A flower is the organ of sexual reproduction in a flowering plant. A typical flower is borne on a stalk and has four whorls of parts attached to the swollen tip of the stalk, the receptacle.
The four whorls
- Sepals (calyx) — usually green leaf-like structures that enclose and protect the flower while it is a bud.
- Petals (corolla) — often brightly coloured and scented in insect-pollinated flowers to attract pollinators.
- Stamens (androecium) — the male parts. Each stamen has an anther that produces pollen grains, held up by a stalk called the filament.
- Carpel or pistil (gynoecium) — the female part. It consists of the stigma (which receives pollen), the style (a stalk), and the ovary which contains one or more ovules. Each ovule holds the female gamete.
Key idea
Male part = stamen (anther + filament) makes pollen. Female part = carpel (stigma + style + ovary), and the ovary holds the ovules. A flower with both male and female parts is a bisexual flower.
Adaptations for pollination
Flower structure reflects the method of pollination.
| Feature | Insect-pollinated | Wind-pollinated |
|---|---|---|
| Petals | large, coloured, scented | small or absent, dull |
| Nectar | present | absent |
| Stamens | inside the flower | long, hanging outside |
| Stigma | small, sticky, inside | large, feathery, hanging out |
| Pollen | sticky, spiky, little | light, smooth, abundant |
Example
A hibiscus flower has large red petals and nectar to attract insects, with sticky pollen and a stigma held inside — typical adaptations of an insect-pollinated flower.
Bisexual and unisexual flowers
A flower that has both stamens and a carpel, like the hibiscus, is a bisexual flower. A flower that has only stamens or only a carpel is a unisexual flower; the maize plant, for example, bears separate male and female flowers. Some flowers also produce nectar in structures called nectaries at the base of the petals; nectar is a sugary reward that attracts insects and encourages them to move between flowers, so that pollen is carried from one flower to another. Studying flower structure therefore helps us predict how a plant is pollinated and how it will reproduce.
Remember
Sepals protect, petals attract, stamens make pollen (male), the carpel receives pollen and holds ovules (female).