Plant Reproduction and Development

1. The angiosperm life cycle

Angiosperms show alternation of generations, but the sporophyte (2n) is dominant and the gametophytes are tiny and dependent. Inside the anther, meiosis produces microspores that develop into pollen grains — the male gametophytes. Inside the ovule, meiosis produces a megaspore that develops into the embryo sac — the female gametophyte, which contains the egg. Fertilization restores the diploid sporophyte, completing the cycle.

2. Flower structure

A typical flower has four whorls of modified leaves. The sepals (calyx) protect the developing bud; the petals (corolla) attract pollinators. The male stamen consists of a filament topped by an anther, which produces pollen. The female carpel (or pistil) consists of a sticky stigma, a style, and an ovary containing one or more ovules, each holding an egg.

3. Pollination and double fertilization

Pollination is the transfer of pollen from an anther to a stigma, by wind, water, or animals; it is not the same as fertilization. Once a pollen grain lands, it grows a pollen tube down the style toward the ovule. In double fertilization, two sperm are delivered: one fuses with the egg to form the diploid zygote (future embryo), and the second fuses with the two polar nuclei of the embryo sac to form the triploid endosperm, which will feed the embryo.

4. Seed and fruit development

After fertilization, the zygote develops into an embryo with one or two cotyledons (seed leaves) and root and shoot meristems. The integuments of the ovule harden into the seed coat, and the ovule as a whole becomes the seed. Meanwhile, the ovary wall thickens into the fruit, which protects the seeds and aids dispersal by wind, water, or animals. A fruit is therefore a mature ovary — not simply a "sweet snack."

5. Germination and asexual reproduction

A seed stays dormant until conditions (water, oxygen, temperature, and sometimes light) are right. Germination begins with imbibition, the uptake of water that swells the seed and breaks dormancy; the embryonic root (radicle) emerges first, followed by the shoot. Many plants also reproduce asexually through vegetative structures — runners (strawberries), rhizomes, tubers (potatoes), and bulbs — producing clones genetically identical to the parent, which is why agriculture can propagate desirable varieties without seeds.

How it works

From flower to seed to seedling:

1. A pollen grain lands on a compatible stigma (pollination) and germinates a pollen tube.
2. The tube grows down the style and delivers two sperm cells to the embryo sac in the ovule.
3. One sperm fuses with the egg → diploid zygote; the other fuses with two polar nuclei → triploid endosperm (double fertilization).
4. The zygote divides into an embryo with cotyledons and root and shoot meristems; the integuments form the seed coat.
5. The ovary develops into fruit, which later disperses the mature seeds.
6. When conditions are right, the seed imbibes water and germinates: the radicle emerges, then the shoot, forming a new sporophyte.

Common confusions

Memory aids

For the flower parts, remember "Stamen = St-a-men (male); Carpel = Carries the eggs (female)". For the chain of events, use "P → F → S → F": Pollination → Fertilization → Seed (from ovule) → Fruit (from ovary), and recall double fertilization as "one sperm for the baby, one for the baby food."

Quick review

Topic Recap

• Angiosperms alternate between a dominant sporophyte and tiny gametophytes (pollen grain and embryo sac).
• A flower's sepals, petals, stamens, and carpels carry out protection, attraction, and reproduction.
• Pollination precedes double fertilization, which makes both zygote and endosperm.
• The ovule becomes the seed and the ovary becomes the fruit.
• Germination resumes growth after dormancy, and many plants also reproduce asexually as clones.

Knowledge Check

1. What is the difference between pollination and fertilization?
2. What two structures does double fertilization produce?
3. Which floral structure develops into the seed, and which into the fruit?
4. What are the three main parts of a seed?
5. What is imbibition, and what happens first when a seed germinates?

Answers and Rationales

1. Answer: Pollination is the transfer of pollen to the stigma; fertilization is the fusion of sperm and egg. Why: Delivery of pollen precedes and enables fusion of gametes.
2. Answer: A diploid zygote and triploid endosperm. Why: One sperm fertilizes the egg; the second fuses with two polar nuclei.
3. Answer: The ovule develops into the seed; the ovary develops into the fruit. Why: Each structure matures after fertilization to protect and disperse the next generation.
4. Answer: The embryo (with cotyledons), stored food (endosperm and/or cotyledons), and the seed coat. Why: These protect and nourish the embryo during dormancy.
5. Answer: Imbibition is water uptake that swells the seed and breaks dormancy; the embryonic root (radicle) emerges first. Why: Water uptake triggers metabolism, and anchoring the root comes before shoot growth.