12th Class Biology Sample Paper Biology - Sample Paper-15

  • question_answer
    (i) Embryo sacs of some apomictic species appear normal but contain diploid cells. Suggest a suitable explanation for the condition.
    (ii) Why is the presence of both chasmogamous and cleistogamous flowers on the same plant beneficial? Explain with examples.
    In plants vegetative propagation occurs naturally also. Explain with the help of examples. Give its significances also.


    (i) Apomixis It is a form of asexual reproduction that mimics sexual reproduction, but produce seeds without fertilisation. It does not involve formation of zygote through the gametic fusion. It occurs in some species of Asteraceae and grasses.
    Apomictic seeds are viable seeds and are produced without pollination or sexual reproduction. These are produced from segments of fruit (mango stem), male gemetic content of pollen (Cyperus) and other vegetative parts.
    In some plant species it is common, e.g. Asteraceae, Poaceae. In some species, the diploid egg cell is formed without reduction division and develops into embryo without fertilisation. It is an asexual reproduction in the absence of pollinators which takes place in extreme environments.
    In some species like Citrus, some of the nuclear cells surrounding the embryo sac start dividing and develop into embryo. It occurs in the megaspore mother cell that does not undergo meiosis, thus produces diploid embryo sac through mitotic divisions.
    (ii) Viola and Commelina can produce both chasmogamous and cleistogamous flowers on the same plant. These flowers produce seeds by out-crossing and selfing, respectively and the mixed breeding system is considered a successful reproductive strategy.
    Vegetative propagation is the formation of new plants form vegetative units or propagules such as buds, tubers rhizomes, etc. It is of two types, i.e. natural and horticultural (artificial).
    Natural Methods of Vegetative Propagation
    In thesemethods, vegetative propagules (somatic parts) of the plant detach fromthe body of themother plant and develop into new plants under suitable conditions. It occurs by following means
    (i) Roots Both tap roots and adventitious roots take part in vegetative propagation. Tap roots of some plants develop adventitious buds from new plants, e.g. Dalbergia (sheesham).
    (ii) Underground stems Different types of underground stem structures can take part in vegetative propagation, e.g. tubers (potato), bulbs (onion), corms (Colocasia), rhizomes (ginger), suckers (mint).
    (iii) Subaerial or Creeping stems They are of three types-runners, stolons, offsets.
    (a) Runners, e.g. lawn grass, Oxalis,
    (b) Stolons, e.g. strawberry.
    (c) Offsets, e.g. Pistia, Eichhornia (water hyacinth).
    (iv) Aerial stems (Aerial shoots) Fleshy phylloclades occur in Opuntia and some other plants. Each segment of such stems can form a new plant. Sugarcane is propagated by planting segments of stems having at least one node.
    (v) Leaves of many plants have adventitious buds and help in vegetative propagation, e.g. Begonia, Bryophyllum, Kalanchoe, Streptocarpus, Saintpaulia, Adiantum caudatum.
    Significance of vegetative reproduction These are as follows
    · It is the only method of propagation in seedless plants, e.g. banana.
    · There is rapid multiplication.
    · Plants thus produced show genetic uniformity.
    · The plant can bemultiplied indefinitely without any change or variation.

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