What this chapter is about
This chapter explores how angiosperms (flowering plants) reproduce sexually. Sexual reproduction involves the formation of male and female gametes, their fusion to form a zygote, and the subsequent development of seeds and fruits. Understanding this process is essential because flowering plants dominate Earth's vegetation and are the primary source of food, fibre, and medicine for humans.
A Class 12 student studies this topic to understand the structural and functional aspects of flowers as reproductive organs, the processes of pollination and fertilisation, and the development of seeds and fruits. This knowledge connects plant biology to agriculture, horticulture, and plant breeding programmes that are vital for food security.
After studying this chapter, you should be able to describe flower structure, explain microsporogenesis and megasporogenesis, distinguish between types of pollination, describe double fertilisation, and understand seed and fruit development.
Key ideas
- The flower is the reproductive unit of angiosperms, containing male parts (stamens producing pollen) and female parts (pistil containing ovules).
- Microsporogenesis is the formation of microspores (which develop into pollen grains) inside the anther through meiosis; megasporogenesis is the formation of megaspores inside the ovule, one of which develops into the embryo sac.
- Pollination is the transfer of pollen grains from the anther to the stigma; it may be self-pollination (same flower or same plant) or cross-pollination (different plant of the same species).
- Double fertilisation is unique to angiosperms: one male gamete fuses with the egg cell to form a diploid zygote (2n), while the second male gamete fuses with the two polar nuclei to form a triploid primary endosperm nucleus (3n).
- The ovule develops into the seed after fertilisation; the ovary develops into the fruit, and the ovary wall becomes the fruit wall (pericarp).
- Outbreeding devices such as self-incompatibility and dioecy promote cross-pollination and genetic variation.
- Apomixis is seed formation without fertilisation, producing offspring genetically identical to the parent; it is significant in agriculture for maintaining hybrid vigour.
Formulas and facts to remember
1. Microspore mother cell (2n) → 4 microspores (n) by meiosis inside each microsporangium of the anther.
2. Megaspore mother cell (2n) → 4 megaspores (n) by meiosis inside the nucellus of the ovule; usually only one megaspore is functional.
3. Mature embryo sac (female gametophyte) is 7-celled and 8-nucleate: 1 egg cell, 2 synergids, 3 antipodal cells, and 1 central cell with 2 polar nuclei.
4. Pollen grain (male gametophyte) at the time of shedding is usually 2-celled (vegetative cell and generative cell) or sometimes 3-celled (generative cell already divided into two male gametes).
5. Double fertilisation: Syngamy (male gamete + egg → zygote, 2n) and triple fusion (male gamete + 2 polar nuclei → primary endosperm nucleus, 3n) occur together.
6. Parts of a typical seed: seed coat (from integuments), cotyledon(s), embryo axis with plumule, hypocotyl, and radicle; endosperm present in endospermic seeds.
7. Agents of pollination: abiotic (wind — anemophily, water — hydrophily) and biotic (insects — entomophily, birds — ornithophily, bats — chiropterophily).
8. Self-incompatibility: a genetic mechanism preventing self-pollen from germinating or fertilising the ovule, promoting outbreeding.
Worked examples
### Example 1: Calculating ploidy after double fertilisation
Question: In a flowering plant, the somatic chromosome number is 24. What is the ploidy of (a) the egg cell, (b) the zygote, and (c) the primary endosperm nucleus after fertilisation?
Solution:
- Somatic cells are diploid (2n = 24), so n = 12.
- (a) The egg cell is formed after meiosis and mitotic divisions; it is haploid, so it has n = 12 chromosomes.
- (b) The zygote forms by fusion of one male gamete (n = 12) with the egg (n = 12). Ploidy = 2n = 24 chromosomes.
- (c) The primary endosperm nucleus forms by fusion of one male gamete (n = 12) with two polar nuclei (each n = 12). Ploidy = n + n + n = 3n = 36 chromosomes.
### Example 2: Identifying pollination type from flower features
Question: A plant species has small, inconspicuous flowers with no fragrance, large feathery stigmas, and abundant light pollen. What type of pollination is likely, and why?
Solution:
- Small, dull flowers with no scent suggest the plant does not attract insects or other animals.
- Feathery stigmas increase surface area to catch airborne pollen.
- Light, abundant pollen can be carried easily by air currents.
- These are adaptations for wind pollination (anemophily). Examples include grasses and maize.
### Example 3: Structure of the embryo sac
Question: Draw a labelled diagram showing the arrangement of cells in a mature embryo sac and state the function of synergids.
Solution (description for diagram):
- At the micropylar end: one egg cell flanked by two synergids (all three together form the egg apparatus).
- At the chalazal end: three antipodal cells.
- In the centre: a large central cell containing two polar nuclei.
- Total: 7 cells with 8 nuclei.
Function of synergids: They guide the pollen tube towards the egg cell. One synergid degenerates, and its filiform apparatus secretes chemicals that attract the pollen tube.
Common mistakes
- Thinking pollination and fertilisation are the same → Pollination is pollen transfer to stigma; fertilisation is gamete fusion inside the ovule, occurring later after pollen tube growth.
- Believing endosperm is always diploid like the zygote → Endosperm is triploid (3n) because it forms from triple fusion.
- Confusing the embryo sac with the ovule → The embryo sac is the female gametophyte located inside the ovule; the ovule also includes integuments and nucellus.
- Assuming all seeds have endosperm → In non-endospermic (exalbuminous) seeds like pea and groundnut, the endosperm is consumed during seed development and food is stored in cotyledons.
- Stating that self-pollination always leads to fertilisation → Self-incompatibility mechanisms can prevent fertilisation even after self-pollination.
Quick revision
- Flower = reproductive unit; stamen produces pollen, pistil contains ovules.
- Microsporogenesis (anther) and megasporogenesis (ovule) both involve meiosis.
- Embryo sac: 7-celled, 8-nucleate; egg apparatus at micropylar end.
- Double fertilisation: syngamy (zygote, 2n) + triple fusion (endosperm, 3n).
- Ovule → seed; ovary → fruit.
- Apomixis = asexual seed production; useful for preserving hybrid traits.