Sexual reproduction in angiosperms (flowering plants) involves the fusion of male and female gametes to produce seeds. The flower is the site of sexual reproduction, and understanding its structure is essential to grasping how plants reproduce.
Structure of a Flower
A typical flower has four whorls: calyx (sepals), corolla (petals), androecium (stamens — male), and gynoecium (pistil/carpels — female). The stamen consists of a filament and an anther where pollen is produced. The pistil has a stigma, style, and ovary containing one or more ovules.
Microsporogenesis and Pollen Development
Pollen grains (male gametophytes) are formed inside the anther through microsporogenesis. Diploid microspore mother cells (2n) undergo meiosis to produce four haploid microspores. Each microspore matures into a pollen grain with two layers: an outer exine (sporopollenin — very resistant) and an inner intine (cellulose). A mature pollen grain contains a vegetative cell and a generative cell (which divides to form two male gametes).
Megasporogenesis and Female Gametophyte
Inside the ovule, a megaspore mother cell (2n) undergoes meiosis to produce four haploid megaspores. Typically, three degenerate and one functional megaspore develops into the female gametophyte (embryo sac) through three rounds of mitosis, producing 8 nuclei arranged as: 3 antipodal cells, 2 synergids, 1 egg cell, and 2 polar nuclei (central cell).
Pollination
- Pollination is the transfer of pollen from anther to stigma. Types:
- Autogamy: self-pollination within the same flower
- Geitonogamy: transfer between flowers on the same plant (genetically self-pollination)
- Xenogamy: cross-pollination between different plants (promotes genetic variation)
Agents: wind (anemophily), water (hydrophily), insects (entomophily), birds (ornithophily), bats (chiropterophily).
Double Fertilisation
- 1.A unique feature of angiosperms is double fertilisation:
- 2.One male gamete fuses with the egg cell → diploid zygote (2n) → embryo
- 3.Second male gamete fuses with two polar nuclei → triploid primary endosperm nucleus (3n) → endosperm (nutritive tissue)
If the polar nuclei are each haploid (n), and the male gamete is also haploid (n), the endosperm ploidy = n + n + n = 3n (triploid). This is why endosperm is triploid in most angiosperms.
A plant has 24 chromosomes (2n = 24). Its pollen grain vegetative cell contains 12 chromosomes. The endosperm formed after double fertilisation contains 12 + 12 + 12 = 36 chromosomes (3n = 36).
Apomixis in dandelion — seeds form without fertilisation, producing plants genetically identical to the mother. This is an advantage for clonal propagation.
In water hyacinth (Eichhornia), pollination occurs above water surface (hydrophily above water), while in Vallisneria, the male flower detaches and floats to the female flower.
Cleistogamous flowers (e.g., Viola — pansy) never open and always self-pollinate, ensuring seed production even without pollinators.
Emasculation in plant breeding — removal of anthers from bisexual flowers before pollen maturity to prevent self-pollination, followed by artificial cross-pollination.
Polyembryony (e.g., citrus) — more than one embryo in a seed, arising from nucellar tissue or multiple fertilisations, producing clonal offspring.
Post-Fertilisation Events
The zygote develops into an embryo. The ovule becomes a seed, and the ovary wall (pericarp) develops into the fruit. Fruits formed without fertilisation are called parthenocarpic fruits (e.g., banana). Seeds may be albuminous (with endosperm: wheat, maize) or non-albuminous (endosperm consumed by cotyledons: pea, beans).
Common mistakes
- Students confuse pollination (pollen transfer) with fertilisation (gamete fusion) — they are separate events.
- The endosperm is triploid (3n), not diploid — caused by triple fusion.
- Geitonogamy appears to be cross-pollination but is genetically self-pollination.
- The exine of pollen is made of sporopollenin, not cellulose.
Summary
Flowering plant reproduction involves microsporogenesis and megasporogenesis forming male and female gametophytes. Pollination transfers pollen to stigma; double fertilisation produces a diploid embryo and triploid endosperm. The ovule becomes a seed and the ovary a fruit.