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(i) Explain the… (Class 12 Biology SQP 2024-25, Q31B)

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BiologyCBSE Class 12 · Sample paper, session 2024-25 · 5 marks · Long answer

CBSE Class 12 Biology sample paper 2024-25: question 31(B)

From the official CBSE sample question paper for 2024-25, section E, question 31(B), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q31(B) · 5 marks · Long answerCompetency-basedSexual Reproduction in Flowering Plants

(i) Explain the significance of each of the following features present in plants given below:
a) In rose-bay plant the stamens ripen before the stigma.
b) In certain species of primrose, the flowers have short stamen and long style.
c) The bisexual flower of mustard exhibits rejection of self-pollen grain.
(ii) Explain how autogamy is prevented in castor and papaya plant respectively.

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2024-25.

Hints

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

Hint 1

These are all outbreeding devices.

Hint 2

Think about timing, position and genetic recognition of pollen.

Hint 3

Castor and papaya differ in whether male and female flowers share a plant.

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)(a) In rose-bay the stamens and stigma mature at different times (dichogamy – protandry). When the pollen is released, the stigma is not yet receptive, so the pollen cannot germinate on the same flower's stigma. This prevents autogamy.

    1 mark

  2. (i)(b) In primrose the anthers and stigma are placed at different positions (short stamens, long style – heterostyly). Pollen cannot easily reach the stigma of the same flower, so self-pollination (autogamy) is prevented.

    1 mark

  3. (i)(c) Mustard shows self-incompatibility, a genetic mechanism of pollen–pistil interaction. The pistil recognises self-pollen and prevents it from germinating or forming a pollen tube, so self-fertilisation is prevented.

    1 mark

  4. (ii) Castor is monoecious: male and female flowers are on the same plant but are separate (unisexual), so autogamy is prevented, though geitonogamy can still occur.

    1 mark

  5. (ii) Papaya is dioecious: male and female flowers are on different plants, which prevents both autogamy and geitonogamy.

    1 mark

Answer: (i)(a) Dichogamy prevents autogamy; (b) different positions of anther and stigma (heterostyly) prevent self-pollination; (c) self-incompatibility rejects self-pollen. (ii) Castor is monoecious (unisexual flowers); papaya is dioecious (male and female flowers on different plants).

Where the marks are

  • Explains that different maturation times prevent autogamy in rose-bay (1 mark)
  • Explains that different positions of anther and stigma prevent autogamy in primrose (1 mark)
  • Explains self-incompatibility as a genetic mechanism preventing pollen tube growth (1 mark)
  • Castor: monoecious with unisexual flowers (1 mark)
  • Papaya: dioecious (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 paperSample paper 2024-25: 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.