External Structure and Importance of a Flowering Plant
Senior 1 · Biology
External Structure and Importance of a Flowering Plant
Lesson 2 of 5
25 min
A few days after the first good rains of the season, the coffee farms around Huye and Nyamasheke turn white: every bush is covered in small, sweet-smelling flowers. Within a week the petals have fallen. From each flower that was pollinated, a green coffee cherry starts to grow, and months later it will turn red and hold two coffee "beans" (seeds).
The flower is the part of the plant that makes seeds. To understand how, you need to know its parts.
You need a large flower (a lily or a hibiscus works well), a hand lens, a razor blade or scalpel, and a sheet of white paper.
Safety: cut on a board, away from your fingers, and hand the blade back to your teacher at once. Some people are allergic to pollen: wash your hands after the practical.
A flower is built in rings (whorls). From the outside inwards: sepals, petals, stamens, and in the centre one or more carpels, all attached to the receptacle at the top of the flower stalk.
Figure 1: Half-section of a typical insect-pollinated flower, cut through the middle from top to bottom.
Definition
Flower: the reproductive shoot of a flowering plant, which produces seeds.
Sepal: a leaf-like, usually green outer part that protects the flower in bud. All the sepals together form the calyx.
Petal: a usually brightly coloured part that attracts insects and birds. All the petals together form the corolla.
Stamen: the male part, made of a stalk (filament) and an anther, which makes pollen grains containing the male sex cells.
Carpel (also called the pistil): the female part, made of the stigma (which receives pollen), the style (which holds up the stigma) and the ovary, which contains the ovules. Each ovule contains a female sex cell and can become a seed.
Receptacle: the top of the flower stalk, to which the flower parts are attached. Nectary: a gland at the base of the petals that makes sugary nectar.
| Part | Sex | Function | After pollination |
|---|---|---|---|
| Sepal | neither | Protects the flower while it is a bud | Usually withers or stays small |
| Petal | neither | Attracts pollinators by colour and scent | Falls off |
| Nectary | neither | Makes nectar, a food reward for pollinators | Dries up |
| Anther | male | Makes and releases pollen | Withers |
| Filament | male | Holds the anther where pollinators touch it | Withers |
| Stigma | female | Sticky surface that catches pollen | Withers |
| Style | female | Holds the stigma up; pollen tubes grow down it | Withers |
| Ovary | female | Contains and protects the ovules | Becomes the fruit |
| Ovule | female | Contains the female sex cell | Becomes a seed |
Warning
Stigma or stamen? The words look alike. Link them to their sex: stamen contains "men": it is the male part. The stigma is the female landing pad at the top of the carpel. Also remember that pollen is made in the anther, not in the stigma.
Worked example 1: stopping a flower from pollinating itself
At RAB, a bean breeder wants to cross two varieties. She must stop the flower of variety A from being pollinated by its own pollen, then dust it with pollen from variety B. Which parts must she remove, and when?
Answer: remove the stamens from the bud before the pollen ripens. Removing stamens like this is called emasculation.
Worked example 2: counting flower parts
A coffee flower has 5 petals and 5 stamens; a lily has 6 coloured parts (tepals) and 6 stamens. What pattern do you see, and what does it tell you?
Answer: the coffee flower follows the dicot pattern and the lily the monocot pattern. Counting flower parts is one quick way to place a plant in its group.
Many Rwandan crops, including coffee, avocado, passion fruit and pumpkins, set more fruit when bees visit their flowers. Bees come for the nectar and carry pollen from anther to stigma as they move. This is why beekeeping cooperatives often place hives near coffee farms: the farmer gets better pollination and the beekeeper gets honey.
Flower Dissection: Reproduction in flowering plants (Science Sauce). Watch the order in which the parts are removed, and compare it with your own dissection.
Tip
Memory hook: from the outside in, the rings of a flower are "Some Plants Seem Clever": Sepals, Petals, Stamens, Carpels.
Practice: the flower
Key points
Sources and credits
Figure 1, the practical, the table, the worked examples and the practice questions are written by Ganzaa.
