Pollination is not fertilisation
These two words are often mixed up, but they describe different events, separated by hours or even days.Pollination is the transfer of pollen from an anther to a stigma. Fertilisation is the fusion of a sperm cell with an egg cell, deep inside an ovule. Between them, the pollen grain must germinate and grow a pollen tube all the way down the style. Pollination can happen without fertilisation, for example when pollen lands on a flower of the wrong species.
The reproductive parts
A flower’s male parts are the stamens: an anther, which makes pollen, on a stalk called the filament. Its female part is the carpel (or pistil): a stickystigma that catches pollen, a style, and an ovary containing one or moreovules. Petals attract pollinators and sepals protect the bud. You can explore these parts, and the tissues of the whole plant, in the Plant Anatomy simulator.
Inside a pollen grain and an ovule
A pollen grain is not a sperm. It is a tiny male gametophyte of two or three cells inside a tough wall: a large tube cell and a small generative cell, which divides by mitosis to maketwo sperm cells. An ovule contains the embryo sac, the female gametophyte. In most flowering plants it has seven cells and eight haploid nuclei:
- the egg cell and two synergids beside it, at the end nearest the opening (the micropyle);
- a large central cell with two polar nuclei;
- three antipodal cells at the far end.
The embryo sac is wrapped in protective layers, the integuments, which will become the seed coat.
Animal and wind pollination
Roughly 88% of flowering plant species rely on animals, mostly insects, but also birds and bats. The rest, including grasses, cereals and many trees such as oak and birch, rely mainly on the wind.
| Animal-pollinated flowers | Wind-pollinated flowers |
|---|---|
| Large, coloured, scented petals | Small, dull flowers, often without petals |
| Nectar as a reward | No nectar |
| Sticky, spiky pollen in modest amounts | Huge amounts of light, smooth pollen |
| Anthers and stigma inside the flower | Dangling anthers and large, feathery stigmas |
| Examples: rose, apple, sage | Examples: wheat, maize, birch |
Animal pollination is targeted: a bee that sticks to one kind of flower carries pollen to the right species. Wind is cheap but wasteful. In the simulator, only a few percent of wind-blown pollen ever reaches a stigma.
Self- and cross-pollination
In self-pollination, pollen reaches a stigma of the same flower or another flower on the same plant. Incross-pollination, pollen comes from a different plant of the same species. Cross-pollination mixes the genes of two parents, so the offspring are more varied and better able to cope with disease and change. Self-pollination is reliable when pollinators are scarce, and peas, wheat and tomatoes do it routinely.
Many plants, including apples, cherries and many clovers, are self-incompatible: the stigma or style recognises the plant’s own pollen and stops its tube from growing. That is why a lone apple tree sets little fruit without a second variety nearby. Other plants prevent selfing by holding the stigma above the anthers, or by ripening anthers and stigma at different times.
Germination and the pollen tube
On a compatible stigma, the pollen grain absorbs water and germinates: the tube cell grows apollen tube that pushes down between the cells of the style. The tube is guided by chemical signals from the synergids and carries the two sperm cells at its tip. Flowering-plant sperm cannot swim, so unlike mosses and ferns they need no water. The tube enters the ovule through the micropyle, grows into a synergid and bursts, releasing both sperm. Each ovule needs its own pollen tube, so a seed-packed fruit such as a tomato needs many grains of pollen.
Double fertilisation
Both sperm cells fuse with something, which is why the process is called double fertilisation:
- Sperm 1 + egg cell (n + n) → a diploid zygote (2n), which becomes the embryo.
- Sperm 2 + central cell with its two polar nuclei (n + n + n) → a triploid primary endosperm nucleus (3n), which develops into the food store.
Double fertilisation is found in flowering plants. It means the plant invests in a food store only when an egg has actually been fertilised.
From ovule to seed, ovary to fruit
- The zygote divides by mitosis into an embryo with a root (radicle), a shoot and one or two seed leaves (cotyledons).
- The endosperm nucleus divides to make endosperm. In cereals it remains as the bulk of the grain (and of white flour); in beans and peas the cotyledons absorb it.
- The integuments harden into the seed coat. The whole ovule becomes the seed.
- The ovary wall becomes the fruit, which protects the seeds and helps disperse them. Petals, stamens, style and stigma wither.
Botanically, a pea pod, a tomato, a cucumber and a cherry are all fruits. What happens when a seed starts to grow is explored in the Seed Germination simulator.
Why it matters
- Food: about three-quarters of the world’s leading food crops benefit from animal pollination, including apples, almonds, coffee and cocoa. Cereals such as wheat, rice and maize are wind-pollinated or self-pollinated.
- Orchards: growers plant compatible varieties side by side and bring in honeybee hives at flowering time.
- Plant breeding: breeders remove the anthers from a flower and dust its stigma by hand to make a chosen cross.
- Hay fever is caused mainly by wind-blown pollen of grasses and trees, which is released in vast amounts.
Common misconceptions
- “Pollination and fertilisation are the same thing.” Pollination moves pollen to a stigma; fertilisation is the fusion of gametes inside the ovule.
- “A pollen grain is a sperm.” It is a tiny male gametophyte that produces two sperm cells.
- “The sperm swims down the style.” The pollen tube grows and carries the sperm to the ovule.
- “Fertilisation is a single fusion.” In flowering plants there are two: one forms the zygote and one forms the endosperm.
- “The ovary becomes the seed.” Each ovule becomes a seed; the ovary becomes the fruit.
- “Any pollen will do.” Pollen of another species, or a self-incompatible plant’s own pollen, does not lead to fertilisation.
What the simulator simplifies
The flowers are generic, with four ovules in the pollination experiment and three in the long section, and nothing is drawn to scale. Real pollen tubes take from under an hour to several days to reach an ovule. In the experiment, pollen of another species never germinates; in reality some foreign pollen does germinate and is stopped later. The bee and wind follow simple rules, and the two plants are either fully self-incompatible or fully self-compatible. Embryo sacs in some species differ from the seven-celled pattern shown, and some fruits (such as strawberries) also grow from parts of the flower other than the ovary.
Key takeaways
- Pollination is pollen transfer to a stigma; fertilisation is the fusion of sperm and egg in the ovule.
- Flowers are pollinated by animals or by wind, and their structure matches the carrier.
- Cross-pollination increases variation; self-incompatibility prevents self-fertilisation.
- A pollen tube delivers two sperm: one fertilises the egg (2n zygote), the other the central cell (3n endosperm).
- The ovule becomes the seed and the ovary becomes the fruit.