In this article we will discuss about:- 1. Occurrence of Coleochaete 2. Plant Body of Coleochaete 3. Cell Structure 4. Features 5. Reproduction.
Occurrence of Coleochaete:
The genus Coleochaete (Gr. Keleon, sheath; chaetos, hair) is represented by about 10 species, out of which 3 species are found in India. They grow in fresh water either as epiphytes on different angiosperms (Trapa, Hydrilla, Ipomea, Sagittaria, Typha etc.) and algae (Oedogonium, Vaucheria) or as endophytes (C. nitellarum) within the cells of Chara and Nitella (Chlorophyceae).
Plant Body of Coleochaete:
Plant body of Coleochaete is multicellular and heterotrichous in habit (Fig. 3.66A, B). They show much variation in their heterotrichous nature. Both erect and prostrate systems are well represented in C. pulvinata. But others may show either well-developed erect system than prostrate system (C. divergens, C. nitellarum) or prostrate system is well-developed than the erect one (C. scutata, C. obicularis).
Due to well-developed prostrate system, it forms discoid thailus and looks like pseudo- parenchyma of one cell in thickness. The discoid thalli may be a circular in C. orbicularis or irregular in C. scutata.
Cell Structure of Coleochaete:
The cells are generally hexagonal or polygonal but may be quadrangular in some region of the thallus. The cells of the erect filament of heterotrichous habit are almost cylindrical, where the length is more than breadth. The cells are uninucleate with single large laminate chloroplast generally with one or rarely with two pyrenoids.
Important Features of Coleochaete:
1. Plant body is thalloid, multicellular and heterotrichous in habit.
2. Cells are uninucleate with single large lamellate chloroplast generally with one or rarely two pyrenoids.
3. Asexual reproduction takes place by biflagellate ovoid zoospores and aplanospores formation.
4. Sexual reproduction is oogamous.
5. The antheridia may be terminal or intercalary. Each antheridium produces single colourless, unicellular, uninucleate, biflagellate antherozoids.
6. The oogonium is a flask-shaped structure with an elongated neck, the trichogyne.
7. Each oospore is converted into spermocarp during elaborate post-fertilisation stages.
8. Oospore undergoes a period of rest and then produces zoospores (16-32) after meiosis.
9. The zoospores on germination gives rise to new individuals.
Fig. 3.70, 3.71 depict life cycle of Coleochaete.
Reproduction in Coleochaete:
Coleochaete reproduces both asexually and sexually.
It takes place by zoospore and aplanospore formation.
During spring or early summer, asexual reproduction takes place by the formation of zoospores (fig. 3.67). Any cell of the plant body may function as zoosporangium (Fig. 3.67A). Each zoosporangium produces single zoospore.
Zoospores are ovoid, unicellular, uninucleate and biflagellate structure with a large parietal chloroplast. Eye-spot is absent (Fig. 3.67B).
The zoospores are liberated from the zoosporangium through a pore on the sporangial wall (Fig. 3.67A). It loses flagella within a short time and secretes a wall on the periphery.
During germination the zoospore divides either transversely (Fig. 3.67C) or vertically and with subsequent divisions (Fig. 3.67D) 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.
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.
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.
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 (Fig. 3.68A).
Antheridia may form on the same filament bearing oogonia in homothallic species. Each antheridium produces single colourless, unicellular, uninucleate, biflagellate antherozoid (Fig. 3.68B). They are liberated by breaking the apical side of the antheridial wall.
The oogonium develops at the apex of an erect branch (Fig. 3.68C). Due to further growth of the lower cell, the oogonium becomes lateral in position (Fig. 3.68D, E). The oogonium is a flask-shaped structure with an elongated, colourless receptive trichogyne (Fig. 3.68E). In discoid species instead of trichogyne, the neck is represented by small papilla. The basal enlarged portion contains a single oosphere or egg.
Before fertilisation the apex of the trichogyne breaks down (Fig. 3.68F) 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 (Fig. 3.68G).
Initially the nucleus of antherozoid is smaller in size than egg and after attaining equal size to the egg, fertilisation takes place resulting in the formation of zygote or oospore.
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 (Fig. 3.69A). The oogonium with pseudoparenchymatous sheath is called spermocarp. The spermocarp is red or reddish-brown in colour. It remains dormant throughout the winter.
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 (Fig. 3.69B). This is followed by subsequent division at right angle to each other and forms octant stage.
Further divisions make 16-32 celled stage (Fig. 3.69C, D). Each cell with its protoplast metamorphoses into single biflagellate zoospore (Fig. 3.69E). 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.
C. scutata, C. orbicularis and C. nitellarum.