Flowers and Pollination

Flowers are the reproductive organs of flowering plants. Pollination transfers pollen from one flower to another, leading to fertilisation and seed production.

Why Do Plants Have Flowers?

Flowers are the reproductive organs of flowering plants (angiosperms). Their purpose is to produce seeds, which grow into new plants. To form a seed, a pollen grain from one flower must reach the female part of another flower of the same species — a process called pollination.

Flowers have evolved remarkable shapes, colours, and scents to attract the animals that carry pollen for them, or to expose their parts to the wind.

Parts of a Flower

Sepals — Small, leaf-like structures at the base of the flower that protect the bud before it opens.

Petals — Often brightly coloured and fragrant to attract pollinators such as bees, butterflies, and birds.

Stamens (male parts) — Each stamen consists of a thin stalk called the filament topped by an anther. The anthers produce and release pollen grains, which contain the male sex cells.

Carpel / Pistil (female part) — Consists of:

Stigma — The sticky top surface that receives pollen.
Style — A tube connecting the stigma to the ovary.
Ovary — Contains one or more ovules, each holding a female sex cell (egg cell). After fertilisation, the ovary develops into a fruit and each ovule develops into a seed.

Note

Some flowers (like tulips) have both male and female parts. Others (like a Holly bush) have separate male and female flowers, either on the same plant or on different plants. This helps prevent self-fertilisation and promotes genetic diversity.

Pollination: Wind vs. Animal

Insect (and animal) pollination — Brightly coloured, scented flowers with nectar attract insects (bees, butterflies) and other animals (hummingbirds, bats). The pollinator picks up pollen on its body while feeding on nectar and transfers it to the next flower it visits.

Wind pollination — Flowers are usually small, dull, and have no nectar or scent (since they do not need to attract animals). They produce huge quantities of light, smooth pollen that floats on the breeze. The stigmas are large and feathery to catch windborne pollen. Examples: grasses, cereals, many trees.

Tip

Hay fever is caused by wind-pollinated plants releasing enormous quantities of pollen into the air. People with hay fever are allergic to this airborne pollen.

Fertilisation and Seed Development

Once pollen lands on a compatible stigma, a pollen tube grows down through the style into the ovary. The male sex cell travels down this tube and fuses with the egg cell in an ovule — this is fertilisation.

The fertilised egg cell develops into an embryo (future plant) inside a seed. Meanwhile, the ovary wall develops into the fruit that protects the seed and aids in its dispersal.

Seeds are dispersed by wind (dandelions), animals (berries), water (coconuts), or explosion (peas). When a seed lands in a suitable spot, it germinates and grows into a new plant.

Real world: Bees and apple orchards

Apple farmers often hire beekeepers to place hives in their orchards during blossom season (April–May). A single honeybee visits up to 1500 flowers per day, picking up pollen on its fuzzy body and depositing it on the sticky stigmas of the next flowers it visits. Without this pollination service, apple yields drop by up to 80%. Many supermarket fruits — apples, strawberries, almonds — depend on managed bee colonies for pollination.

Real world: Hay fever season

In spring, wind-pollinated trees like birch and alder release billions of tiny pollen grains into the air. Each grain is smooth and lightweight to travel on air currents. When these grains land on the eyes, nose, or throat of a person allergic to pollen, the immune system overreacts — causing the runny nose, itchy eyes, and sneezing of hay fever. Pollen count forecasts help sufferers plan their day.

Real world: Vanilla farming

Vanilla orchids, originally from Mexico, are normally pollinated by a specific native bee. On vanilla plantations outside Mexico (e.g., Madagascar), that bee is absent, so workers hand-pollinate each flower individually using a small stick — transferring pollen from the anther to the stigma by hand. This is why natural vanilla is so expensive.

Tip Where do you see this in real life? Next time you eat a strawberry, apple, or cucumber, remember that a pollinator likely visited that flower. Watch bees in a garden — you can often see yellow pollen packed onto their back legs (pollen baskets). Many garden flowers are specifically designed to advertise to pollinators with colour, scent, and nectar.

Practice Problems

What is the difference between pollination and fertilisation?

Show answer

Pollination is the transfer of pollen from an anther to a stigma. Fertilisation is the fusion of the male sex cell (from the pollen) with the female egg cell in the ovule. Pollination must happen before fertilisation can occur.

Give two differences between insect-pollinated and wind-pollinated flowers.

Show answer

Insect-pollinated flowers have bright petals, nectar, and scent to attract pollinators, and produce less pollen. Wind-pollinated flowers are small and dull with no scent or nectar; they produce large amounts of light, smooth pollen and have feathery stigmas.