Chapter 4: The Flower
In the 1690s, a professor of medicine at Tübingen in Germany noticed something odd about a mulberry tree growing near his garden. It flowered every year and it carried fruit every year, yet the fruit held no good seeds. There was no other mulberry tree anywhere near it.
Rudolf Jakob Camerarius began to wonder whether flowers, like animals, needed something male and something female before a seed could form. So he went looking for a plant on which he could test the idea. He chose the castor plant, which carries its dust-shedding flowers and its seed-forming flowers separately on the same shoot. Before the dust could be released, he cut the dust-shedding flowers away.
The plant went on growing. It still produced the other kind of flower. But when the season ended, the capsules that formed were empty — no seeds inside. He repeated the removal on other plants and saw the same empty result each time. In 1694 he wrote up his observations in a letter that is still remembered, and drew the conclusion the empty capsules pointed to: the fine dust from those flowers must reach the other flowers, or no seed is made.
A flower, he had shown, is not simply the prettiest part of a plant. It is where the plant reproduces.
Diagram
Fig. 4.0-1 — A Castor Shoot Bearing Two Kinds Of Flowers Line drawing of a single upright castor shoot with its flower cluster; show the dust-shedding flowers grouped in the lower part of the cluster and the seed-forming flowers in the upper part, with a young three-lobed capsule below; label upper flowers, lower flowers, developing capsule, leaf, stem.
4.1 Structure Of A Bisexual Flower
Reproduction is the one function no species can do without. An organism that fails to reproduce may live a full life, but it leaves nothing behind it. In flowering plants, this work is done by the flower.
The flower is usually the most noticeable part of a plant. Its size, colour and scent are what we notice first. All three, as you will see, exist for reasons that have nothing to do with our liking them.
4.1.1 The Flower As A Modified Shoot
A shoot is a stem bearing leaves. In a flower, the stem is very short and the leaves are no longer leaves at all. They have been altered in shape, in colour and in job.
That is why a flower is described as a modified shoot. Reading it this way explains something you will meet again and again in this chapter: floral parts are arranged in rings around a central axis, exactly as leaves are arranged around a stem.
4.1.2 The Four Floral Whorls
From The Stalk Upwards
A flower is usually held away from the stem on a stalk. This stalk is the pedicel. It lifts the flower clear of the leaves, where insects and wind can reach it.
Some flowers have no stalk at all and sit directly on the stem. Such a flower is described as sessile.
The tip of the pedicel is often widened into a small cup or disc. This swollen tip is the receptacle, also called the thalamus. Every floral part is attached to it, so it works rather like a base plate.
The floral parts are borne on the thalamus in four rings, or whorls. The order in which they sit, from the outside inwards, is fixed. The order of the whorls, from the outside in, is set out in FC 4.1.
Flowchart
FC 4.1 — Floral Whorls From The Outside Inwards Pedicel → thalamus → first whorl: calyx (sepals) → second whorl: corolla (petals) → third whorl: androecium (stamens) → fourth whorl: gynoecium (pistil, made of carpels)
The First And Second Whorls
The outermost whorl is made of small green parts called sepals. Taken together, the sepals form the calyx. Being outermost, they are the parts that enclose the flower while it is still a bud.
Inside the calyx lies the second whorl, made of the petals. Petals are usually larger than sepals and brightly coloured. All the petals together form the corolla.
The Third Whorl — The Male Parts
The third whorl is made of long thread-like structures that project outwards, each ending in a swollen tip. Each of these is a stamen, and the stamens together form the androecium. The male parts of a flower are marked by the symbol ♂.
A stamen has two parts. The thread-like stalk is the filament, and the two-lobed tip it carries is the anther.
The Fourth Whorl — The Female Parts
At the very centre stands the pistil, the fourth whorl. The female parts of a flower are marked by the symbol ♀.
The pistil may be a single female unit, called a carpel, or several carpels fused together. When several carpels make up the pistil, they are together called the gynoecium.
Each carpel has three parts, one above the other. The swollen base is the ovary, the slender middle stalk is the style, and the tip is the stigma. The whole arrangement of a bisexual flower is shown in Fig. 4.1-1.
Diagram
Fig. 4.1-1 — Generalised Arrangement Of The Parts Of A Bisexual Flower Vertical section through a generalised bisexual flower, cut through the middle so that the whorls are seen one inside the next; label pedicel, receptacle, sepal, petal, stamen with its filament and anther marked separately, and pistil with its ovary, ovule, style and stigma marked separately; bracket the filament and anther together as one stamen and bracket the ovary, style and stigma together as the pistil; keep all lead lines straight and non-crossing.
4.1.3 Complete And Incomplete Flowers
Not every flower carries all four whorls. This gives us a simple way of sorting flowers into two groups.
A flower that contains all four floral whorls is a complete flower. It is also called a perfect flower.
If one or more whorls are missing, the flower is an incomplete flower, also called imperfect. Note that the test is about whorls being absent, not about which whorl is absent — a flower with no petals and a flower with no stamens are both incomplete.
4.1.4 Essential And Non-Essential Parts
The four whorls do not all do the same kind of work. Two of them make the cells that join to form a seed. The other two do not.
The essential parts of a flower are the parts directly concerned with reproduction. These are the stamens, which are the male parts, and the carpels, which are the female parts.
The non-essential parts are the helping parts. They either protect the reproductive parts or make the flower attractive so that pollination can take place. These are the sepals and the petals. The two groups are compared in Table 4.1.
The word ‘non-essential’ is easily misread. It does not mean useless — a flower stripped of its sepals as a bud, or of its petals in the open, is a flower in trouble. It means only that these parts do not themselves make gametes.
4.1.5 Tepals And The Perianth
In some flowers the sepals and the petals look so alike that you cannot tell one whorl from the other. Where this happens, the parts are not called sepals or petals at all. They are called tepals, and all the tepals together form the perianth.
A perianth is described by the colour it takes. When it is green, like a set of sepals, it is a sepaloid perianth. When it is not green, it is a petaloid perianth.
4.1.6 Bracts
A flower does not always arise bare from the stem. It may arise in the axil of a small leaf-like structure — the angle between that structure and the stem. Such a structure is a bract.
Some bracts are green and look like ordinary small leaves. Others are large and brightly coloured, and these are the ones that mislead the eye. In Bougainvillea, the coloured parts most people call petals are in fact three broad bracts, as Fig. 4.1-2 shows.
The real flowers of Bougainvillea are small, narrow and cylindrical, and sit attached to the inner face of each bract. Once you know where to look, they are easy to find.
Diagram
Fig. 4.1-2 — Bracts Of Bougainvillea Line drawing of one Bougainvillea unit: three large, broad, papery bracts arranged around three small tubular flowers, each flower attached to the inner face of a bract; label bract, flower, stem.
4.1.7 Nectaries
Most flowers produce a sweet, fragrant liquid called nectar. It is secreted by groups of cells called nectaries, which usually lie at the base of the pistil or at the bases of the petals.
Nectar is food, and that is the whole point of it. Insects such as honeybees come to the flower for the nectar and leave carrying pollen, which is how pollen reaches another flower of the same kind.
In some plants the nectary is large enough to see without a lens. Nasturtium is a good example, and Fig. 4.1-3 shows one of its flowers cut down the middle so that the nectar-holding spur is visible at the bottom.
Diagram
Fig. 4.1-3 — Nectary Of A Nasturtium Flower Line drawing of a nasturtium flower cut medially from top to bottom, showing the long backward-pointing spur at the base with clear liquid collected in it; label petal, sepal, stalk, nectary.
4.1.8 Sexuality In Flowers
4.1.8.1 Bisexual, Unisexual And Neuter Flowers
Before flowers can be sorted by sex, two structures must be named. The anthers of the stamens produce pollen, a fine powder whose grains carry the male cells, or male gametes. The ovary of the carpel contains the ovule, and each ovule encloses an egg cell, the female gamete.
So the stamens are the male parts of the flower and the carpels are the female parts. A flower may carry both, or only one, or neither. The three possibilities are set out in FC 4.2.
Flowchart
FC 4.2 — Flowers Sorted By Their Essential Whorls Flower → both stamens and carpels present → bisexual (hermaphrodite) flower Flower → only one of the two present → unisexual flower → branch 1: only stamens → male (staminate) flower; branch 2: only carpels → female (pistillate) flower Flower → neither present → neuter flower
A flower that contains both stamens and carpels is a bisexual or hermaphrodite flower. Hibiscus, rose, lily and tomato are examples.
A flower that has only one of the two is a unisexual flower, and it is therefore also an incomplete flower. Papaya and palm bear unisexual flowers.
Unisexual flowers are further named by the whorl they do carry. One with only stamens is a male or staminate flower. One with only carpels is a female or pistillate flower.
A neuter flower is one in which both male and female reproductive parts are lacking. Such a flower can never form a seed, which raises an obvious question — why does the plant make it at all? In the sunflower, the large strap-shaped florets around the rim are sterile, and their work is to advertise: they make the head conspicuous, while the small fertile florets packed in the centre do the seed-making.
4.1.8.2 Monoecious And Dioecious Plants
In most plants the flowers are bisexual, and both kinds of essential part sit in the same flower. In many others the male and female flowers are separate. Where they are separate, the plant may be of either of two types.
In monoecious plants, male and female flowers grow on the same plant. The word says so: mono- means one and oecium means house, so both kinds of flower live in one house. Maize, cucumber and pumpkin are monoecious.
In dioecious plants, male flowers and female flowers grow on different plants. Here di- means two, so the two kinds of flower live in two separate houses. Palm and papaya are dioecious.
This has a practical consequence a gardener notices at once. A single dioecious plant standing alone can never set fruit with seeds, because there is no partner nearby to supply pollen.
4.2 General Description Of The Floral Parts
You have met the four whorls in order. We now go through them one at a time and look at how each varies from plant to plant, and at what each is for.
Two words will keep coming back. Poly- means many and points to parts that are separate from one another. Gamo- means joined and points to parts that are fused together.
4.2.1 Calyx
There are usually five sepals in a flower. In some plants there are fewer, and in a few there are more.
The sepals may be separate from one another or joined. When they are free, the calyx is described as polysepalous. When they are fused into a cup or tube, it is gamosepalous. Both conditions are drawn in Fig. 4.2-1.
Diagram
Fig. 4.2-1 — Free And Fused Types Of Calyx Three line drawings side by side of a calyx alone, with the rest of the flower removed: (a) five separate spreading sepals joined only at the base; (b) sepals fused into a smooth cup; (c) sepals fused into a tube with the tips still free and pointed; label (a) polysepalous, (b) and (c) gamosepalous.
Some flowers carry a second, outer set of sepal-like parts below the calyx. These extra parts are called episepals, and together they form an epicalyx. Hibiscus is a familiar plant that has one.
When the flower opens, the sepals do not all behave in the same way. In some plants they fall off; in others they persist and stay attached even as the fruit develops.
Sepals are usually green, but not always. Brightly coloured sepals are called petaloid sepals, because they look like petals. Those of the Gul Mohur are red.
What The Calyx Does
The calyx is outermost, and its work follows from that position. It encloses and protects the young flower bud while the delicate parts inside are still forming.
Green sepals do a second job as well. Being green means they contain chlorophyll, so they carry out photosynthesis like an ordinary leaf.
4.2.2 Corolla
The petals are generally arranged in a single whorl. In some plants there is a double whorl, as in the poppy, and in others the petals are arranged in a spiral, as in the water lily.
Like sepals, petals may be separate or joined. Free petals make a polypetalous corolla, and united petals make a gamopetalous corolla, which often forms a tube. Fig. 4.2-2 shows one free and two joined forms.
Diagram
Fig. 4.2-2 — Kinds Of Corolla Three line drawings of whole flowers seen from the side: A, a flower with four broad separate petals spreading from a narrow base; B, a flower whose petals are fused into a narrow tube with small free lobes at the top; C, a flower whose petals are fused into a wide funnel; label A free, B tubular, C funnel shaped.
Petals are the advertising whorl. Their colour and their fragrance together attract insects, and insects carry pollen from flower to flower.
They protect as well. Where the petals are joined into a tube, that tube encloses the stamens and the pistil and shields them.
4.2.3 Androecium
4.2.3.1 Stamen And Pollen Grains
The number of stamens differs widely between plants. Some flowers carry only a few; others carry a very large number.
Each stamen is built of a long filament with an anther attached at its far end. The anther is usually two-lobed.
Each lobe of the anther holds two pollen sacs, so a single anther has four in all. Inside the pollen sacs the pollen grains are formed and stored, as Fig. 4.2-3 shows.
When the pollen is fully matured, the pollen sacs rupture and the grains are set free. This is the moment the anther has been building towards; until it happens, the pollen is sealed away and can reach nothing.
Pollen grains are fine powdery particles. Under a microscope, grains from different plants are seen to differ in shape, in size and in the sculpturing of their surface.
Diagram
Fig. 4.2-3 — Anther And Pollen Grains Two panels. A: transverse section of a two-lobed anther held on its filament, cut to show four pollen sacs with rounded pollen grains inside them; label pollen sac, pollen grains, filament. B: a ring of eight separate pollen grains from different plants, drawn to show differences in outline and surface — smooth, spiny, ridged, pitted, oval and three-lobed forms; label as pollen grains from different plants.
4.2.3.2 Cohesion Of Stamens
Stamens may stand quite separate from one another, or their filaments may be joined into bundles. Joining of like parts in this way is called cohesion.
When the stamens are free, the condition is polyandrous, as in Petunia. When they are joined, they may be organised into one, two or several groups, all three of which are drawn in Fig. 4.2-4.
- Monadelphous — the filaments are united into one group, and only the anthers stay free. The joined filaments form a staminal tube, as in the china rose and in cotton.
- Diadelphous — the filaments are united in two bundles. In the pea, nine of the ten stamens form a staminal tube and the tenth stands free.
- Polyadelphous — the filaments are united in several groups, as in Bombax.
Diagram
Fig. 4.2-4 — Cohesion Of Stamens Three line drawings of the androecium alone, the rest of the flower removed: (a) all filaments fused into a single central tube with free anthers at the top; (b) filaments in two bundles, a large bundle of nine and a single separate stamen beside it; (c) filaments fused into several separate bundles arising from a common base; label (a) monadelphous, one group, (b) diadelphous, two groups, (c) polyadelphous, several groups.
4.2.4 Gynoecium
The gynoecium is also known as the pistil. It is composed of one or more carpels, and every carpel has the same three parts, one above the other. The whole arrangement, from gynoecium down to the ovules, is set out in FC 4.3.
Flowchart
FC 4.3 — Parts Of The Gynoecium Gynoecium (pistil) → one or more carpels → each carpel: stigma (top) → style (middle) → ovary (base) → cavity of the ovary, single or divided into locules → ovules attached to the placenta
4.2.4.1 Stigma And Style
The stigma is the knob-like part at the very top. It may be simple, or divided into two or more lobes, and in some plants it is spread out into a feathery shape.
Its surface is never smooth and bare. It is covered with hair or with glandular papillae, which are tiny sticky outgrowths.
That surface explains the stigma’s job. It is the landing place for pollen during pollination, and a sticky, hairy, spread-out top holds a grain that arrives instead of letting it blow away.
The style is the slender tubular stalk below the stigma. It connects the stigma to the ovary, and so it decides how far a pollen grain must travel after landing.
4.2.4.2 Ovary, Ovules And Placenta
The ovary is the swollen part at the base of the pistil. It is made of one carpel, or of many carpels fused together.
The cavity inside the ovary is not the same in every plant. It may be a single chamber, or it may be divided into several chambers called locules.
Each chamber holds a number of small rounded bodies. These are the ovules, and each of them holds the egg cell.
The ovules are not loose in the chamber. They are attached to the ovary wall by a tissue called the placenta, which holds them in position and connects them to the parent plant. Fig. 4.2-5 shows this attachment in section.
Diagram
Fig. 4.2-5 — Section Of An Ovary Two panels. A: longitudinal section of a whole pistil showing stigma, style and ovary with two ovules attached inside; label stigma, style, ovary, ovule, placenta. B: transverse section of an ovary divided into three locules, each holding two ovules attached to the wall; label locule, ovary wall, ovule, placenta. Keep all lead lines straight and non-crossing.
4.3 Inflorescence And Placentation
So far we have looked at one flower on its own. Most plants, however, do not carry their flowers singly. They carry them in groups, arranged on the stem in a definite pattern.
4.3.1 Inflorescence
The flowers of a plant are borne on an axis, and different plants arrange them on that axis in different ways. This manner of arrangement is called inflorescence.
Ways Flowers Are Arranged
The arrangement can be recognised by asking two things: where the flower is attached, and whether the flowers are spread out or gathered together. Four common patterns are drawn in Fig. 4.3-1.
- Singly, either at the apex of the main stem or at the tip of a lateral branch.
- Singly, in the axils of the leaves.
- Growing out from the axils of different leaves but reaching the same level, so that the flowers form a cluster.
- On an axis that is flattened sideways into a disc, as in the sunflower head you met earlier. Here the youngest flowers are in the centre and the oldest at the edge.
Diagram
Fig. 4.3-1 — Four Ways In Which Flowers Are Arranged Four simple line drawings of shoots in a row. (a) A shoot with a single flower at the tip of the main stem. (b) A shoot with single flowers in the axils of the leaves along its length. (c) A shoot whose flower stalks arise from the axils of leaves at different heights but end at the same level, forming a flat-topped cluster. (d) A shoot ending in a flattened disc crowded with small flowers, the smallest at the centre and the largest at the rim. Label each drawing (a) to (d).
4.3.2 Placentation
You have already met the placenta as the tissue that attaches the ovules to the wall of the ovary. Just as flowers are arranged in a pattern on the stem, the ovules are arranged in a pattern inside the ovary, and that pattern differs from plant to plant. It is called placentation.
From Ovule To Seed
The pattern is easiest to see after the flower is over. As the ovary matures it becomes the fruit, and the ovules inside it become the seeds.
So the seeds of a fruit sit exactly where the ovules sat. Cut open a pea pod, a tomato, a mango or a lotus and the arrangement of the seeds shows you the placentation of the flower that made it, as in Fig. 4.3-2.
Diagram
Fig. 4.3-2 — Seeds Lying Where The Ovules Lay Two line drawings side by side. A: a pea pod split open along one side and laid flat, showing round seeds attached in a single row to one edge of the pod; label pod wall, seed, point of attachment. B: a tomato cut across, showing chambers with seeds attached to a central fleshy mass; label fruit wall, chamber, seed, central tissue.
4.4 Summary
Reproduction keeps a species in existence, and in flowering plants the flower is the organ that carries it out.
A flower is a specialised shoot in which the leaves are modified into floral structures.
The flower is usually held on a stalk called the pedicel; a flower without a stalk is sessile. The widened tip of the pedicel is the receptacle, or thalamus, on which all floral parts are borne.
The floral parts are arranged in four whorls. From the outside inwards these are the calyx of sepals, the corolla of petals, the androecium of stamens (♂) and the pistil or gynoecium of carpels (♀).
Each stamen has a filament and an anther. Each carpel has a basal ovary, a middle style and a terminal stigma.
A flower with all four whorls is complete, or perfect. A flower with one or more whorls missing is incomplete, or imperfect.
The essential parts are the stamens and the carpels, which are directly concerned with reproduction. The non-essential parts are the sepals and the petals, which protect the reproductive parts and attract pollinators.
Where sepals and petals cannot be told apart, the parts are called tepals and together form the perianth. A green perianth is sepaloid and a non-green perianth is petaloid.
A bract is a leaf-like structure in whose axil a flower arises. Bracts may be green, or large and coloured as in Bougainvillea, where they are easily mistaken for petals.
Nectaries are groups of cells, usually at the base of the pistil or of the petals, that secrete nectar. Nectar attracts insects such as honeybees, which then carry pollen between flowers.
Anthers produce pollen, which carries the male gametes. The ovary contains ovules, each enclosing an egg cell, the female gamete.
A flower with both stamens and carpels is bisexual or hermaphrodite. A flower with only one of them is unisexual, and therefore also incomplete; it is staminate if only stamens are present and pistillate if only carpels are present. A flower lacking both is neuter.
In monoecious plants the male and female flowers grow on the same plant. In dioecious plants they grow on different plants.
Sepals may be free (polysepalous) or fused (gamosepalous). An extra outer set of sepal-like parts forms an epicalyx. Brightly coloured sepals are petaloid sepals. Sepals protect the bud and, when green, photosynthesise.
Petals may be free (polypetalous) or fused into a tube (gamopetalous). Their colour and fragrance attract insects, and a fused corolla also shields the inner whorls.
An anther is usually two-lobed, each lobe holding two pollen sacs, four in all. On ripening, the sacs rupture and release the pollen grains, which are powdery and differ in shape and size between plants.
Free stamens are polyandrous. Joined stamens are monadelphous in one group, diadelphous in two groups, or polyadelphous in several groups.
The stigma is knob-like, often lobed or feathery, and covered with hair or papillae, so that it holds pollen landing on it. The style connects the stigma to the ovary.
The ovary is the swollen base of the pistil. Its cavity may be a single chamber or divided into locules, and the ovules within are attached to the ovary wall by the placenta.
Inflorescence is the mode of arrangement of flowers on the axis of a plant, and placentation is the manner in which ovules are arranged on the wall of the ovary.
As the ovary matures into the fruit, the ovules within it become the seeds, so the arrangement of seeds in a fruit reveals the placentation of the flower.
Flowchart
FC 4.4 — The Chapter In One Chain Flower (a modified shoot) → borne on pedicel → attached to thalamus → four whorls → non-essential: calyx (sepals) and corolla (petals) → essential: androecium (stamen = filament + anther → pollen sacs → pollen grains) and gynoecium (carpel = stigma → style → ovary → locules → ovules on placenta) → ovary matures into fruit and ovules become seeds
4.5 Key Terms
Androecium — the third whorl of a flower, made up of all its stamens.
Anther — the two-lobed tip of a stamen, in which pollen is formed.
Bisexual flower — a flower that carries both stamens and carpels; also called hermaphrodite.
Bract — a leaf-like structure in whose axil a flower arises; it may be green or large and coloured.
Calyx — the outermost whorl of a flower, made up of all its sepals.
Carpel — a single female unit of a flower, consisting of ovary, style and stigma.
Complete flower — a flower that contains all four floral whorls; also called perfect.
Corolla — the second whorl of a flower, made up of all its petals.
Diadelphous — the condition in which the filaments of the stamens are united into two bundles.
Dioecious plant — a plant in which the male and female flowers grow on different individual plants.
Epicalyx — an extra whorl of sepal-like parts, the episepals, lying outside the calyx.
Episepals — the individual sepal-like parts that together form an epicalyx.
Essential parts — the parts of a flower directly concerned with reproduction, namely the stamens and the carpels.
Female flower — a unisexual flower that carries only carpels; also called pistillate.
Filament — the thread-like stalk of a stamen, bearing the anther at its tip.
Flower — a specialised shoot in which the leaves are modified into floral structures.
Gamopetalous — the condition in which the petals are fused, often forming a tube.
Gamosepalous — the condition in which the sepals are fused into a cup or tube.
Gynoecium — the innermost whorl of a flower, made up of one or more carpels; also called the pistil.
Hermaphrodite flower — another name for a bisexual flower.
Incomplete flower — a flower in which one or more of the four whorls are missing; also called imperfect.
Inflorescence — the mode of arrangement of flowers on the axis of a plant.
Locules — the separate chambers into which the cavity of an ovary may be divided.
Male flower — a unisexual flower that carries only stamens; also called staminate.
Monadelphous — the condition in which the filaments of all the stamens are united into a single group.
Monoecious plant — a plant in which the male and female flowers grow on the same individual plant.
Nectar — the sweet, fragrant liquid produced by a flower, which attracts insects.
Nectaries — groups of cells, usually at the base of the pistil or the petals, that secrete nectar.
Neuter flower — a flower in which both the male and the female reproductive parts are lacking.
Non-essential parts — the sepals and petals, which protect the reproductive parts and attract pollinators but make no gametes.
Ovary — the swollen basal part of a carpel, enclosing the ovules.
Ovule — a small rounded body inside the ovary that encloses the egg cell and later becomes the seed.
Pedicel — the stalk that supports a flower.
Perianth — the whorl formed by tepals, where sepals and petals cannot be told apart.
Petaloid — describing a non-green perianth, coloured like petals.
Petaloid sepals — sepals that are brightly coloured instead of green.
Petals — the parts forming the corolla, usually large and brightly coloured.
Pistil — the female part of a flower, made of one or more carpels; another name for the gynoecium.
Pistillate flower — see female flower.
Placenta — the tissue that attaches the ovules to the wall of the ovary.
Placentation — the manner in which the ovules are arranged or attached on the wall of the ovary.
Pollen — the fine powder produced in the anthers, whose grains carry the male gametes.
Pollen sacs — the four chambers within an anther in which the pollen grains are formed and stored.
Polyadelphous — the condition in which the filaments of the stamens are united into several groups.
Polyandrous — the condition in which the stamens are free from one another.
Polypetalous — the condition in which the petals are free from one another.
Polysepalous — the condition in which the sepals are free from one another.
Receptacle — the widened tip of the pedicel on which the floral parts are borne; also called the thalamus.
Sepaloid — describing a green perianth, coloured like sepals.
Sepals — the parts forming the calyx, usually green and enclosing the bud.
Sessile — describing a flower borne directly on the stem, without a stalk.
Staminal tube — the tube formed when the filaments of stamens are fused together.
Staminate flower — see male flower.
Stamen — a unit of the androecium, made of a filament and an anther.
Stigma — the knob-like tip of a carpel, on which pollen lands.
Style — the slender stalk of a carpel, connecting the stigma to the ovary.
Tepals — floral parts in which sepals and petals cannot be told apart from one another.
Thalamus — see receptacle.
Unisexual flower — a flower that carries only stamens or only carpels.
Whorls — the rings in which the floral parts are arranged on the thalamus.
4.6 Points To Ponder
Petals and sepals are called non-essential parts, yet a plant spends a great deal of material and energy building them. If they take no part in making gametes, what is the plant buying with that expense?
A flower is described as a modified shoot, and its parts as modified leaves. What features of a flower still betray its leafy origin, and which parts have travelled furthest from a leaf?
A dioecious plant can never fertilise itself, while a bisexual flower carries both sexes in one place. What might a plant gain by giving up that convenience, and what does it risk?
Wind-pollinated grasses have small, dull, scentless flowers with long feathery stigmas, while insect-pollinated flowers are large, coloured and fragrant. Working only from what this chapter says about the jobs of each part, how would you explain each of those differences?
Both the stigma and the ovary receive something from outside the flower — one receives pollen, the other must eventually receive the male gamete. Why might it be an advantage for these two structures to be separated by a long style, rather than sitting one on top of the other?
If the coloured bracts of a Bougainvillea were removed but the small flowers left untouched, what would you expect to happen over the following weeks, and what would that tell you about the work bracts do?
4.7 Objective Questions
- The part of the flower on which all four whorls are borne is the: (1 mark)
- pedicel
- thalamus
- bract
- placenta
- A flower in which the filaments of the stamens are united into two bundles is described as: (1 mark)
- polyandrous
- monadelphous
- diadelphous
- polyadelphous
- The total number of pollen sacs present in a typical two-lobed anther is: (1 mark)
- one
- two
- four
- eight
- A flower that lacks both stamens and carpels is called: (1 mark)
- unisexual
- bisexual
- staminate
- neuter
- For questions a. and b., choose from the following options:
- Both A and R are true, and R is the correct explanation of A.
- Both A and R are true, but R is not the correct explanation of A.
- A is true but R is false.
- A is false but R is true.
- Assertion (A): Green sepals can carry out photosynthesis. Reason (R): Sepals are the outermost whorl of the flower and enclose the bud. (1 mark)
- Assertion (A): Every unisexual flower is an incomplete flower. Reason (R): A unisexual flower lacks one of the two essential whorls. (1 mark)
- Match the term in Column A with the correct description in Column B. (4 marks)
| Column A | Column B |
|---|---|
| a. Epicalyx | i. Tissue attaching the ovules to the ovary wall |
| b. Placenta | ii. Chamber of a divided ovary |
| c. Locule | iii. An outer whorl of sepal-like parts |
| d. Perianth | iv. Whorl in which sepals and petals cannot be told apart |
- Fill in the blanks. (4 marks)
- A flower borne directly on the stem, without a stalk, is said to be __________.
- Sepals fused into a cup make the calyx __________.
- The slender stalk connecting the stigma to the ovary is the __________.
- Male and female flowers borne on different plants describe a __________ plant.
- State whether each statement is true or false. (4 marks)
- The androecium is the innermost whorl of a flower.
- Nectaries may be situated at the bases of the petals.
- The prefix ‘gamo-’ is used when parts of a floral whorl are fused.
- A complete flower must be bisexual.
- Name the following. (4 marks)
- The collective name for all the petals of a flower.
- The leaf-like structure in whose axil a flower arises.
- The tube formed by the fusion of the filaments of stamens.
- The structure within the ovary that encloses the egg cell.
- Pick the odd one out in each group and give a reason for your choice. (3 marks)
- Sepal, petal, anther, tepal
- Stigma, style, ovary, filament
- Monadelphous, diadelphous, gamosepalous, polyadelphous
4.8 Check Your Concepts
Define inflorescence and placentation. (2 marks)
Name the four floral whorls in order from the outside inwards. (2 marks)
Give a reason for each of the following. (2 marks)
- The stigma is sticky and often feathery.
- The sepals of a bud are tough and green.
State two functions of the petals of a flower. (2 marks)
Why is a flower described as a modified shoot? (3 marks)
Explain, with one example each, the terms monoecious and dioecious. (3 marks)
Differentiate between the following, stating the basis of difference in each case. (4 marks)
- Essential parts and non-essential parts of a flower
- Polysepalous calyx and gamosepalous calyx
Describe the structure of a stamen, and explain how pollen grains are set free from it. (4 marks)
What would happen if the anthers of every flower on an isolated papaya plant bearing only pistillate flowers were the only source of pollen available? Explain your answer. (4 marks)
Describe the structure of a carpel from its base to its tip, stating one function of each of its three parts. (5 marks)
A gardener finds that a single plant standing alone in a courtyard flowers year after year but never sets seed. Suggest two different explanations from what you have learnt in this chapter, and state what the gardener could do in each case. (5 marks)
4.9 Figure-Based Questions
Study Fig. 4.1-1. (4 marks)
- Name the two parts that together make up one stamen.
- Name the whorl that lies immediately inside the calyx.
- State one function of the part labelled receptacle.
- Is the flower shown complete or incomplete? Give a reason.
Refer to Fig. 4.2-3. (3 marks)
- Name the structure of which panel A is a transverse section.
- How many pollen sacs are shown, and how are they distributed between the lobes?
- State one observation that panel B allows you to make about pollen grains.
Fig. 4.2-4 shows three arrangements of stamens. (3 marks)
- Name the condition shown in each of the three drawings.
- In which of them do the anthers alone remain free?
- Name one plant that shows the arrangement in drawing (b).
Redraw the transverse section of the ovary shown in Fig. 4.2-5 and label the following: ovary wall, locule, ovule and placenta. (3 marks)
Look at Fig. 4.3-1. (2 marks)
- Name the mode of arrangement of flowers that the whole figure illustrates.
- In drawing (d), where are the youngest flowers situated?