Flower | ICSE Class 9 Biology Notes

This note covers the topic Flower from the ICSE Class 9 Biology unit on Flowering Plants. The CISCE syllabus asks for the structure of a bisexual flower and the functions of its parts, complete and incomplete flowers, the essential and non-essential whorls, and the meaning of inflorescence and placentation. The note covers that scope and adds a few related descriptive terms, such as the position of the ovary, so that you can recognise them. The plant examples given here are the standard textbook ones.

What is a flower?

The flower is the reproductive unit in the angiosperms (the flowering plants). It is meant for sexual reproduction. After fertilisation it gives rise to the fruit and the seeds.

Definition: A flower is a modified shoot. The shoot apical meristem changes to a floral meristem, the internodes do not elongate, and the axis gets condensed. The apex then produces different kinds of floral appendages laterally at successive nodes instead of leaves.

Three points in this definition explain why a flower is called a modified shoot.

• A shoot grows from a shoot apical meristem. In a flower this meristem has changed into a floral meristem.
• A shoot has nodes separated by internodes. In a flower the internodes do not elongate, so the nodes lie very close together.
• A shoot bears leaves at its nodes. A flower bears floral appendages, the sepals, petals, stamens and carpels, at its nodes instead of leaves.

Two parts of the flower belong to the stalk and not to the whorls.

Because the axis is condensed, the floral parts are not spread out along a stem as leaves are. They sit close together on the thalamus in rings, one inside the other. Each ring is called a whorl.

Note: "Thalamus" and "receptacle" are two names for the same part. "Pedicel" is the stalk of a single flower. Do not use "petiole" for it: a petiole is the stalk of a leaf.

What are the four whorls of a flower?

A typical flower has four different kinds of whorls. From the outside to the centre, they are:

1. Calyx, made up of sepals.
2. Corolla, made up of petals.
3. Androecium, made up of stamens.
4. Gynoecium, made up of one or more carpels.

A flower is shown whole, with five labels listed from top to bottom: androecium, gynoecium, corolla, calyx and pedicel. Beside it the four whorls are drawn separately in order: the calyx as a ring of small green sepals, the corolla as a ring of large coloured petals, the androecium as a group of stamens each with a stalk and a head, and the gynoecium as a single flask-shaped structure. When you draw a flower, label these five and show the whorls one inside the other.

See Fig. 5.10 in your NCERT textbook

The order never changes. The calyx is always the outermost whorl and the gynoecium is always at the centre, with the corolla and the androecium between them. A useful check when labelling a diagram is that the two outer whorls are the accessory ones and the two inner whorls are the reproductive ones.

Which whorls are essential and which are non-essential?

The calyx and the corolla are accessory organs. The androecium and the gynoecium are reproductive organs. In ICSE language these two groups are the non-essential and the essential whorls.

In some flowers, such as the lily, the calyx and the corolla are not distinct. The two together are then termed the perianth.

Note: "Non-essential" does not mean useless. Sepals protect the bud and petals attract insects. The word only means that these whorls do not themselves form the pollen grains or the ovules.

What are bisexual, unisexual, complete and incomplete flowers?

The two pairs of terms describe different things. "Bisexual" and "unisexual" look only at the two essential whorls. "Complete" and "incomplete" look at all four whorls.

• A complete flower is always bisexual, because it has both the androecium and the gynoecium.
• A unisexual flower is always incomplete, because one of the essential whorls is missing.
• A bisexual flower may still be incomplete, if it lacks the calyx or the corolla.

A unisexual flower with only stamens is a male (staminate) flower. A unisexual flower with only carpels is a female (pistillate) flower. The china rose (Hibiscus), the flower named in the ICSE practical list, is bisexual and complete.

What are the calyx and the corolla?

Calyx

The calyx is the outermost whorl of the flower, and its members are called sepals. Generally, sepals are green and leaf-like, and they protect the flower in the bud stage.

Corolla

The corolla is composed of petals. Petals are usually brightly coloured to attract insects for pollination. The shape and colour of the corolla vary greatly in plants: it may be tubular, bell-shaped, funnel-shaped or wheel-shaped.

Sepals and petals compared

The way the sepals or petals are arranged in the floral bud, with respect to the other members of the same whorl, is called aestivation. In china rose the margin of each petal overlaps that of the next one, and this arrangement is called twisted aestivation.

Note: The prefixes give the meaning. "Gamo" means united and "poly" means free. The same prefixes are used for sepals and petals.

What is the structure of the androecium?

The androecium is composed of stamens. Each stamen represents the male reproductive organ.

An anther that is bilobed with two pollen-sacs in each lobe has four pollen-sacs in all. A sterile stamen is called a staminode.

There may also be a variation in the length of the filaments within one flower, as in Salvia and mustard.

The stamens of a flower may remain free, or they may be united among themselves or with other parts.

What is the structure of the gynoecium?

The gynoecium is the female reproductive part of the flower. It is made up of one or more carpels. A carpel consists of three parts.

A gynoecium with a single carpel has one stigma, one style and one ovary. When more than one carpel is present, the carpels may be free or fused.

From flower to fruit

1. Pollen grains are produced in the pollen-sacs of the anther.
2. The stigma, the receptive surface of the carpel, receives the pollen grains.
3. After fertilisation, the ovules develop into seeds.
4. The ovary matures into a fruit.

Note: Keep the three levels apart. The gynoecium is the whole female whorl. A carpel is one unit of it. The ovary is only the basal part of a carpel. In the same way, the androecium is the whole male whorl, a stamen is one unit of it, and the anther is only the head of a stamen.

How are flowers described by the position of the ovary?

Flowers are described as hypogynous, perigynous or epigynous, based on the position of the calyx, corolla and androecium in respect of the ovary on the thalamus.

Four flowers are shown cut lengthwise. In (a), hypogynous, the ovary sits on top of the thalamus and the sepals, petals and stamens are attached below it. In (b) and (c), perigynous, the thalamus forms a cup: the ovary sits in the centre of the cup and the other parts arise from its rim. In (d), epigynous, the thalamus completely encloses the ovary, and the sepals, petals and stamens arise above the ovary.

See Fig. 5.9 in your NCERT textbook

Note: The name of the flower and the name of the ovary go in opposite directions. In a hypogynous flower ("hypo", below) the other parts are below the ovary, so the ovary is superior. In an epigynous flower ("epi", upon) the other parts are above the ovary, so the ovary is inferior.

How else can a flower be described?

By symmetry

By the number of floral parts

A flower may be trimerous, tetramerous or pentamerous when its floral appendages are in multiples of 3, 4 or 5 respectively.

By bracts

A bract is a reduced leaf found at the base of the pedicel. Flowers with bracts are called bracteate, and those without bracts are called ebracteate.

What does inflorescence mean?

Definition: Inflorescence is the arrangement of flowers on the floral axis.

When a shoot tip transforms into a flower, the flower is always solitary. When many flowers are borne on an axis, the inflorescence is of one of two major types, depending on whether the apex develops into a flower or continues to grow.

A branched flowering axis in which the main axis ends in a flower. The branches below it also end in flowers, some open and some still in bud. Because the tip of each axis is used up in forming a flower, the axis cannot keep growing. NCERT Figure 5.7 shows the other type, a racemose inflorescence, as a photograph of a long flowering axis carrying many flowers and buds along its sides.

See Fig. 5.8 in your NCERT textbook

Note: The ICSE syllabus asks for the meaning of inflorescence only. Learn the definition first. The two types are given here so that you can recognise the terms racemose and cymose if they appear.

What does placentation mean?

Definition: Placentation is the arrangement of ovules within the ovary.

Each ovary bears one or more ovules attached to a flattened, cushion-like placenta. The way the ovules are arranged on the placenta inside the ovary differs from plant to plant.

Five drawings. In (a), marginal, a pod is opened to show a row of ovules along one edge; the placenta ridge bears two such rows, one on each side. In (b), axile, a cross-section of an ovary is divided into chambers by walls that meet at the centre, with the ovules attached at the centre. In (c), parietal, the ovules are attached to the inner wall of the ovary. In (d), free central, the ovules are attached to a central column and there are no dividing walls. In (e), basal, a single ovule is attached at the base of the ovary.

See Fig. 5.12 in your NCERT textbook

Note: The ICSE syllabus asks for the meaning of placentation only. The definition is what you must be able to write. The types and examples are for recognition.

How do you examine and draw a flower such as Hibiscus?

The ICSE practical list asks for a cross-pollinated flower, for example Hibiscus (china rose), to be examined and identified, and its parts studied and labelled. Work from the outside to the centre.

1. Find the pedicel, and the thalamus at its tip, on which all the whorls are borne.
2. Look at the outermost whorl, the calyx, and note the sepals. (Hibiscus also has a ring of small leaf-like bracteoles, the epicalyx, just outside the calyx; it is not one of the four whorls.)
3. Look at the corolla, and note the large, brightly coloured petals.
4. Find the androecium. Note the many stamens, each with an anther. In Hibiscus the filaments are united into one column (monoadelphous).
5. Find the gynoecium at the centre. Identify the stigma, the style and the ovary.
6. Cut the ovary across and look for the ovules attached to the placenta.

The main points about china rose are these.

Glossary

• Flower — The reproductive unit in angiosperms; a modified shoot meant for sexual reproduction.
• Pedicel — The stalk of a flower, whose swollen end bears the floral whorls.
• Thalamus — The swollen end of the pedicel, also called the receptacle, on which the four whorls are arranged.
• Calyx — The outermost whorl of the flower, made up of sepals that protect the flower in the bud stage.
• Corolla — The whorl made up of petals, usually brightly coloured to attract insects for pollination.
• Perianth — The calyx and corolla together when they are not distinct from each other, as in the lily.
• Androecium — The male reproductive whorl of the flower, composed of stamens.
• Stamen — The male reproductive organ, consisting of a stalk or filament and an anther.
• Staminode — A sterile stamen, that is a stamen which does not produce pollen grains.
• Gynoecium — The female reproductive part of the flower, made up of one or more carpels.
• Carpel — One unit of the gynoecium, consisting of three parts: stigma, style and ovary.
• Bisexual flower — A flower that has both the androecium and the gynoecium.
• Unisexual flower — A flower having either only stamens or only carpels.
• Complete flower — A flower in which all four whorls are present: calyx, corolla, androecium and gynoecium.
• Incomplete flower — A flower in which one or more of the four whorls is missing.
• Essential whorls — The androecium and the gynoecium, which take a direct part in reproduction.
• Non-essential whorls — The calyx and the corolla, the accessory whorls, which protect the flower and attract pollinators.
• Inflorescence — The arrangement of flowers on the floral axis of a plant.
• Placentation — The arrangement of ovules within the ovary of a flower.

Common errors and misconceptions

• Misconception: The petals are the essential part of a flower because they are the most visible. Correct: The essential whorls are the androecium and the gynoecium. The corolla is a non-essential, accessory whorl.
• Misconception: A bisexual flower is the same as a complete flower. Correct: Bisexual refers only to having both androecium and gynoecium. A complete flower has all four whorls.
• Misconception: Gamosepalous means the sepals are free. Correct: Gamosepalous means sepals united. Polysepalous means sepals free.
• Misconception: The stigma produces pollen grains. Correct: Pollen grains are produced in the pollen-sacs of the anther. The stigma is the receptive surface for pollen grains.
• Misconception: In a hypogynous flower the ovary is inferior. Correct: In a hypogynous flower the ovary is superior. The ovary is inferior in an epigynous flower.
• Misconception: Inflorescence is another word for a flower. Correct: Inflorescence is the arrangement of flowers on the floral axis.
• Misconception: Placentation is the arrangement of seeds in a fruit. Correct: Placentation is the arrangement of ovules within the ovary.
• Misconception: The ovary becomes the seed. Correct: After fertilisation the ovules develop into seeds, and the ovary matures into a fruit.

Exam-style questions with model answers

Q1. Define inflorescence. [1 mark]

1. Inflorescence is the arrangement of flowers on the floral axis. It is of two major types, racemose and cymose, depending on whether the main axis continues to grow or ends in a flower.

Q2. Define placentation. [1 mark]

1. Placentation is the arrangement of ovules within the ovary. The ovules are attached to a flattened, cushion-like tissue called the placenta, for example along the central axis in axile placentation.

Q3. Name the essential whorls of a flower and state why they are called essential. [2 marks]

1. The essential whorls are the androecium and the gynoecium.
2. They are the reproductive organs and take a direct part in reproduction: the androecium produces pollen grains and the gynoecium bears the ovules.

Q4. Distinguish between a complete flower and an incomplete flower. [2 marks]

1. A complete flower has all four whorls: calyx, corolla, androecium and gynoecium.
2. An incomplete flower lacks one or more of these four whorls.

Q5. Name the parts of a stamen and state the function of each. [3 marks]

1. A stamen is the male reproductive organ of the flower. It consists of a stalk, called the filament, and an anther at its tip.
2. The filament holds the anther up.
3. The anther is usually bilobed, and each lobe has two chambers, the pollen-sacs, in which the pollen grains are produced.

Q6. Name the three parts of a carpel and give one function of each. [3 marks]

1. Stigma: usually at the tip of the style; it is the receptive surface for pollen grains.
2. Style: the elongated tube that lies on the ovary; it connects the ovary to the stigma.
3. Ovary: the enlarged basal part of the carpel; it bears one or more ovules attached to the placenta, and the ovules develop into seeds after fertilisation.

Q7. Distinguish between hypogynous, perigynous and epigynous flowers, with the position of the ovary in each. [3 marks]

1. Hypogynous: the gynoecium occupies the highest position and the other parts are below it; the ovary is superior, as in mustard and china rose.
2. Perigynous: the gynoecium is in the centre and the other parts are on the rim of the thalamus at almost the same level; the ovary is half inferior, as in plum and rose.
3. Epigynous: the thalamus grows upward and encloses the ovary, and the other parts arise above it; the ovary is inferior, as in guava and cucumber.

Q8. Describe the structure of a typical bisexual flower, naming the four whorls in order and stating the function of each. [5 marks]

1. A flower is borne on a stalk, the pedicel, whose swollen end, the thalamus, carries four whorls arranged successively.
2. Calyx: the outermost whorl, made of sepals, which are generally green and leaf-like and protect the flower in the bud stage.
3. Corolla: made of petals, which are usually brightly coloured to attract insects for pollination.
4. Androecium: the male whorl, made of stamens. Each stamen has a filament and an anther, and pollen grains are produced in the pollen-sacs of the anther.
5. Gynoecium: the female whorl, made of one or more carpels. Each carpel has a stigma, a style and an ovary; the ovary bears the ovules. The calyx and corolla are the non-essential whorls, and the androecium and gynoecium are the essential whorls.

Key takeaways

• A flower is a modified shoot and the reproductive unit of angiosperms; its whorls are borne on the thalamus at the tip of the pedicel.
• The four whorls, from outside to centre, are the calyx of sepals, the corolla of petals, the androecium of stamens and the gynoecium of carpels.
• The calyx and corolla are non-essential, accessory whorls; the androecium and gynoecium are the essential, reproductive whorls.
• A bisexual flower has both androecium and gynoecium; a complete flower has all four whorls, and an incomplete flower lacks one or more.
• A stamen has a filament and an anther, and the pollen grains are produced in the pollen-sacs of the anther.
• A carpel has a stigma, a style and an ovary; after fertilisation the ovules develop into seeds and the ovary matures into a fruit.
• The ovary is superior in a hypogynous flower, half inferior in a perigynous flower and inferior in an epigynous flower.
• Inflorescence is the arrangement of flowers on the floral axis, and placentation is the arrangement of ovules within the ovary.

Test yourself

What is the swollen end of the pedicel called?

The swollen end of the pedicel is called the thalamus or receptacle, and it bears the four whorls of the flower.

Which whorl protects the flower in the bud stage?

The calyx, which is made up of sepals, protects the flower in the bud stage.

What is the perianth?

The perianth is the name given to the calyx and corolla together when they are not distinct, as in the lily.

Where are pollen grains produced?

Pollen grains are produced in the pollen-sacs of the anther, which is the head of the stamen.

What is a sterile stamen called?

A sterile stamen, one that does not produce pollen grains, is called a staminode.

Which part of the carpel receives the pollen grains?

The stigma, usually at the tip of the style, is the receptive surface for pollen grains.

What kind of ovary does a hypogynous flower have?

A hypogynous flower has a superior ovary, because the other floral parts are situated below the gynoecium.

What is the condition of the stamens in china rose?

In china rose the stamens are monoadelphous, that is united into one bunch or one bundle.