and some colourless cytoplasm is exuded out which attracts the
antherozoids. Although many antherozoids get entangled in the exuded
cytoplasm,
but only one enters into oogonium and reaches to the egg through broken
trichogyne
Initially the nucleus of antherozoid is smaller in
size than egg and after attaining equal size to the egg, fertilization takes
place resulting in the formation of zygote or oospore.
Post-Fertilization Changes:
After fertilization a septum is laid down which
separates the basal region from the trichogyne. Gradually the zygote becomes
enlarged and secretes a thick wall around itself.
The neighbouring cells of the oogonium get
stimulated and form pseudoparenchymatous covering around it. The oogonium with
pseudoparenchymatous sheath is called spermocarp. The spermocarp is red or
reddish-brown in colour. It remains dormant throughout the winter.
👉Life cycle of volvox, Sexual Reproduction in Volvox,
👉Protista Kingdom प्रोटिस्टा जगत
During favourable condition the spermocarp becomes
active. The colour of spermocarp changes to green and then it germinates.
During germination its nucleus undergoes meiotic division. First it divides by
a transverse division into two cells. This is followed by subsequent division
at right angle to each other and forms octant stage.
Further divisions make 16-32 celled stage. Each cell
with its protoplast metamorphoses into single biflagellate zoospore. The
zoospores are liberated by breaking the zygote wall and sheath into two halves.
After liberation the zoospores swim freely for some time. Later on, they
germinate and form new haploid plant body of Coleochaete.
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Identification:
Sub-division—Algae.
(1) Thallus construction simple,
(2) Presence of chlorophyll,
(3) Cell wall of cellulose.
Class—Chlorophyceae.
(1) Chloroplasts green in colour,
(2) Reserve food in the form of starch,
(3) Motile structures flagellated,
(4) Flagella equal in length.
Order—Chaetophorales.
(1) Plant body heterotrichous,
(2) Presence of setae.
Family—Coleochactaceac,
(1) Vegetative cells with long cytoplasmic — hair
(setae),
(2) Cells uninucleate,
(3) Filaments branched,
(4) Each cell with a single parietal and laminate
chloroplast.
Genus—Coleochaete.
(1) Plant body multicellular,
(2) Thallus parenchymatous,
(3) Presence of spermocarp.
Hints for collection:
It occurs as freshwater alga, mostly as an epiphyte
on submerged water plants (e.g. pomoea, Typha, Polygonum, etc.) or on other
algae. It also grows endophytically inside the cells of Charales.
👉Life cycle of volvox, Sexual Reproduction in Volvox,
👉Protista Kingdom प्रोटिस्टा जगत