The flower: structure, importance and the site of sexual reproduction
Walk through any Indian flower market before Diwali and you meet heaps of orange and yellow marigold strung into garlands, jasmine woven into gajras for the hair, rose petals sold by the kilo for weddings and temples, and lotus buds for worship.
People have bred, traded and treasured flowers for thousands of years.
Yet none of those colours, scents and shapes evolved for us.
To the plant, a flower is a piece of reproductive equipment: everything about it serves the making of fruits and seeds.
This unit follows that equipment from the moment a bud forms to the moment a seed is shed.
You need to know the structure of a typical flower, why the flower is called the site of sexual reproduction in flowering plants (angiosperms), what flowers mean to people culturally, socially and economically, and how to draw and label a flower cut lengthwise, naming the androecium as the male organs and the gynoecium as the female organs.
Why flowers matter to people
Flowers carry meaning in almost every human culture.
They are used for decoration and beauty (aesthetic and ornamental value), in social events such as weddings, welcomes and farewells, in religious offerings, and in festivals.
They also stand for feelings: a bouquet for congratulations, a wreath for mourning, roses for affection.
In India, think of the marigold torans over doorways at Diwali, the lotus as a national symbol, tulsi and hibiscus offered at home shrines, and the gajra of jasmine (mogra).
There is an economic side too.
Growing flowers and ornamental plants for sale, as cut flowers, potted plants, seeds and bulbs, is called floriculture.
Marigold, rose, tuberose, chrysanthemum, gerbera and jasmine are grown commercially across India, and a farmer who grows them earns from a crop that is valued for its beauty, not for food.
Flowers also feed industries indirectly: honey comes from bees collecting nectar, perfumes and attars use petals, and almost every fruit and grain crop starts life as a flower.
Why biologists call the flower the site of sexual reproduction
To a biologist the flower is remarkable for two reasons.
It is a morphological marvel, because its parts are modified leaves arranged in tight whorls on a short axis, and an embryological marvel, because inside it two tiny gametophytes develop, gametes meet and an embryo begins.
Every step of sexual reproduction in an angiosperm happens in or on the flower: making pollen, making eggs, pollination, fertilisation and the growth of seed and fruit.
Two parts of the flower matter most for reproduction.
- The androecium is the whorl of stamens. It is the male reproductive organ. The pollen grains (male gametophytes) develop inside the anthers of the stamens.
- The gynoecium is made of one or more pistils (carpels). It is the female reproductive organ. The embryo sac (female gametophyte) develops inside the ovules, which are inside the ovary of the pistil.
The sepals and petals do not make gametes.
They protect the bud and attract pollinators, so they are often called accessory whorls, while the androecium and gynoecium are the essential whorls.
Structure of a typical flower
A flower sits on a stalk, the pedicel.
The swollen top of the pedicel is the thalamus (also called the receptacle).
The floral parts are borne on the thalamus in four whorls, from outside to inside:
Whorl
Units
Usual role
Calyx
sepals, usually green
protect the flower in the bud
Corolla
petals, often coloured
attract animal pollinators
Androecium
stamens; each a filament and an anther
male: makes pollen
Gynoecium
pistils; each a stigma, style and ovary
female: holds ovules
A flower with both stamens and pistils is bisexual (hermaphrodite), like the hibiscus, mustard or pea.
A flower with only one of them is unisexual: a staminate (male) flower or a pistillate (female) flower, as in maize, cucumber and papaya.
Whether flowers are bisexual or unisexual matters later, when you study how plants avoid self-pollination and how breeders cross them.
The flower in longitudinal section
The labelled diagram below is the drawing to learn.
It is a lengthwise cut through a bisexual flower, so you see the pistil in the middle and the stamens, petals and sepals on either side.
Read the diagram from the bottom up.
The pedicel ends in the thalamus.
The sepals are the outermost, smallest parts.
The petals are inside them.
Each stamen is a thin stalk (the filament) carrying a swollen anther at its tip.
In the centre stands the pistil: a swollen ovary at the base holding ovules, a slender style above it, and the stigma at the top where pollen lands.
How to draw it in an answer:
1. Draw the pistil first, in the centre, as a large oval ovary with a tube (style) rising from it and a knob (stigma) on top. Put a few small ovules inside the ovary.
2. Draw the thalamus as a broad base under the ovary and the pedicel below it.
3. Draw two or three stamens on each side, each a line ending in an oval anther, attached at the thalamus.
4. Draw the petals outside the stamens and the sepals outside and below the petals.
5. Label with straight ruled lines drawn outside the drawing, each line touching the right part, and write the labels horizontally.
Labels that are credited: stigma, style, ovary, ovule, anther, filament, petal, sepal, thalamus, pedicel.
Brackets for androecium (or stamen) and gynoecium (or pistil) are useful when a question asks you to show the male and female organs.
Connect it. The anther and the ovule are the two places where the gametophytes form.
The rest of this unit zooms into each in turn: the anther and pollen in the next two sections, the pistil and ovule after that, and the embryo sac inside the ovule.
Then the two meet in pollination and double fertilisation.
Where a term is given with a short lead-in and then a sentence in italics, the italic sentence is the wording to learn for the examination.
- Flower: Flowers are the seat of sexual reproduction in angiosperms.
- Androecium and gynoecium: In the flower, androecium consisting of stamens represents the male reproductive organs and gynoecium consisting of pistils represents the female reproductive organs.
- Floriculture: the commercial growing of flowers and ornamental plants for sale.
- Thalamus (receptacle): the swollen tip of the flower stalk on which the floral whorls are borne.
- Bisexual flower: a flower that has both stamens and pistils; a unisexual flower has only one of them.
- Essential whorls: the androecium and gynoecium, which take part directly in reproduction; the calyx and corolla are accessory whorls.
Draw a labelled diagram of the longitudinal section of a bisexual flower and label any six parts, including the two parts in which the male and female gametophytes develop.
(½ x 6) [3]
How to answer: draw the outline as in the labelled flower above: pistil in the centre, stamens on either side, petals and sepals outside, thalamus and pedicel below.
Labels that earn ½ each (any six): anther (½); filament (½); stigma (½); style (½); ovary (½); ovule (½); petal; sepal; thalamus.
The question asks for the parts where gametophytes develop, so anther (male gametophyte, pollen) and ovule (female gametophyte, embryo sac) must be among your six.
A label line that stops short of the part, or touches the wrong one (an "anther" line ending on the filament), earns nothing for that label.
No mark is given for the outline itself unless the question says so.
Read the following statements about a typical flower.
(i) The stamens together form the androecium, the male reproductive organ.
(ii) Ovules are borne inside the anther.
(iii) The thalamus is the swollen end of the pedicel on which the whorls are borne.
(iv) Sepals and petals are the essential whorls of a flower.
Which of the statements are correct?
(A) (i) and (ii)
(B) (i) and (iii)
(C) (ii) and (iv)
(D) (i), (iii) and (iv)
Judge each statement first:
1. (i) True: stamens make up the androecium, the male organ.
2. (ii) False: ovules are inside the ovary of the pistil; the anther holds pollen.
3. (iii) True: the thalamus (receptacle) tops the pedicel.
4. (iv) False: sepals and petals are accessory; the essential whorls are the androecium and gynoecium.
5. Only (i) and (iii) are true, so (A), (C) and (D) are out.
Answer: (B) / (i) and (iii) (1)
A Class XII student says that flowers are "useful only to bees".
Give two ways in which flowers are important to people in India, one social or cultural and one economic.
(1 + 1) [2]
Answer:
- Social or cultural: flowers are used in religious offerings, weddings, festivals and to express feelings such as affection, happiness or grief (for example marigold garlands and torans at Diwali) (1).
- Economic: floriculture, the commercial growing of flowers such as marigold, rose, tuberose and jasmine, gives farmers and traders an income; flowers are also the source of perfumes and, through bees, of honey (1).
A weak answer writes "flowers are beautiful and useful" for both marks.
It names no use, so it earns nothing.
Each mark needs one concrete use with an example.
Name the part of a flower that (a) receives pollen grains, (b) protects the flower in the bud, (c) bears the ovules, (d) holds the anther up.
(½ x 4) [2]
Answer: (a) stigma (½); (b) sepal / calyx (½); (c) ovary (½); (d) filament (½).
One-word answers are enough here.
"Pistil" for (c) is not precise enough: the ovules are in the ovary, one part of the pistil.
A flower diagram scores only through its labels, at ½ each.
Rule the label lines with a scale, keep every label outside the drawing on the left or right, and give exactly the number of labels asked.
If the question asks for the male and female parts, make sure the anther and ovary (or ovule) are among them.
- Calling the whole pistil the "ovary". The pistil has three parts, stigma, style and ovary; the ovary is only the swollen base. Mixing them up loses the label mark.
- Writing that petals are the female part because they surround the centre. Petals make no gametes; the female organ is the gynoecium, the male organ the androecium.
- Drawing the stamens growing out of the ovary. In a typical flower they arise from the thalamus (or from the petals in some species), not from the pistil.
- Treating "thalamus" and "pedicel" as one part. The pedicel is the stalk; the thalamus is its swollen tip that carries the whorls.
Check your understanding
1. The whorls of a flower that take part directly in sexual reproduction are
(A) calyx and corolla
(B) corolla and androecium
(C) androecium and gynoecium
(D) calyx and gynoecium [1]
2. In a longitudinal section of a mustard flower, the ovules are seen inside the
(A) anther
(B) stigma
(C) style
(D) ovary [1]
3. Assertion (A): The flower is called the site of sexual reproduction in angiosperms.
Reason (R): The sepals and petals of a flower protect the bud and attract pollinators.
Choose the correct option.
(A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation for Assertion (A).
(B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation for Assertion (A).
(C) Assertion (A) is true, but Reason (R) is false.
(D) Assertion (A) is false, but Reason (R) is true. [1]
4. Assertion (A): The petals of a flower are its female reproductive organs.
Reason (R): The gynoecium, made of one or more pistils, is the female reproductive organ of a flower.
Choose the correct option.
(A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation for Assertion (A).
(B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation for Assertion (A).
(C) Assertion (A) is true, but Reason (R) is false.
(D) Assertion (A) is false, but Reason (R) is true. [1]
5. Draw a neat diagram of the longitudinal section of a flower and label the part that receives pollen, the part that holds the ovules, the stalk of the stamen and the swollen top of the pedicel. Name the parts of this flower that will become the fruit and the seeds after fertilisation. (½ x 4 + ½ + ½) [3]
6. A marigold grower near Bengaluru sells most of her crop as garlands during the festival season. Mention one social or cultural use and one economic use of flowers shown by this example, and name the term for growing flowers commercially. (1 + ½ + ½) [2]
7. Attempt either option (A) or (B):
(A) Differentiate between the androecium and the gynoecium with respect to (i) the units they are made of and (ii) the gametophyte that develops in them. [2]
OR
(B) Differentiate between a bisexual and a unisexual flower, giving one example of each. [2]
8. A papaya plant bears flowers that have pistils but no stamens. Explain why such a flower can still form seeds, and state what it needs from another plant to do so. [2]
Answers
1. (C) / androecium and gynoecium (1)
2. (D) / ovary (1)
3. (B) / Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation for Assertion (A). (1) Both statements are true, but protecting the bud and attracting pollinators is not why the flower is the site of sexual reproduction; the reason is that the pollen grain (male gametophyte) forms in its anther and the embryo sac (female gametophyte) in its ovule.
4. (D) / Assertion (A) is false, but Reason (R) is true. (1) Petals are accessory whorls; the female organ is the gynoecium, so A is false and R is true.
5. Diagram as in the labelled flower above; labels ½ each: stigma, ovary, filament, thalamus (½ x 4). No marks for a label line that does not touch the named part. The ovary becomes the fruit (½); the ovules become the seeds (½).
6. Social or cultural: garlands for festivals, worship, weddings or decoration (1); economic: income from selling the flowers (½); floriculture (½).
7. (A) Any two pairs, 1 each:
Androecium
Gynoecium
made of stamens
made of one or more pistils (carpels)
male gametophyte (pollen grain) develops in the anther
female gametophyte (embryo sac) develops in the ovule
OR (B):
Bisexual flower
Unisexual flower
has both stamens and pistils (1)
has only stamens or only pistils (1)
Examples: bisexual, hibiscus / mustard / pea (½); unisexual, papaya / maize / cucumber (½).
8. The gynoecium (pistil) contains the ovary with ovules, and each ovule holds an embryo sac with an egg, so the flower has all it needs on the female side to form seeds (1); it needs pollen from the stamens of a male papaya plant to reach its stigma (1).