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Study the diagram given… (Class 12 Biology 2023, Q33a)

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BiologyCBSE Class 12 · Board exam 2023, set 57/1/1 · 5 marks · Long answer

CBSE Class 12 Biology 2023: question 33(a)

Set in the CBSE Class 12 board exam of 2023, paper 57/1/1, section E, question 33(a), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q33(a) · 5 marks · Long answerCompetency-basedSexual Reproduction in Flowering Plants

Study the diagram given below showing the modes of pollination. Answer the questions that follow.
(i) The given diagram shows three methods of pollen transfer in plants. What are the technical terms used for pollen transfer methods '1', '2' and '3' ?
(ii) How do the following plants achieve pollination successfully ?
(1) Water lily
(2) Vallisneria
(iii) Flowering plants have developed many devices to avoid inbreeding depression. Explain one hereditary and one physiological device which helps plants to achieve this target.

Figure in the paper: Three flowers: arrow 1 shows pollen transfer within the same flower (anther to its own stigma); arrow 2 shows transfer between two flowers on the same plant; arrow 3 shows transfer from a flower of a different plant to a flower of another plant. See it in the official paper.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2023 board paper, set 57/1/1.

Hints

Open one hint at a time and try again before the next.

Hint 1

The three terms differ by whether the pollen comes from the same flower, the same plant or a different plant.

Hint 2

Water lily flowers rise above the water, but Vallisneria flowers float on the surface. Think about which agent carries the pollen in each case.

Hint 3

One device is controlled by genes and acts at the pistil. The other depends on timing, that is, when the anther and the stigma mature.

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

  1. (i) Method 1: transfer of pollen from the anther to the stigma of the same flower is autogamy.

    ½ mark

  2. Method 2: transfer of pollen between different flowers of the same plant is geitonogamy.

    ½ mark

  3. Method 3: transfer of pollen from a flower of one plant to the stigma of a flower on a different plant is xenogamy.

    ½ mark

  4. (ii)(1) Water lily: its flowers emerge above the water surface. They are therefore pollinated by insects or wind, as in most land plants, and not by water.

    ½ mark

  5. (ii)(2) Vallisneria: the female flowers reach the water surface on long stalks. The male flowers or pollen grains are released onto the water surface and are carried passively by water currents. Some of them reach the female flowers and their stigmas, which brings about pollination.

    1 mark

  6. (iii) Hereditary (genetic) device – self-incompatibility: this is a genetic mechanism that stops pollen from the same flower, or from other flowers of the same plant, from fertilising the ovules. It works by inhibiting pollen germination or pollen-tube growth in the pistil.

    1 mark

  7. Physiological device – non-synchronised maturation (dichogamy): pollen release and stigma receptivity do not happen at the same time. Either the pollen matures before the stigma becomes receptive, or the stigma becomes receptive much before pollen is released. This prevents self-pollination.

    1 mark

Answer: (i) 1 – Autogamy, 2 – Geitonogamy, 3 – Xenogamy. (ii) Water lily – by insects/wind; Vallisneria – female flowers reach the surface on long stalks and water currents carry the released male flowers/pollen to them. (iii) Hereditary: self-incompatibility; Physiological: pollen release and stigma receptivity not synchronised.

Where the marks are

  • Names autogamy, geitonogamy and xenogamy correctly (½ × 3 = 1½ marks)
  • Water lily pollinated by insects/wind (½ mark)
  • Vallisneria: female flowers on long stalks at the surface; pollen/male flowers carried by water currents (1 mark)
  • Self-incompatibility explained as inhibition of pollen germination/pollen-tube growth (1 mark)
  • Physiological device: non-synchrony of pollen release and stigma receptivity, explained (1 mark)

Revise the chapter

Practise this chapterBoard-style questions from Sexual Reproduction in Flowering Plants, one at a time, marked against the answer key and marking points.Back to the full paper2023 board paper, set 57/1/1: every question, by section.

The question is transcribed from the official paper; figures are described in words. The solution, hints and chapter tag are ours. Spotted an error? Write tosupport@questionbuddy.in.