SolveItClass 9 · NCERT

NCERT Solutions · Class 9 Science Reproduction: How Life Continues

24 questions · 22 still being checked

Revise, Reflect, Refine 11.7–11.8 (part 11 of 19)

  1. Exercise 11.7

    Why do vegetatively propagated plants tend to be more vulnerable to diseases than sexually reproduced plants?

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    Because vegetatively propagated plants are clones — genetically identical, so a disease able to attack one plant can attack every plant in the field.
    NCERT_Solution_Class9_Science_Ch11_RRR_Q11-7
    Vegetative propagation uses a single parent and mitosis, so there is no variation among the new plants; not one of them carries a different combination of characters that might resist the pathogen.
    Sexually reproduced plants come from the fusion of gametes formed by meiosis, so the chromosomes of two parents are shuffled into new combinations and the seedlings differ from one another.
    In such a mixed population, some individuals happen to be resistant and survive an outbreak, and the crop is not wiped out together.
    The chapter states the general principle: variation helps some individuals adapt better to changing environments, and is important for the survival of a species.
    This is also why tissue-cultured banana plantlets are deliberately raised from healthy shoot tips — an identical crop has to be started virus-free, because it has no built-in variation to fall back on.

  2. Exercise 11.8

    If all flowers in a type of plant were only capable of self-pollination, how would it affect the genetic diversity over several generations? Explain.

    Not cross-checked

    NCERT prints no numerical answer for this exercise, so this working has not been cross-checked against the book.

    Genetic diversity would fall steadily generation after generation, and the plants would become more and more alike.
    NCERT_Solution_Class9_Science_Ch11_RRR_Q11-8
    In self-pollination the pollen reaches the stigma of the same flower or another flower of the same plant, so both sets of chromosomes come from a single parent — no new genetic material ever enters the line.
    The only source of variation left is the reshuffling of that one plant's own chromosomes during meiosis. The same limited set of characters is recombined over and over, so genuinely new combinations soon stop appearing.
    Compare this with cross-pollination, where chromosomes from two different plants combine and every seed receives a unique mix — the mechanism the chapter shows with the three bead pairs, where just three pairs already give eight combinations.
    With little variation left, hardly any individual carries a character useful in a changed situation, so a new disease, pest, drought or temperature change could damage the entire population at once.
    Since variation is what lets some individuals adapt and what drives evolution over time, a strictly self-pollinating species would be much less able to survive a changing environment.