In this article we will discuss about:- 1. Occurrence of Coleochaete 2. Thallus Structure of Coleochaete 3. Cell Structure 4. Growth 5. Reproduction 6. Life Cycle 7. Affinities.
Occurrence of Coleochaete:
The genus Coleochaete is the only representative of the family Coleochaetaceae. It is represented by about ten species of which three species—C. orbicularis, C. nitellarum and C. scutata are common in India.
Coleochaete is fresh water epiphyte which grows epiphytically upon other algae e.g., Oedogonium, Vaucheria or on submerged parts of aquatic angiosperms like Hydrilla, Nymphaea, Sagittaria, Trapa and Typha. It also grows endophytically (e.g., C. nitellarum) within the cells of Nitella and Chara.
Thallus Structure of Coleochaete:
The thallus is multicellular, branched and heterotrichous in nature. The truly heterotrichous habit with well-developed prostrate and erect system is best represented in C. pulvinata (Fig. 2). In most of the species the basal prostrate system is well developed and erect system is reduced e.g., in C. scutata and C. orbicularis.
The lateral branches of the prostrate system form discoid thalIi (Fig. 1) which may be irregular (e.g., C. scutata) or regular (e.g., C. orbicularis) in outline. In C. nitellarum and C. divergens there is well developed erect system and the prostrate system is reduced or filamentous.
Cell Structure of Coleochaete:
The cells of the Coleochaete may be quadrangular, polygonal or cylindrical. The cells are uninucleate and each cell has a large parietal chloroplast of irregular shape that partially or wholly encircles the protoplast. Chloroplasts contain one or two prominent pyrenoids.
Some cells of thallus bear a single, long, un-branched cytoplasmic seta or hair (Fig. 2) the base of which is covered with a cylinder of gelatinous material. The development of seta takes place after the formation of a pore on the wall and the appearance of a deep strained granule just below the pore. The sheath of seta remains attached with the thallus even when the seta breaks off.
Growth of Coleochaete:
The apical growth takes place in the branches of erect system. The apical cell divides transversely to increase the length. The growth of the discoid prostrate system takes place by marginal meristem. Erect branches originate as lateral outgrowths of prostrate system.
Reproduction in Coleochaete:
The reproduction in Coleochaete takes place by asexual and sexual methods:
(i) Asexual Reproduction:
Asexual reproduction takes place by means of zoospores which are large, ovoid and biflagellate. The zoospores are usually formed in the spring and beginning of the summer season.
Any vegetative cell of the thallus may function as zoosporangium. A single zoospore is formed in each zoosporangium (Fig. 3 A). The large, ovoid, biflagellate zoospores are uninucleate. The chloroplast is parietal and there is no eye spot (Fig. 3 D, E).
The zoospore escapes by moving in amoeboid manner through a pore in parental cell wall and then moves about for an hour or so. Then it loses flagella, comes to rest and secretes a wall (Fig. 3 B, C). After a short period of rest it germinates either horizontally or vertically.
In C. scutata, the first division is transverse, the upper cell develops into hair or seta and the lower cell forms the embryonal disc. The embryonal disc forms the discoid thallus (Fig. 3 F, G).
In C. pulvinata and C. nitellarum, the first division is vertical and both the daughter cells divide repeatedly to form prostrate system. The erect system develops from the prostrate system. According to Wesley (1928), during unfavorable season thick walled aplanospores may develop instead of zoospore in zoosporangium. When favourable conditions approach the aplanospore germinates to form new thallus.
(ii) Sexual Reproduction in Coleochaete:
The dwarf species of Coleochaete reproduce exclusively by zoospores. In all other species of Coleochaete the sexual reproduction is of advanced oogamous type. The plants can be homothallic e.g., C. divergens and C. pulvinata or heterothallic e.g., C. scutata. The male and female sex organs are antheridia and oogonia respectively. The arrangement and structure of sex organs are different in discoid and cushioned species.
Structure and Development of Antheridia:
In discoid forms like C. scutata, antheridia develop midway between the centre and periphery of the thallus. In cushioned forms like C. pulvinata, the antheridia are borne at the tips of the erect branches as club shaped cells in clusters. The vegetative cell divides into two, of which only one functions as the antheridial mother cell which forms antheridium by another division (Fig. 4 A).
Each antheridium forms a single, uninucleate and biflagelate antherozoid (Fig. 4 B). The antherozoid may be green e.g., C. scutata or colourless e.g., C. pulvinata. The antherozoid is liberated by the rupture of the antheridial wall.
Structure and Development of Oogonia:
In discoid forms oogonia develop near the margin of the radiatory rows of the vegetative cells. The oogonial initial cells differ only in size and later become hemispherical structure with a neck represented by a short papilla. After some marginal growth of thallus, oogonia come a little distance interior to the margin of thallus.
In cushioned forms the oogonia develop at the tips of erect branches. As the cell just below the oogonium continues the growth of branch, the oogonium appears lateral in position.
The oogonium, also called carpogonium, is a flask shaped structure with a swollen base called carpogonium and a long colourless neck called trichogyne. The carpogonium contains a single egg and one or more chloroplasts. In discoid forms e.g., C. scutata the trichogyne is absent and neck is represented by a short papilla (Fig. 4 C-E).
Fertilization in Coleochaete:
Just before fertilization the tip of trichogyne breaks and some colourless substance comes out of the opening.
Many antherozoids are chemically attracted to the tip but only one antherozoid passes down into the oogonium. The fusion of male and female nucleus does not take place immediately. The male nucleus increases in size and becomes equal to the female nucleus. At this stage male and female nuclei fuse to form a diploid zygote (Fig. 4 F, G).
Post Fertilization Changes:
After formation of zygote the basal swollen part of the oogonium is separated from the trichogyne by the formation of a septum. The trichogyne degenerates and the oogonium increases in size.
The zygote secretes a thick brown wall around it. The cells below and around the oogonium are stimulated and they form a pseudoparenchymatous layer around the oogonium. This structure of reddish brown colour is now called spermocarp or fruiting body (Fig. 4 H, I).
Germination of zygote:
The spermocarp is shed in water by the degeneration of basal cells. It remains dormant during unfavorable winter season. When favourable conditions return, the spermocarp again turns green. The zygote nucleus first divides meiotically and then by simple division to make 8-32 daughter cells.
The first division of zygote is followed by a transverse wall and the subsequent divisions are at right angle to each other. Each daughter protoplast metamorphosis’s into a biflagellate zoospore. The spermocarp and the zygote wall break into halves and the zoospores are released (Fig. 4 I-K). The zoospores swim freely for some time, then develop into new thalli.
Life Cycle of Coleochaete:
The life cycle of Coleochaete is haplontic type (Fig. 5, 6). The thallus is haploid and represents the gametophytic generation. It produces antheridia and oogonia on erect branches. The antherozoids and oogonia take part in fertilization and fuse to form a diploid zygote.
After post fertilization a diploid spermocarp is formed which represents diploid sporophytic generation. The zygote divides meiotically to form 8-32 haploid zoospores. The haploid zoospores germinate to form haploid gametophytic thalli.
Affinities of Coleochaete:
Coleochaete possesses advanced characters like terrestrial tendency, heterotrichous thallus differentiated into prostrate and erect system. The male and female sex organs are distinct and the sexual reproduction is advanced oogamous type (Fig. 5, 6).
The post fertilization changes in Coleochaete are parallel to Rhodophyceae and certain characters like presence of setae are comparable to Phaeophyceae. According to Fritsch (1935) Chaetophorales may represent the possible ancestors from which the terrestrial higher plants originate.