Q. 10. Describe the life cycle patterns in algae.
Related Question -
Q. Illustrate different types of life cycle in algae studied by you. (2004)
Q. Describe different life cycle pattern in algae. (2002)
Ans. Life cycle pattern in algae: -
In algae, as in thallophyta, there is no regular and fixed alternation of generation comparable with the higher plants. In most of the simpler forms, viz. Blue green algae, Protococcus, Scenedesmus, etc. reproduction is asexual and there is no alternation of generations.
In the case of the simple unicellular algae like Chlamydomonas, Sphaerella, etc. which reproduce sexually, there is no distinct recognisable sporophytic generation, though the zygospore or the oospore, which is formed by the fusion of the gametes, is regarded by some people as a rudimentary development of sporophytic generation, because it is diploid and during germination undergoes reduction, producing either haploid zoospores or germinating directly to produce the haloid plant (gametophyte) bearing reproductive organs. The life-cycle of these primitive algae consists, therefore, of an alternation of one-celled haploid phase with a one-celled diploid phase. Since in these forms haploid phase is predominant and diploid one rudimentary, this type of alternation of generations is designated as haploid predominant type. This is also appeared in filamentous algae. e.g., Ulothrix, Spirogyra, Oedogonium, Vaucheria, etc. In which the haploid phase in filamentous showing vegetative cell division and alternates with a unicellular diploid phase the zygospore or oospore. Such haloid filamentous plants or the haplonts reproduce asexually by zoospores or aplanospore.
In Bryopsis and Codium, there is alternation of one-celled haploid phase with diploid coenocyte, the morphological equivalent of multicellular diploid generation or diplont. Certain Ulvaceae and Cladophoraceae are, however,described as diplohaplonts, becaus in their life cycle, there is an alternation of many-celled haploid generation with a many celled generation. In Cladophoraceae and a Ulvaceae the meiosis which is usually characteristic of the zygotes delayed and the nuclei in the cells of the diploid plants usually do and divide reductionally until the plant is fully mature, though sometime may take place while the diploid plant is still juvenile. Meiosis may take place in the nuclei of all the cells of diploid plant, e.g., Ulva or may be restricted to the actively growing terminal cells at the branch apices as in Cladophora. In one species of cladophora, e.g., C. glomerata, reduction is reported to occur in the nuclear division immediately preceeding gamete-formation; the gametes then are the only cells with in chromosomes, all rest of the life-cycle having 2n.
In Fucus, Sargassum, etc. Chromosome reduction occurs during the gamete formation in the “antheridia” and “oogonia” the only haploid stage being the sperms and eggs. The sperms and eggs of fusion form zygote, which develops into a new diploid plant again. Fucus has, therefore, reached a condition in which the haploid or gametophytic structure are rudimentary and reduced to a few nuclei. Since diploid stage is prominant in these plants, they show diploid predominant type of alternation of generations.
In Dictyota, one of the brown Algae, there is a distinct alternation of a flat ribbon-like sexual plant with on identical asexual plant. The two generations are morphologically idential but differ in physiology and chromosomes numbers. The sexual plants with haploid number of chromosomes bear antheridia or oogonia, while the asexual plant, which is produced from the oospore is diploid and shows reduction division at the first division of the spore mother-cell during the tetraspore formation. The tetraspores, which are haploid, on germination produce the haploid sexual plants. Bearing antheridia or oogonia. Since there are only two phases, i.e., diploid in the life-cycle, it is called biphasic. The two phases being morphologically identical this type of alternation of generations is called or homologous. In Ectocarpus also, alternation of generations is isomorphic or homologous.
In other brown algae, the sporophyte of Laminaria is a large complex plant and the gametophytes (male and female) are small and simple. This types of life-cycle also is biphasic but the type of alternation of generation is heteromorphic or heterologous as the two phases are morophologically different.
In the red algae, there is a great variation. Although the female sex organs and sexual reproduction are more complicated, alternation of generation in some of the lower genera, such as Batrachspermum and Nemalion is approximately on the level of the green algae where the plant-body is characteristically in the haploid condition and, therefore, a gametophyte. The zygote formed as a result of the fusion of egg and sperm is the only diploid structure having 2n chromosomes. The fruit body or the cystocarp produced from the oospore. Though appearantly sporophytic, is haploid because during its formation the zygote nucleus undergoes reduction division. Thus in the lower red algae, while there is a definite alternation of chromosomes numbers from haploid to diploid and back to haploid, there is no corresponding alternation of plants namely, gametophytes and sporphytes with different functions. This type of life-cycle is triphasic as it involves three phases, namely Batrachospermum plant, cytoscarp and zygote. The first two are haploid while third one is diploid. Thus there is an alternation of two gametophytic (haploid) generations with one sporophytic (diploid) generation. This type of life cycle is also called haploidiontic.
In the higher red algae, polysiphonia, there is a distinct alternation of generations. The plants have not only different chromosome numbers but are different in functions as well. There are definite sexual plants bearing male and female sex organs and asexual plants bearing tetraspores. The spores on germination produce sexual plants. On fertilization, the number of chromosomes is doubled by the fusion of gametes. The diploid zygote on germination produces the cytocarp, an asexual phase bearing carposporangia, which produce diploid carpospores. The carpospores produce tetrasporic plant bearing terasporangia. During tetraspore formation, however, reduction division takes place so that the tetraspores are haploid. The tetraspores on germination produce sexual plants bearing male or female reproductive organs. This type of life-cycle is also triphasic but is called diploid or sporophytic generations.
Q.11. Describe foliaceous forms of thallus with example.
Ans. Foliceous forms or parenchymatous thallus: - The plant body occurs in the form of multicellular, flat, leaf like or plate like morphology. It develops a parenchymtous thallus in which the cell divide in more than one plane. Examples - UIva (Chlorophyta), Saragassum, Laminaia (Phaeophyta), Prophyra (Rhodophyta), etc.
Q.12. Write a short note on ‘Heterotrichous filament’
Ans. Heterotrichous filament: -
Heterotrichous filamentous are the characteristic of the higher class of algae. The thallus of these algae consists of two types of branches, which are situated at the right angle of each other. These branches are called heterotrichous filaments. The filaments are prostrate and erect type.They are Polysiphonia, Ectocarpus, Stigeoclonium etc.