10 min readPathway sheetPart III (4 marks) on pollination; Part IV (7 marks) on double fertilisation or the L.S. of a flower

Sexual reproduction in flowering plants

A flower carries male stamens and female pistils. After pollination, double fertilisation takes place in the ovule; the ovule becomes the seed and the ovary becomes the fruit.

PollinationTransfer of pollen grains from the anther to the stigma

Double fertilisationMale gamete + egg → zygote (2n); male gamete + secondary nucleus → primary endosperm nucleus (3n)

After fertilisationOvule → seed; ovary → fruit; zygote → embryo

Embryo sac7 cells, 8 nuclei: egg, 2 synergids, 3 antipodals and a central cell with 2 polar nuclei

Parts of a flower

Whorl	Made of	Job
Calyx	Sepals (usually green)	Protect the flower in the bud
Corolla	Petals (often bright and scented)	Attract insects for pollination
Androecium (male)	Stamens. Each has a stalk (filament) and a head (anther) that makes pollen grains.	Pollen grains give the male gametes
Gynoecium (female)	Pistil (carpels): stigma (catches pollen), style (tube) and ovary (holds ovules)	Ovules hold the egg (female gamete)

Self-pollination vs cross-pollination

Self-pollinationPollen reaches the stigma of the same flower or another flower on the same plant. Sure to happen and needs no agent, but gives little variation. Example: pea.

Cross-pollinationPollen reaches the stigma of a flower on another plant of the same kind. Needs an agent and gives variation. Agents: wind (maize, grasses), water (Vallisneria), insects (honeybees on sunflower), birds (sunbirds on Canna) and other animals (squirrels on the silk cotton tree).

Wind-pollinated flowers are small and dull with light, dry pollen. Insect-pollinated flowers are bright, scented and have nectar.

Fertilisation in a flowering plant

1
A pollen grain lands on the sticky stigma of a flower of the same kind.
2
It takes up sugar and water and grows a pollen tube.
3
The pollen tube grows down through the style into the ovary.
4
It enters the ovule through a tiny opening, the micropyle, and reaches the embryo sac.
5
The tube bursts and releases two male gametes.
6★
One male gamete fuses with the egg → zygote (2n). This is syngamy.
7★
The other fuses with the secondary nucleus (2n) → primary endosperm nucleus (3n). This is triple fusion.
8
Syngamy and triple fusion together are double fertilisation. It happens only in flowering plants.

★ key step

After fertilisation: what becomes what

Before	After
Zygote	Embryo (the future plant)
Primary endosperm nucleus	Endosperm (stored food for the embryo)
Ovule	Seed
Integuments (coats of the ovule)	Seed coat
Ovary	Fruit
Ovary wall	Fruit wall (pericarp)
Sepals, petals, stamens, style and stigma	Usually wither and fall (the sepals stay on in brinjal and tomato)

How to draw the L.S. of a flower

1
Draw a short stalk with a swollen top. Label pedicel and thalamus.
2
On the thalamus, draw small leaf-like parts at the bottom. Label sepals.
3
Above them, draw large petals on both sides. Label petals.
4
Inside the petals, draw a few stalks with oval heads. Label filament, anther and stamen.
5
In the centre, draw a flask: a round base, a thin neck and a flat top. Label ovary, style, stigma and pistil.
6
Inside the ovary, draw small ovals. Label ovules. Add a pollen grain on the stigma with a tube growing down to an ovule. Label pollen tube.

In real life

Honeybees cross-pollinate many crops, like sunflower and mustard, so protecting bees protects our food. The rice grain you eat is mostly endosperm, the food store made by triple fusion.

Exam angle

2 marks: “What is double fertilisation?” One male gamete fuses with the egg (syngamy, giving the zygote); the other fuses with the secondary nucleus (triple fusion, giving the endosperm). Together these are double fertilisation.

4 marks: “Distinguish between self-pollination and cross-pollination.” Same flower or plant vs another plant; no agent vs agent needed; little vs more variation; examples (pea; maize, sunflower).

7 marks: “Describe fertilisation in flowering plants with a labelled diagram.” Pollination; pollen tube through the style; entry through the micropyle; two male gametes; syngamy and triple fusion; diagram of the ovule with the pollen tube; changes after fertilisation.

Remember it

One plus one makes the baby: male gamete + egg = embryo (2n). One plus two makes the food: male gamete + 2 polar nuclei = endosperm (3n).