Reproductive System in Flowering Plants – SS2 Biology Lesson Note (Third Term)

The flower is the reproductive structure of a flowering plant. It carries both male and female sex organs, so it can carry out sexual reproduction. A flower has four floral parts: the calyx, corolla, androecium and gynoecium, joined to the plant by a stalk called the pedicel. After fertilisation, the flower produces seeds.

Behavioural objectives

By the end of the lesson, students should be able to:

• name the parts of a typical flower and state the function of each;
• describe the three types of ovary position and give an example of each;
• distinguish between monoecious and dioecious plants;
• explain what placentation is;
• describe the five types of placentation and give an example of each.

What are the reproductive organs of a flowering plant?

The flower is the reproductive structure of a flowering plant. It has both male and female sex organs, so it can carry out sexual reproduction. After fertilisation has taken place in the flower, seeds are produced.

A flower has two major parts:

1. The floral part (whorls)
2. The flower stalk (pedicel)

The floral part of the flower is formed by:

1. calyx (sepals)
2. corolla (petals)
3. androecium (stamens)
4. gynoecium (carpels)

What are the parts of a typical flower?

The calyx

The calyx consists of sepals, which are usually small and green. They protect the flower while it is in the bud. Sepals are usually 3-5 in number. They may be separate (polysepalous) or joined to form a cup (gamosepalous). An epicalyx may be present, e.g. in hibiscus flowers.

The corolla

The corolla consists of petals inside the sepals. Petals are the attractive part of the flower and may number 4-10. They may be separate (polypetalous) or joined to form a tube (gamopetalous). They are generally coloured and scented to attract pollinators (e.g. insects).

The androecium

The androecium is the male reproductive organ of a flower. It is a group of stamens, each of which has two parts: the filament and the anther. Stamens may be 3 or more, free or joined together.

The anther is a 2-4 lobed structure that produces the pollen grains (fine yellowish particles), which in turn produce the male gamete. At maturity, the anther lobes open to release the pollen grains.

The gynoecium

The gynoecium is the female reproductive organ of a flower. It is the innermost floral part. It consists of carpels, which may be one or many. A gynoecium with one carpel is monocarpous, e.g. flamboyant. When there are many carpels, they may be separate (apocarpous) or fused (syncarpous), e.g. hibiscus.

A pistil or carpel consists of three parts: the ovary, the style and the stigma.

The ovary contains an ovule, which produces the female gamete. After fertilisation, the ovary develops into the fruit, while the ovule develops into the seed.

What are the types of ovary?

1. Superior ovary: the ovary is above the other floral parts. Such a flower is called a hypogynous flower, e.g. hibiscus.
2. Inferior ovary: the ovary is below the other floral parts. Such a flower is called an epigynous flower, e.g. sunflower.
3. Half inferior ovary: the ovary is at the same level as the other floral parts. Such a flower is called a perigynous flower, e.g. rose.

What are the types of flower?

Generally, flowers differ in the following:

1. number of floral parts
2. colour of petals
3. size and shape
4. symmetry

Ways of grouping flowers

1. Flowers can be radially symmetrical or regular (actinomorphic), or irregular (zygomorphic).
2. A flower is complete or perfect when all four floral parts are present. It is imperfect or incomplete if one or more of the floral parts are absent.
3. Flowers can be grouped by the position of the ovary (hypogynous, epigynous or perigynous flowers).
4. Flowers may occur singly (solitary flower) or in groups (inflorescence).
5. Flowers may be borne in the axils of leaves (axillary flowers) or at the end of the stem or branches (terminal flowers).

What are the sexes in plants?

Most flowers are bisexual, i.e. they have both stamens and carpels, while a few are unisexual (either male or female). A plant can therefore be monoecious or dioecious.

• Monoecious: male and female flowers occur on the same plant, e.g. maize.
• Dioecious: male and female flowers are found on different plants, e.g. pawpaw.

What is placentation?

Placentation is the arrangement of the ovules within the ovary of a flower. The ovules are attached to the ovary by fleshy structures called placenta, through short stalks called funicles.

Types of placentation

1. Marginal placentation: ovules are arranged at the margin of the ovary, e.g. beans, flamboyant flowers.
2. Parietal placentation: ovules are arranged at the side of the ovary, within a single chamber or cavity, e.g. pawpaw.
3. Free central placentation: ovules are borne on a knob projecting from the base of the ovary, e.g. water lily.
4. Axile placentation: carpels meet at the centre to form the placenta for attachment of ovules, e.g. tomatoes.
5. Basal placentation: ovules are attached to the base of the ovary, e.g. sunflower.

Evaluation

1. Name the four floral parts of a flower.
2. State the difference between a monoecious plant and a dioecious plant, with one example of each.
3. Name the three types of ovary position and give one example of each.
4. What is placentation?

Assignment

Section A

1. The reproductive parts of the flower include all the following except A. corolla B. androecium C. gynoecium D. stigma
2. A plant that has both male and female flowers on the same plant is referred to as A. monoecious B. dioecious C. hypogynous D. axillary
3. An example of a flower with a half inferior ovary is A. hibiscus B. sunflower C. rose D. flamboyant
4. The arrangement of ovules in the ovary is referred to as A. pollination B. placentation C. fertilisation D. solitary
5. An example of axile placentation is found in A. sunflower B. lily C. pawpaw D. tomato

Section B

6. With the aid of diagrams, distinguish between a monocarpous ovary and a syncarpous ovary.
7. Describe five types of placentation and give one example in each case.