Question and Answer - Part - 1
Question 1. What is the basis of classification of algae?
Answer: The main basis of classification of algae has been done on the basis of presence or absence of pigments, which impart an algae its colour. Chlorophyceae contains chlorophyll a and b, giving it the green colour and the name ‘blue-green algae’. Phaeophyceae contains chlorophyll a and c and fucoxanthin. The fucoxanthin gives it the brown colour and hence the name ‘brown algae’. Rhodophyceae contains chlorophyll a and d and phycoerythrin. The phycoerythrin gives the disitinct red colour and hence the name ‘red algae’.
Question 2. When and where does reduction division take place in the life cycle of a liverwort, a moss, a fern, a gymnosperm and an angiosperm?
Answer:- Reduction division or meiosis takes place in gametic cells. The role of meiosis is to halve the number of chromosomes so that once fertilization takes place the embryo thus formed will be having similar number of chromosomes as in parent somatic cells. In plants sometimes the dominant phase will contain all of diploid stage or all of haploid stage. There can be a brief interspersal of other stage. This is known as the alteration of generation.
1. Sporophytic generation is represented only by the one-celled zygote. There are no free-living sporophytes. Meiosis in the zygote results in the formation of haploid spores. The haploid spores divide mitotically and form the gametophyte. The dominant, photosynthetic phase in such plants is the free-living gametophyte. This kind of life cycle is termed as haplontic. e.g., algae
2. On the other extreme, is the type wherein the diploid sporophyte is the dominant, photosynthetic, independent phase of the plant. The gametophytic phase is represented by the single to few-celled haploid gametophyte. This kind of lifecycle is termed as diplontic. All seed-bearing plants i.e. gymnosperms and angiosperms, follow this pattern. Meiosis results in formation of male and female gametes.
3. Bryophytes and pteridophytes, interestingly, exhibit an intermediate condition (Haplo-diplontic); both phases are multicellular and often free-living. However, they differ in their dominant phases. The meiosis takes place in the sporophytes to produce haploid spores.
Question 3. Name three groups of plants that bear archegonia. Briefly describe the life cycle of any one of them.
Answer:- Bryophytes, Pteridophytes and Gymnosperms bear distinct archegonia. In angiosperms the archegonia is reduced to just a few cells so the term is not used in case of angiosperms.
Life Cycle of Gymnosperms:
Reproduction:- The gymnosperms are heterosporous; they produce haploid microspores and megaspores. The two kinds of spores are produced within sporangia that are borne on sporophylls which are arranged spirally along an axis to form lax or compact strobili or cones.
Male Gamete:- The strobili bearing microsporophylls and microsporangia are called microsporangiate or male strobili. The microspores develop into a male gametophytic generation which is highly reduced and is confined to only a limited number of cells. This reduced gametophyte is called a pollen grain. The development of pollen grains takes place within the microsporangia.
Female Gamete:- The cones bearing megasporophylls with ovules or megasporangia are called macrosporangiate or female strobili. The male or female cones or strobili may be borne on the same tree (Pinus) or on different trees (Cycas). The megaspore mother cell is differentiated from one of the cells of the nucellus. The nucellus is protected by envelopes and the composite structure is called an ovule. The ovules are borne on megasporophylls which may be clustered to form the female cones. The megaspore mother cell divides meiotically to form four megaspores. One of the megaspores enclosed within the megasporangium (nucellus) develops into a multicellular female gametophyte that bears two or more archegonia or female sex organs. The multicellular female gametophyte is also retained within megasporangium.
Fertilization: The pollen grain is released from the microsporangium. They are carried in air currents and come in contact with the opening of the ovules borne on megasporophylls. The pollen tube carrying the male gametes grows towards archegonia in the ovules and discharge their contents near the mouth of the archegonia. Following fertilisation, zygote develops into an embryo and the ovules into seeds.
Class eleven science: Plant Kingdom: Classification - Alga
Class eleven science: Plant Kingdom: Bryophytes
Class eleven science: Plant Kingdom: Pteridophytes and Gymnosperms
Class eleven science: Plant Kingdom: Angiosperms
Class eleven science: Plant Kingdom: Question and Answer-part-2