CBSE Class 12 Biology – Sexual Reproduction in Flowering Plants
NCERT-aligned chapter resource | Biology Code 044
1. Flower as the Reproductive Organ
A typical flower contains four whorls: calyx, corolla, androecium and gynoecium. The androecium produces pollen, while the gynoecium contains the ovary with ovules. The ovule contains the embryo sac, where female gametophyte development occurs.
2. Male Gametophyte: Microsporogenesis
Inside the anther, sporogenous tissue gives rise to microspore mother cells. Meiosis produces haploid microspores, which develop into pollen grains. A mature pollen grain commonly has a vegetative cell and a generative cell; the generative cell ultimately forms two male gametes.
3. Female Gametophyte: Megasporogenesis
A megaspore mother cell in the ovule undergoes meiosis to form four megaspores. Usually one remains functional and develops into the embryo sac. The mature embryo sac is typically seven-celled and eight-nucleate: egg apparatus, three antipodals and the central cell containing two polar nuclei.
4. Pollination
Pollination is transfer of pollen from anther to stigma. It may be autogamy, geitonogamy or xenogamy. Agents include wind, water and animals. Outbreeding devices such as dichogamy, herkogamy, self-incompatibility and unisexuality reduce self-pollination.
5. Pollen–Pistil Interaction
The stigma recognises compatible pollen. A compatible pollen grain germinates and the pollen tube grows through the style towards the ovule. The two male gametes enter the embryo sac.
6. Double Fertilisation
Male gamete + two polar nuclei → primary endosperm nucleus → endosperm
Syngamy forms the diploid zygote. Triple fusion forms the triploid primary endosperm nucleus. The occurrence of both events in the same embryo sac is double fertilisation, a defining feature of angiosperms.
7. Post-Fertilisation Changes
The zygote develops into the embryo; the primary endosperm nucleus develops into endosperm; ovule becomes seed and ovary becomes fruit. The integuments form the seed coat.
8. Apomixis, Parthenocarpy and Polyembryony
Apomixis is seed formation without normal fertilisation. Parthenocarpy produces fruits without fertilisation, often resulting in seedless fruits. Polyembryony refers to formation of more than one embryo in a seed.
NCERT Diagram Practice
- Longitudinal section of anther
- Mature embryo sac
- Path of pollen tube
- Double fertilisation
- Dicot embryo
Chapter-Specific MCQs
- The two male gametes participate in: (A) two synergids (B) syngamy and triple fusion (C) two pollen tubes (D) two ovaries. Answer: B.
- The functional megaspore develops into the: (A) pollen grain (B) embryo sac (C) endosperm (D) seed coat. Answer: B.
- Which process can produce seedless fruit? (A) Parthenocarpy (B) Syngamy (C) Microsporogenesis (D) Xenogamy. Answer: A.
Competency Questions
- Why is geitonogamy genetically similar to autogamy even though pollen transfer occurs between two flowers?
- Predict the immediate product if triple fusion fails but syngamy occurs.
- Explain how self-incompatibility promotes outbreeding.