Biology Class Eleven  



Plant Kingdom

NCERT Solution

Question – 1 - What is the basis of classification of algae?

Answer: The presence or absence of pigments is the main basis of classification of algae.

Chlorophyceae: Chlorophyll a and b are present in them and impart green colour. Chlorophyceae are also called ‘blue-green algae’.

Phaeophyceae: Chlorophyll a and c and fuxoxanthin are present. Fucoxanthin imparts brown colour. Phaeophyceae are also called ‘brown algae’.

Rhodophyceae: Chlorophyll a and d and phycoerythrin are present. Phycoerythrin imparts red colour. Rhodophyceae are also called ‘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: In case of liverwort, moss and fern; the saprophytic plant produces haploid spores after meiosis. In case of gymnosperm and angiosperm, meiosis takes place in antheridium and ovary; for the formation of pollen grains and ovules.

Question – 3 - Name three groups of plants that bear archegonia. Briefly describe the life cycle of any one of them.

Answer: Gymnosperms are heterosporous. Haploid microspores and megaspores are produced. Spores are produced within sporangia. Sporangia are borne on sporophylls which are arranged spirally along an axis. The Sporophylls form lax or compact strobili or cones.

Male Cone: The male strobili are called microsporangiate. Microspores develop into a male gametophytic generation which is highly reduced and is confined to only a limited number of cells. This small-sized gametophyte is called pollen grain.

Female Cone: The female strobili are called megasporangiate. The male or female cones can be found on the same tree (pinus) or on different trees (cycas). The megaspore mother cell is differentiated from one of the cells of the nucellus. Nucellus is protected by envelopes and the composite structure and is called an ovule. Meiotic division in the megaspore mother cell produces four megaspores. One of the megaspores develops into a multicellular female gametophyte. The female gametophyte bears two or more archegonia or female sex organs. The female gametophyte is retained within the megasporangium.

Fertilization: The pollen grains are released from the microsporangium and carried by air currents. They come in contact with the opening of the ovules on the female cone. A pollen tube develops in the pollen grain. The male gamete travels through the pollen tube to reach near the mouth of archegonia. After fertilization, zygote develops into an embryo. The ovule develops into seed.

Question – 4 - Mention the ploidy of the following:

  1. Protonemal cell of a moss;
    Answer: Haploid

  2. Primary endosperm nucleus in dicot,
    Answer: Triploid

  3. Leaf cell of a moss;
    Answer: Haploid

  4. Prothallus cell of a ferm;
    Answer: Haploid
     
  5. Gemma cell in Marchantia;
    Answer: Diploid
     
  6. Meristem cell of monocot,
    Answer: Diploid
     
  7. Ovum of a liverwort,
    Answer: Diploid
     
  8. Zygote of a fern.
    Answer: Zygote

Question – 5 - Write a note on economic importance of algae and gymnosperms.

Answer: Economic Importance of Algae:

  • Almost 50% of carbon fixation on earth is carried out by algae. Thus, they are among the large producers on the earth; especially in aquatic habitat. About 70 species of marine algae are used as food, e.g. Porpyra, Laminaria and Sargassum.
  • Some marine brown and red algae produce large amounts of hydrocolloids. Hydrocolloids are water holding substances and are used commercially.
  • Agar; which is obtained from Gelidium and Gracilaria is used as culture medium in laboratories. It is also used in making ice-cream and jelly.
  • Chlorella and Spirullaina are used as food supplements; even during space travel.

Economic Importance of Gymnosperms:

  • They are widely used as ornamental plants.
  • Fibres from conifer are used as paper pulp.
  • Conifers are widely used in reforestation projects.
  • Turpentine and rosin are made from conifer resin.
  • Commercially useful oils are produced from many conifers, e.g. juniper, pine, fir, etc.

Question – 6 - Both gymnosperms and angiosperms bear seeds, then why are they classified separately?

Answer: The seeds of gymnosperm are naked, while those of angiosperms are covered. Hence, they are kept in different groups.

Question – 7 - What is heterospory? Briefly comment on its significance. Give two examples.

Answer: Some pteriophytes are heterosporous, e.g. Selaginella and Salvinia. In heterospory, the megaspore germinates to produce female gametophyte, while the microspore germinates to produce the male gametophyte. This event is a precursor of the seed habit in higher plants.

Question – 8 - Explain briefly the following terms with suitable examples:-

(a) Protonema

Answer: A protonema forms the earliest stage of a bryophyte life cycle. It is a thread-like chain of cells. Protonema is formed just after the germination of spores. It subsequently develops into a leafy gametophyte.

(b) Antheridium

Answer: An antheridium is the male sex organ of a plant. It produces the male gametes. It is present in the gametophyte phase of lower plants, but is present in the sporophyte phase of higher plants.

(c) Archegonium

Answer: Archegonium is a structure which is present in the gametophyte phase of some plants. The archegonium produces the female gamete.

(d) Diplontic

Answer: In this case, the diploid sporophyte is the dominant photosynthetic phase of the plant. The gametophyte is represented by the single to few-celled haploid gametophyte. Gymnosperms and angiosperms show this pattern.

(e) Sporophyll

Answer: A leaf or cluste of leaves which bears sporangia is called sporophyll. Sporophylls can produce microspores or megaspores as the case may be.

(f) Isogamy

Answer: In this case, the gametes are similar in size, e.g. Spirogyra. The gametes can be flagellated or non-flagellated.


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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


   
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