Q.13. How are caps formed in the genus oedogonium ? Explain.
Ans. The oedogonium grows by means of cell divisions, which occur only in some specialized cells. These cells are characterized by the presence of cap cells at the apical ends.
At the time of cell division, nucleus of the cell move towards the upper side. This is followed by the appearence of ring-like in growth near the septum of upper side of the cell. The ring gradually increases and extends towards the lower side the cell. The nucleus divides by mitotic division to form two daughter nuclei. Now the spetum is laid down between two daughter nuclei. In this way, two daughter cells are formed. The upper daughter cell is smaller whereas, the lower daughter cell larger. Then the upper daughter cell streches to attain normal size. In doing so, the outer wall ruptures near the upper end the cell. It remains present as a cap-like structure over this and is termed as cap cell. This newly formed cell is provided with two-layered wall only, later, the third layer develops and it attaches with the cell of lower old cell. In this way, two cells are produced. The number of cap cells indicates the numbers of divisions in the cell.
Q.14. Draw a well labelled diagram of a nannandrous oedogonium filament. (2012)
Ans.
Q.15. Describe the life history of Coleochaete.
Ans. systematic position: -
Class - Chlorophyceae
Order - Chaetophorales
Family - Coleochaetaceae
Genus - Coleochaete
Ocurrence: - Coleochaete is a fresh water algae which grows epiphytically or on submerged aquatic angiosperms like Hydrila, Nymphaea, Lemna etc. Thalli of Coleochaete are somtimes epiphytic on Oedogonium and Vaucheria. some species are endophytic in the members of the order Charales, e.g., C. nitellarum grow endophytically within the cells of Nitella and Chara.
The genus is represented in India by three species, C. orbicularis, C. scutata and C. nitellarum.
Thallus structure: - The thallus is multicellular, heterotrichous and branched. Heterotrichous habit is best known in C. pulvinata which has well-developed prostrate and erect systems. In the other species one of the two systems (prostrate or erect) is either absent or reduced, e.g., in C. nitellarum and C. divergens there is a well-eveloped erect system but the prostate system is filamentous or reduced and in C. scutata C. orbicularis erect system is absent and the lateral branches of the prostrate system form discoid thalli which may be irregular (e.g., C. scutata) or regular (C. orbicularis) in outline.
Cell structure: - The cells of the thallus may be quadrangular, polygonal or cylindrical. Each cell is uninucleate and has a single parietal laminate chloroplast with a large pyrenoid. Some of the cells of the thallus have a long unbranched cytoplasmic thread, called setae. The setae is enclosed in a gelatinous material. The setae develop from a blepharoplast, occuring in each cell below a pore in its cell wall. In older cells, the setae may be broken off.
Growth: - The erect branches show apical growth. The terminal cell of the erect filament divides transversely and adds to its height. The growth in the prostrate system takes place by means of a marginal meristem. The erect branches originate as lateral outgrowths of the prostrate system.
Reproduction: -
Coleochaete reproduces both asexually and sexually.
(a) Asexual reproduction: - The asexually reproduction takes place by means of biflagellate zoospores in summer or spring season. Any vegetative cell of the thallus may function as zoosporangium. A single large zoospore is formed in each zoosporangium.
The zoospore is ovoid, biflagellate and uninucleate. It has a single chloroplast but there is no eye spot. The zoospore escapes through a pore at the apex of the zoosporangium. It loses two flagella soon after liberation and secretes a wall.
At the time of germination the zoospore divides into two cell by a horizontal or vertical divisions. In C. scutata (which is a discoid form) the first division is transverse, the upper cell forms seta and the lower divides repeatedly and forms the discoid thallus. In heterotrichous species (e.g., C. pulvinata) the first division is vertical and both the daughter cells divides repeatedly to form prostrate system. The erect or projecting system develops from the prostrate system.
During unfavourable conditions the zoosporangium, instead of zoospore, forms farily thin walled non-motile aplanospores. The aplanospores germinate under favourable conditions and form new thalli.
(b) Sexual reproduction: - The sexual reproduction is advanced oogamous type. The species of Coleochaete are homothallic (e.g., C. pulvinata) or heterothallic (e.g., C. scutat). The male and female sex organs are called antheridia and oogonia (or carpogonia) respectively.
(i). Antheridium: - The antheridia are borne in form of clusters at the tips the erect branches, usually in groups. In discoid forms (e.g., C. scutata) the antheridia develop midway between the antheridium develops as an outgrowth of the terminal cell of the lateral branch. This outgrowth is separated from rest of the filament by septum and develops into an antheridium. Each antheridium forms a single, uninucleate and biflagellate antherozoid which may be green or collourless. The antherozoid is liberated by the rupture of the antheridial wall.
(ii). Oogonium or Carpogonium: - The oogonia develop at the tips of the erect branches terminally. But the cell lying just below the oogonium continues the growth of the branch and as such their oogonium appears lateral in position. The Carpogonium is a flask-shaped structure with a long neck, known as trichogyne. However, in discoid species trichogyne and the neck are absent and the neck is represented by a short papilla. The basal part of the oogoonium contains a prominent nucleus and one or more chloroplasts. The protoplast of the basal part rounds off to form a single egg or ovum.
(c). Fertilization: - Just before fertilization the tip of the trichogyne breaks down and some colourless protoplasm in extruded from the broken tip. Although many antherozoids are attracted towards the oogonium, only one enters the oogonium. The male and female nuclei do not fuse immediately. The male nucleus is smallar than the female nucleus. So male nucleus increases in size as soon as it becomes equal to the size of the female nucleus, both the nuclei fuse to form a diploid zygote.
Fig. - Coleochaete: Sexual reproduction, A. A branch with antheridium, B. Antherozoid, C-E. Successive stages in the development of oogonium, F. Fertilization, G. Oospore, H-J. Stages in germination of oospore, K. zoospore.
Post-fertilization changes: - After fertilization the basal swollen part of the oogonium is separated from the trichogyne by the formation of a septum. The zygote greatly increases in size and secretes a thick wall. Now the cells below and around the oogonium are stimulated and form a pseudoparenchymatous layer is called spermocarp. spermocarp is reddish-brown in colour it is shed in water by degeneration of the basal cell and remains dormant throughout winter.
Germination of (oospore) spermocarp: - On favourable conditions the contents of the spermocarp again turn green. The zygote nucleus divides meiotically. The first division of the zygote is by a transverse wall. and the subsequent divisions are at right angles to each other. Thus 8-32 daughter cells are formed. Each cell metamorphoses into a single biflagellate zoospore. The zoospores are liberated by breaking of the spermocarp and the zygote walls breaks into two halves. They swim freely for a short time, then settle down and eventually develop into new thalli.
Alternation of generation: - A distinct alternation of generation are not found in Coleochate. The haploid thallus bears male and female gametes on erect branches which fuse to form diploid zygote. The zygote nucleus divides meiotically, forming 16-32 haploid zoospores. The zoospores, on germination, again give rise to haploid thllai. Thus diploid phase is represented only by zygote. such a life cycle where diploid phase is restricted only zygote is known as haplontic.
Q.16. Explain the affinities of Coleochaete. (2012)
Ans. Afinities of Coleochaete: - The photosynthetic pigments-chlorophyll and reserve food starch, non-jacketed unicellular sex organs and haplontic life cycle are typically chlorophycean features of Coleochaete. it shows resemblance with the members of the order Chaetophorales in its cell structure and heterotrichous thallus organisation. However, elaborate oogamous reproduction retention of the zygote on the haploid plant, presence of a protective layer of vegetative cells around the mature zygotes, elaborate post-fertilization events (development of spermocarp) are some of the marked advanced feature exhibited by Coleochaete.
The occurrence of a discoid thallus in some species of Coleochaete; e.g., C. scutata; of liverworts, such as riccis. Bower (1908) compared the spermocarp of Coleochaete to a simple or primitive sporophyte of early land plants. The more recent studies of marchant and Pickett-Heaps (1973), Graham and Mcbride (1979), Frederick et al. (1973), Graham and Wilcox (1983) have pointed out that Coleochaete possesses certain features like the presence of phragmoplast during cell division, presence of glycolate oxidase and peroxisomes enzymes, and parenchyma in some species, which indicates its affinities with early land plants might have originated from Coleochaete like ancestors.
There are remarkable analogies between the sexual reproduction of Coleochaete and the Nemalionales (e.g., batrachospermum) among the red algae. The structure of the oogonium of Coleochaete resembles the carpogonium of rhodophyceae. Furthermore, in coleochaete the oospore is retained within the oogonium and development of sheath around it forms a short of a fructification (spermocarp) which is analogous to the cystocarp of Batrachospermum. Pringsheim (1860) and Lambert (1910) reported that the swarmers (meiozoospores) of Coleochaete produce several generations of dwarf asexual plants before sexual plants are produced. A more or less similar type of life cycle is found in Batrachospermum. These similarities, however, do not throw any light on the phylogenetic relationship of Coleochaete and Rhodophyceae but simply indicate homoplasy.