A flower has four whorls: calyx, corolla, androecium and gynoecium. Flowers differ in symmetry, in the number of members and in the position of the ovary. Sepals and petals differ in union and aestivation, stamens in union and attachment, and ovules in placentation.
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The flower: the reproductive unit
The flower is the part of a flowering plant that carries out sexual reproduction. Take a China rose and look at it from the outside to the centre. You find four rings of parts. Each ring is called a whorl.
The calyx is the outermost whorl. It is made of sepals.
The corolla is made of petals.
The androecium is made of stamens, the male parts.
The gynoecium is made of carpels, the female parts.
The androecium and the gynoecium are the reproductive parts. The calyx and the corolla are accessory parts. They protect the flower and attract insects.
In some flowers, like the lily, the calyx and the corolla look alike and cannot be told apart. Together they are called the perianth.
Kinds of flower
Bisexual or unisexual. A flower with both an androecium and a gynoecium is bisexual. A flower with only stamens or only carpels is unisexual.
Bracteate or ebracteate. At the base of the flower stalk some flowers have a small, reduced leaf called a bract. Such flowers are bracteate. Flowers without a bract are ebracteate.
Symmetry. Cut a flower through its centre. How many ways give two matching halves?
Actinomorphic, or radial: any plane through the centre gives two equal halves. Mustard, datura and chilli are actinomorphic.
Zygomorphic, or bilateral: only one vertical plane gives two similar halves. Pea, gulmohur, bean and Cassia are zygomorphic.
Asymmetric, or irregular: no plane gives two similar halves. Canna is asymmetric.
Number of members. A flower is trimerous when its floral parts are in multiples of 3. It is tetramerous for multiples of 4 and pentamerous for multiples of 5.
The position of the ovary
Where does the ovary sit compared with the other parts? The answer gives three kinds of flower.
Three flowers cut lengthways, side by side, each with a green stalk at the bottom, a tan thalamus, a pale green ovary with three small yellow ovules, a green style with a stigma, two pink petals, two green sepals and two stamens with yellow anthers. On the left, the hypogynous flower: the thalamus is a small flat dome, the ovary sits on top of it in the highest position, and the sepals, petals and stamens are fixed below the ovary. It is labelled ovary superior, with mustard, China rose and brinjal as examples. In the middle, the perigynous flower: the thalamus is a wide cup, the ovary sits free in the centre of the cup, and the sepals, petals and stamens are fixed on the rim of the cup at about the level of the ovary. It is labelled ovary half inferior, with plum, rose and peach as examples. On the right, the epigynous flower: the thalamus forms a round ball that completely encloses the ovary and is fused with it, and the sepals, petals and stamens arise from the top of the ball, above the ovary. It is labelled ovary inferior, with guava, cucumber and the ray florets of sunflower as examples.
Hypogynous. The gynoecium is in the highest position, and the other parts are below it. The ovary is superior. Examples: mustard, China rose and brinjal.
Perigynous. The gynoecium is in the centre, and the other parts are on the rim of the thalamus, almost at its level. The ovary is half inferior. Examples: plum, rose and peach.
Epigynous. The margin of the thalamus grows upward, encloses the ovary completely and fuses with it. The other parts arise above the ovary. The ovary is inferior. Examples: guava, cucumber and the ray florets of sunflower.
The calyx and the corolla
The calyx is the outermost whorl. Its members are the sepals. They are generally green and leaf-like, and they protect the flower in the bud stage. The sepals may be united, which is gamosepalous, or free, which is polysepalous.
The corolla is made of petals. Petals are usually brightly coloured to attract insects for pollination. They may be united, which is gamopetalous, or free, which is polypetalous. The corolla may be tubular, bell-shaped, funnel-shaped or wheel-shaped.
Aestivation
The way the sepals or petals are arranged in the bud, with respect to the other members of the same whorl, is called aestivation.
Valvate: the margins just touch one another, without overlapping, as in Calotropis.
Twisted: one margin of each member overlaps the next one, all round, as in China rose, lady's finger and cotton.
Imbricate: the margins overlap one another, but not in any fixed direction, as in Cassia and gulmohur.
Vexillary: pea and bean flowers have five petals. The largest, the standard, overlaps the two side petals, the wings. The wings overlap the two smallest petals, the keel. This is also called papilionaceous.
The androecium
The androecium is made of stamens, the male reproductive organs. A stamen has a stalk, the filament, and an anther.
Each anther is usually bilobed, and each lobe has two chambers, the pollen sacs. The pollen grains are made in the pollen sacs. So a typical anther has four pollen sacs. A sterile stamen, which makes no pollen, is a staminode.
Stamens may join other parts of the flower, or one another.
Epipetalous: the stamens are attached to the petals, as in brinjal.
Epiphyllous: the stamens are attached to the perianth, as in lily.
Polyandrous: the stamens are free from one another.
Monoadelphous: the stamens are united in one bundle, as in China rose.
Diadelphous: the stamens are united in two bundles, as in pea.
Polyadelphous: the stamens are united in more than two bundles, as in citrus.
The filaments in one flower may differ in length, as in Salvia and mustard.
You should find that a flower with ten stamens has forty pollen sacs. Move the stamens slider and watch the last bar. It always stands at four times the number of stamens.
The gynoecium
The gynoecium is the female reproductive part. It is made of one or more carpels. A carpel has three parts.
The ovary is the enlarged base. It holds the ovules.
The style is the long tube above the ovary.
The stigma is at the tip of the style. It receives the pollen grains.
The ovules sit on a flat, cushion-like placenta inside the ovary. When there is more than one carpel, they may be free, as in lotus and rose. The flower is then apocarpous. They may be fused, as in mustard and tomato. The flower is then syncarpous.
After fertilisation the ovules become seeds, and the ovary becomes a fruit.
Placentation
The way the ovules are arranged inside the ovary is called placentation.
Five small round drawings of an ovary cut across, in a row, each with a bold name above it and examples below. First, marginal placentation: a one-chambered ovary with a tan ridge along its left edge and three yellow ovules attached to the ridge, as in pea. Second, axile placentation: a round ovary divided by three walls into three chambers that meet at a tan central axis, with one ovule next to the axis in each chamber, as in China rose, tomato and lemon. Third, parietal placentation: a round ovary with four tan cushions on its inner wall, each with an ovule, and a dashed line across the middle for the false partition, as in mustard and Argemone. Fourth, free central placentation: a round ovary with no walls inside and a tan column in the middle that carries five ovules, as in Dianthus and primrose. Fifth, basal placentation: a long section of an ovary shaped like a pentagon, with a tan placenta at its base and a single ovule on it, as in sunflower and marigold.
Marginal: the placenta is a ridge along one side of a one-chambered ovary. The ovules form two rows on it, as in pea.
Axile: the placenta is on the central axis of an ovary with several chambers, as in China rose, tomato and lemon.
Parietal: the ovules grow on the inner wall of the ovary. There is one chamber, but a false partition may split it into two, as in mustard and Argemone.
Free central: the ovules sit on a central column, and there are no partitions, as in Dianthus and primrose.
Basal: the placenta is at the base of the ovary, and a single ovule is attached to it, as in sunflower and marigold.
Try it in Python
The program keeps a few flowers in a dictionary, with the symmetry and the position of the ovary. Then it says whether each ovary is superior, half inferior or inferior.
You should see that the plum has a half inferior ovary. The brinjal line says that it is actinomorphic, with a superior ovary.
Practise with new numbers
Common mistakes
Saying that a hypogynous flower has an inferior ovary. Hypogynous means superior. Epigynous means inferior.
Mixing up epipetalous and epiphyllous. Epipetalous stamens are attached to the petals. Epiphyllous stamens are attached to the perianth.
Counting two pollen sacs in an anther. The anther is bilobed and each lobe has two chambers, so there are four.
Mixing up gamo- and poly-. Gamosepalous sepals are united. Polysepalous sepals are free.
Thinking that parietal placentation means two chambers. The ovary has one chamber. A false partition may divide it.
Remember these
The four whorls, from outside to inside: calyx, corolla, androecium, gynoecium.
Actinomorphic: radial. Zygomorphic: bilateral. Asymmetric: no symmetry.
Hypogynous: superior ovary. Perigynous: half inferior. Epigynous: inferior.
Aestivation: valvate, twisted, imbricate and vexillary.
Stamens: epipetalous, epiphyllous; free, or united in one, two or many bundles.
Placentation: marginal, axile, parietal, free central and basal.
Check yourself
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
Name the four whorls of a flower, from the outside inwards.
A flower can be cut into two similar halves in only one vertical plane. What kind of symmetry is this? Give an example.
In a guava flower the thalamus has grown up around the ovary and fused with it. Name the position of the ovary, and say if the ovary is superior or inferior.
The stamens of a flower are joined into two bundles. What is this called, and in which flower is it seen?
Which kind of placentation has one ovule attached at the base of the ovary?
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