NCERT Solutions Class 9 Science Chapter 11 Reproduction How Life Continues

F R E E S T U D Y M AT E R I A L F O R E V E R Y S T U D E N T

CLASS 9 · SCIENCE

NCERT Solutions

Chapter 11: Reproduction: How
Life Continues

NCERT Textbook — Exploration

BOOK PAGES SECTIONS QUESTIONS MEDIUM

208 – 227 25 66 English

Solutions, notes, sample papers & more at 75 pages

Class 9 Science Chapter 11 Reproduction: How Life Continues AglaSem · NCERT Solutions

CLASS 9 · SCIENCE · EXPLORATION

NCERT Solutions — Chapter 11: Reproduction: How Life
Continues
Every organism dies, yet life does not stop — because every organism makes new individuals of its own kind
before it goes. This chapter follows that single idea through two very different routes: asexual reproduction,
which runs on mitosis and makes exact copies, and sexual reproduction, which runs on meiosis plus
fertilisation and makes individuals that are like their parents but never identical to them.

TEXTBOOK BOOK PAGES

Exploration (Class 9) 208 – 227

SECTIONS QUESTIONS

25 66

MEDIUM

English

Think It Over — Page 208
Chapter opener

THINK IT OVER

Q1 When does a farmer prefer asexual or sexual methods of reproduction for crops
production?

A farmer uses an asexual (vegetative) method when the plant must be an exact copy of a
chosen parent, and a sexual (seed) method when he wants seed that stores well, travels
cheaply and carries new combinations of characters.

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Class 9 Science Chapter 11 Reproduction: How Life Continues AglaSem · NCERT Solutions

FARMER METHOD CROPS REASON
WANTS… CHOSEN

Every plant identical Vegetative — Sugarcane setts, potato Only mitosis is involved, so all
to one elite mother cutting, grafting, tubers, banana offspring are clones: same
plant layering, tissue plantlets, grafted sweetness, same fruit size,
culture mango and rose, ginger same ripening date

A crop that sets no Vegetative only Banana, seedless grape, There is no other route — the
usable seed sugarcane plant cannot be raised from
seed at all

A quick harvest Vegetative Sugarcane, potato A cutting or tuber already has
nodes and stored food, so it
skips seed germination and
seedling stages

Cheap, storable, Sexual — seed Wheat, rice, maize, Seeds are small and dry; every
transportable planting mustard, pulses seed carries a fresh
material and new combination of characters, so
varieties better plants can be selected

Why it happens: the two routes differ in one step. In vegetative propagation the
new plant is built by mitosis from cells of the parent, so the chromosome sets are
copied without any reshuffling — the daughter plant is a clone. In seed production
the parent first makes gametes by meiosis (chromosome pairs separate
independently) and two gametes then fuse. That reshuffling is exactly what the
farmer wants when he is breeding, and exactly what he does not want when he is
multiplying a variety he has already perfected.

Tip: the convenience of clones has a price. A field of clones has no genetic variation,
so a disease that can attack one plant can attack every plant in it. That is why the
chapter recommends virus-free tissue-culture banana plantlets, and why a wise
farmer keeps more than one variety.

Q2 Why do you think most complex animals and flowering plants use sexual
reproduction, while many simple organisms, like yeast and hydra mainly reproduce
asexually?

Sexual reproduction buys variation; asexual reproduction buys speed. Complex organisms
need variation more, simple organisms need speed more.

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Class 9 Science Chapter 11 Reproduction: How Life Continues AglaSem · NCERT Solutions

Why complex organisms go sexual. A mango tree or an elephant lives for years, produces
relatively few offspring and meets a changing world — new pests, new diseases, changing
rainfall. Sexual reproduction gives every offspring a new combination of characters, so some
individuals in a family will happen to cope with the new condition.

number of gamete types from n chromosome pairs = 2n

in humans, n = 23 → 223 = 8,388,608 kinds of gamete

zygote combinations = 223 × 223 = 246 ≈ 7.0 × 1013

Seven lakh crore possibilities — and that is before counting the extra mixing the chapter hints
at. No two children of the same parents are alike, and that is the point.
Why simple organisms go asexual. A yeast cell in a warm sugar solution or a hydra in a pond
is small, short-lived and sitting in food. What matters there is filling the space before the food
runs out. Budding needs no partner, no gamete, no meeting of two cells — a single cell simply
divides by mitosis. A yeast cell can put out a bud within a couple of hours, so the population
climbs steeply while conditions last.

Why it happens: finding a mate and making gametes costs time and energy, and
only half of a parent's genes reach any one offspring. Asexual reproduction avoids
all of that. The cost is that all offspring are identical, so one unfavourable change
hits the whole population. A long-lived, slow-breeding organism cannot afford that
risk; a fast-breeding one can, because it turns over so many generations that it will
meet the next good patch anyway.

Did you know? Yeast and hydra are not only asexual. When food runs short or the
water cools, both can switch to sexual reproduction — exactly when variation
becomes worth its cost.

Activity 11.1 — Pages 209 – 210

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Class 9 Science Chapter 11 Reproduction: How Life Continues
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Let us explore — 11.1.1 How is vegetative propagation in plants helpful in agriculture?
co m
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ACTIVITY 11.1

.co a g l
a s emthe following points while observing the technique of cutting in the field. (i)
gl
Note
a
Q1
Does the gardener, scientist or horticulturist cut the overgrown branches of a plant
at the end of its growing season? (Different plants have different growing seasons).

. com
(ii) Observe them prepare the cuttings from a plant for the purpose of growing new
ag
a s
plants. Note the average length of emthe cuttings. (iii) Count the number of nodes and
internodes on the cuttings. gl
a
. c om
m and they
(i) Yes. Gardeners prune the overgrown branches at the end of the growingeseason,
s
o m
take their ccuttings gl a
. from that pruned material.
a
a s em(iii) — record what you actually measure. A typical shoot cutting is about 15 – 20 cm
l and carries 3 – 5 nodes, which means 2 – 4 internodes between them.
(ii) and
aglong
a s
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.(CM) agl
PLANT GROWING SEASON LENGTH OF CUTTING NODES INTERNODES
ENDS
se m
g l a
Money plant a
All year in warm places 15 3 2

m
.co
Rose After the winter flush 20 4 3

3se
m
(sett).co
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Sugarcane At harvest 30
g l a 2

m a
ase
agl
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Why the end of the growing season: by then the shoot has finished elongating and

a se
com l
has stored food in it, and its buds are mature but still dormant. A cutting taken then
. a g
e m
has enough reserve to live on until roots form, and it will not waste that reserve on
as taken in mid-growth is soft, watery and wilts
making new leaves first. Alcutting
g
before it can root. a

. c om
s
Why nodes are counted: a node is the point where a leaf ande man axillary bud are
. cattached, la that can make new
om and it is the only place on the stem where theagtissue
as em roots and new shoots is present. The internode between two nodes has none. So a
agl cutting with no node cannot become a plant, however long it is — the count of
c
m .
nodes, not the length, is what decides whether the cutting will take.
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Class 9 Science Chapter 11 Reproduction: How Life Continues AglaSem · NCERT Solutions

Tip: collect cuttings in the morning (step 2 of the activity). The stem is fullest of
water at that hour, before the day's transpiration has drawn it down, so the cutting
starts with the largest possible water reserve.

Q2 Insert the cuttings up to approximately half of their length in the soil mixed with
compost at an angle of about 45– 60° from the soil surface (Fig. 11.2). Water them
regularly and observe the change, if any.

slanting cut

soil surface 45°–60°

node

Fig. 11.2, page 209 — redrawn sketch: a stem cutting pushed into compost-mixed soil at
a slant, with about half its length below the surface. The buried part is shown through a
cut-away of the soil.

In about two to three weeks, roots appear from the buried nodes and new shoots push
out of the nodes above the soil. Compost is mixed into the soil because it holds moisture and
keeps the soil loose enough for young roots to push through.

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Class 9 Science Chapter 11 Reproduction: How Life Continues AglaSem · NCERT Solutions

Why slant the cutting instead of pushing it straight down: a slanted cutting
keeps a longer stretch of stem inside the soil for the same digging depth, so more
nodes are in contact with moist soil. For a cutting pushed to a depth h at an angle θ
to the surface, the buried length is

L = h ÷ sin θ

at θ = 45°: L = h ÷ 0.707 = 1.41 h

at θ = 60°: L = h ÷ 0.866 = 1.15 h

straight down (θ = 90°): L = h ÷ 1 = h

So planting at 45° buries about 40 per cent more stem than planting vertically to the same
depth — more nodes in the rooting zone, and the buried nodes stay in the warm, moist upper
layer of the soil instead of going down into cold subsoil.

Why the lower leaves are removed (step 3): leaves lose water by transpiration.
Until roots form, the cutting has no way of replacing that water, so every leaf left on
it is a leak. Removing the lower leaves cuts the loss; a few upper leaves are kept so
that photosynthesis can still supply sugar for root growth. Buried leaves would also
rot and invite fungi.

Check it yourself: plant two money-plant cuttings of the same length, one with all
its leaves and one with only the top two. The second usually roots first — evidence
that water loss, not leaf number, is the limiting factor at this stage.

Q3 Water the plant regularly and observe the growth of Plant B along with Plant A.
[Grafting]

Within two to three weeks the graft heals and the buds on the Plant B piece sprout. From
then on the shoot, the leaves and the flowers are all of Plant B's variety, while everything
below the graft joint stays Plant A. If Plant B was a yellow rose, the plant now carries yellow
roses on a wild-rose root system.

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Class 9 Science Chapter 11 Reproduction: How Life Continues AglaSem · NCERT Solutions

Why it works: the piece of Plant B (the scion) is a vegetative part, so it grows by
mitosis and keeps every character of Plant B exactly. It has no roots of its own, so it
must be joined to Plant A (the stock) in such a way that water and minerals can flow
up into it and sugars can flow down. That happens only if the cambium — the thin
dividing layer just under the bark — of the two pieces touches. New cells from both
cambia meet, form a callus and then join up the xylem and phloem, so the two
stems become one plumbing system.

Why the other branches of Plant A are cut (step 4): a branch that is left on the stock
competes for the water and food coming up from the roots, and being already established it
wins. Removing those branches sends the whole supply into the scion.
Why the joint is wrapped (step 4): a fresh cut is an open wound. The cloth or film keeps it from
drying, holds the two pieces pressed together while the callus forms, and keeps insects and
fungal spores out until the wound is healed.

Tip: a grafted plant is genetically two plants living as one. If a shoot ever sprouts
from below the graft joint, it belongs to the wild stock and will carry the stock's
flowers, not the variety you wanted — cut it off as soon as you see it.

Q4 Water it regularly and observe the growth of new leaves on the twig buried in the
soil. After 10 – 15 days, the roots will develop from the area of the twig buried in the
soil. [Layering]

New leaves keep coming out on the buried twig, and after 10 – 15 days roots grow from
the buried portion. Once those roots are established, the twig is cut off from the parent
and becomes an independent plant — a clone of the parent tree.

Why layering is more reliable than a cutting: a cutting is on its own from the
moment it is separated — it must find water with no roots. A layered twig is still
attached to the parent the whole time, so water and minerals keep arriving from the
parent's roots and sugars keep arriving from the parent's leaves. It cannot wilt. That
is why the new leaves keep growing even before any roots have formed.

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Class 9 Science Chapter 11 Reproduction: How Life Continues AglaSem · NCERT Solutions

Why roots form exactly at the buried bend: two things meet there. The soil
provides darkness and steady moisture, which is what root primordia need to grow
out. And the bend slows the sugars moving down the phloem, so food collects in
that stretch of stem and is available to build roots. Nodes buried at that point are
where the roots actually emerge.

Try this: lemon, guava, jasmine, bougainvillea and pomegranate all layer easily.
Choose a thin, flexible twig, scrape a little bark off the part you bury (this slows the
sugar flow further), peg it under the soil and keep it watered — and be patient for a
fortnight.

Activity 11.2 — Page 211

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Class 9 Science Chapter 11 Reproduction: How Life Continues
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Let us explore — 11.1 Asexual Reproduction
co m
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m as
ACTIVITY 11.2

.co a g l
a s em
gl
Do you observe any small, round outgrowths (buds) emerging from the parent
a
Q1
yeast cells as shown in Fig. 11.6?

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e m . ag
g l as
a

co m
em.
m l as
m .co a g
l a se
a g
m a s
m.co agl
l a se
a g

co m
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Fig. 11.6 “Yeast with outgrowths”, page 211 — redrawn sketch of the micrograph: yeast

m l as
.co g
cells as they appear in a drop of the mixture under the compound microscope.

m a
l a se
ag
se m
com g l a
. a
Yes. Under the compound microscope you see oval yeast cells, and many of them carry a
m
ase
smaller round swelling attached at one end — the bud. Sometimes a bud already carries a bud

agl
of its own, so short chains of two, three or four cells are seen.

co m
.
sugar solution used = 1 g in 10 mL
em
m l as
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= 10 g per 100 mL = 10% (w/v)
a g
a s em sugar in the 20 mL taken = (1 g ÷ 10 mL) × 20 mL = 2 g

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Class 9 Science Chapter 11 Reproduction: How Life Continues AglaSem · NCERT Solutions

Why this set-up gives buds within an hour or two: yeast needs a respiratory
substrate, warmth and time. Two grams of sugar in the tube is far more than the
pinch of yeast can use up, so energy is not the limit. Warmth speeds up the enzymes
that drive respiration and cell division. The cotton plug lets carbon dioxide escape
while keeping other microbes out, and leaving the tube undisturbed lets the cells
settle and multiply without being knocked apart.

Tip: look at the edge of the drop under the coverslip, where the cells are spread in a
single layer. In the crowded middle the cells lie on top of one another and buds are
easy to miss.

Q2 Do these features indicate that the yeast is duplicating?

Yes — the yeast is multiplying, and each bud is a genetic duplicate of the parent cell. But
note carefully how it duplicates: the parent does not split into two equal halves. It pushes out a
small bud at one point on its wall.
The sequence is:

1. The parent cell's DNA is copied and its nucleus divides by mitosis.
2. A small outgrowth appears on the cell wall.
3. One of the two identical nuclei moves into the outgrowth, along with some cytoplasm.
4. A wall forms across the neck and the bud is pinched off. It then grows to full size and starts
budding itself.

Why the copies are identical: mitosis replicates every chromosome once and then
separates the two copies, so each daughter nucleus receives the same number of
chromosomes carrying the same information as the parent. There is no meiosis and
no second parent, so nothing is reshuffled and nothing new is added. The offspring
are therefore clones.

Tip: the word "duplicating" is a fair description of the genetic outcome but not of the
shape. Amoeba splits into two nearly equal cells (binary fission); yeast makes a small
bud on a large parent (budding). Both are asexual, both use mitosis, but the division
of the cytoplasm is equal in one and unequal in the other.

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Class 9 Science Chapter 11 Reproduction: How Life Continues AglaSem · NCERT Solutions

Q3 How do these observations help you in understanding reproduction in yeast?

They show, directly under your own microscope, that yeast reproduces asexually by
budding — from one parent, without gametes, without a partner, and very fast.

One parent. Every bud is attached to a single cell. No two cells were seen fusing, so no
gamete and no fertilisation is involved.
Genetically identical offspring. The bud is built by mitosis from the parent's own material,
so the population in the tube is a colony of clones.
Speed. Buds appear within 1 – 2 hours of the yeast becoming active. Since every new cell can
itself bud, the number of cells rises very steeply while sugar and warmth last.
Conditions matter. Nothing happened until the tube was warm and sugar was available —
asexual reproduction is fastest when the environment is favourable, exactly as the chapter
says.

Why this is worth knowing outside the laboratory: the same budding is what
makes dough rise and what ferments sugar in idli and dosa batter. Carbon dioxide
from the respiring, multiplying yeast is trapped in the dough and puffs it up. A warm
kitchen makes the batter rise quicker for exactly the reason your test tube had to be
kept warm.

In-text Questions — Page 211
11.1 Asexual Reproduction

Q1 Are there other methods of asexual reproduction in organisms?

Yes. Budding is only one of several. All of them share the same core — a single parent and
cell division by mitosis — and differ only in how the parent body is divided up.

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Class 9 Science Chapter 11 Reproduction: How Life Continues AglaSem · NCERT Solutions

METHOD WHAT HAPPENS ORGANISMS NAMED IN
THIS CHAPTER

Binary fission The parent cell divides into two nearly equal Bacteria, amoeba
daughter cells

Budding An outgrowth forms at one spot, grows and Yeast, hydra
separates

Spore formation Millions of light spores are made in a sac or on a Rhizopus, Aspergillus and other
vesicle and released into the air fungi

Vegetative A new plant grows from a stem, leaf or Potato, ginger, money plant,
propagation underground part of the old one sugarcane, Bryophyllum

Why they are all called asexual: not one of them involves a gamete or a fusion of
two cells. The chromosome number is never halved and never restored — it simply
stays the same at every division, because mitosis gives each daughter cell the same
set as the parent. That is why every product is a clone.

Q2 You have learnt that food like fruits get rotten by microbes (fungi). Where else do
we find these microorganisms?

Almost everywhere — wherever there is moisture, warmth and something organic to feed
on. The chapter itself points to several of these places.

In the air. Spores drift on air currents. This is the source that seeded your bread in Activity
11.3.
On cooked food left out. A day on the kitchen counter in warm weather is enough for a
smell and fuzzy patches to appear.
On damp walls. The black patches so often seen in the monsoon are fungal colonies.
In soil and in manure heaps. Fungi there break down dead leaves, straw and dung — this is
how compost forms.
On rotting fruit and vegetables, on old leather and damp clothes, and on our own skin and
inside our gut.

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Class 9 Science Chapter 11 Reproduction: How Life Continues AglaSem · NCERT Solutions

Why they are so widespread: a spore is lightweight and usually single-celled, so
even a slow air current lifts it. Millions are released from a single mould colony, so
the air over any inhabited place always carries some. They do not need to stay alive
and active while travelling — a spore can wait, dry and dormant, until it lands
somewhere moist and nourishing, and only then germinate.

Q3 Where do these microorganisms come from?

From other living microorganisms — from spores and cells that were already present in
the air, in dust, in water or on the surface. They are never produced by the non-living food
itself.

Why we are sure: for a long time people believed in spontaneous generation — that
maggots arose from meat and moulds from bread. Louis Pasteur settled the
question by experiment. He showed that a nutrient broth stays clear indefinitely if
microorganisms from the air are kept out of it, and clouds over as soon as they are
allowed in. So new life always comes from pre-existing life. From this came his germ
theory of disease — that particular microorganisms cause particular diseases —
and support for the cell theory, that all cells arise from pre-existing cells.

What follows from it in daily life: if microbes always come from outside, then keeping them
out, or killing the ones already present, must preserve food and prevent infection. That is why
we boil water, pressure-cook and sterilise surgical instruments, and why a hospital insists on
sterile dressings.

Tip: notice how carefully Activity 11.3 is designed around this idea. The cotton and
tissue are moistened with pre-boiled water, so the water carries no live microbes. Any
mould that then grows on the bread can only have come from the air — the activity
is a small Pasteur experiment of your own.

Activity 11.3 — Pages 211 – 212

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Class 9 Science Chapter 11 Reproduction: How Life Continues
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Let us experiment — 11.1 Asexual Reproduction
co m
e m.
m as
ACTIVITY 11.3

.co a g l
a s em three days, observe the surface of the bread or roti carefully using a
gl
After
a
Q1
magnifying glass. Do you notice the growth of mould?

com
m . ag
l a se
ag
Yes. By the third day the surface carries cottony white or grey threads, and on them tiny dark
pin-heads — black in Rhizopus, greenish-black in Aspergillus. As more days pass the patch
spreads and darkens.

co m
se m.
m l a
Why the moist chamber is built the way it is: mould spores need three things
o
.c and the chamber supplies each one. a g
m
together,

l a seMoisture — from the wet cotton and tissue paper. A dry spore cannot germinate;
a g
that is why you add a few drops of water if the bread begins to dry.

a s
com slowly or not at all, which is exactly why agl
Warmth, 25 – 35 °C — the temperature at which the fungal enzymes work
.
a s em
fastest. Below this the spores germinate

a gl
we refrigerate perishable food.
Nutrients — the starch in the bread or roti, which the hyphae digest and absorb.

. c om
The chamber is kept dark and away from direct sunlight because sunlight both dries
the bread and carries ultraviolet radiation that damages spores.em
c o m g l as
m . a
e
as Check it yourself: put an identical moist chamber into a refrigerator. After three
agl days it will show little or no mould, while the warm one is covered. Same bread,
se m
m single comparison is a fair test.
same water, same spores in the air — only the temperature differs, so temperature
coThat g l a
. a
em
must be what made the difference.
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Class 9 Science Chapter 11 Reproduction: How Life Continues AglaSem · NCERT Solutions

Q2 Observe the mould under the microscope and draw its diagram based on your
observations. Compare the diagram you have drawn with Fig. 11.8 and share your
observations with your classmates.

Sac Spores

Hyphae

(a) Rhizopus (b) Aspergillus

Fig. 11.8 “Fungi”, page 212 — redrawn sketch: (a) Rhizopus, with the labels Sac, Spores
and Hyphae as printed; (b) Aspergillus, whose stalks end in dark rounded heads.

Under the microscope, after staining with cotton blue, you see a mat of branched, thread-
like hyphae with upright stalks rising from it, each stalk ending in a rounded structure
packed with dark spores. Match what you have drawn against Fig. 11.8.

FEATURE RHIZOPUS — FIG. 11.8 (A) ASPERGILLUS — FIG. 11.8 (B)

Body Network of colourless branched Network of branched hyphae
hyphae spreading over the bread

Spore-bearing A closed round sac (sporangium) at the A swollen vesicle at the tip of a long stalk, with
structure tip of an upright stalk chains of spores radiating from it

Spores Tiny round spores packed inside the Tiny round spores in chains on the outside
sac; released when it bursts of the vesicle

Colour of the White cottony mat with black pin- Powdery, greenish-black or yellow-green
patch heads

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Class 9 Science Chapter 11 Reproduction: How Life Continues AglaSem · NCERT Solutions

sac (sporangium) chains of
spores inside spores

vesicle

stalk

hyphae
(a) Rhizopus (b) Aspergillus
The two moulds of Fig. 11.8 side by side. Both keep their spores high above the surface, but Rhizopus
encloses them in a closed sac while Aspergillus carries them in open chains on a swollen vesicle.

Why cotton blue is added: fungal hyphae are nearly colourless and almost invisible
against a bright field. Cotton blue is a dye that binds to the chitin of the fungal wall
and stains it deep blue, so the outline of the hyphae and the spores stands out
sharply.

Why the spore sacs are held up on stalks: a spore lying on the bread would go
nowhere. Raised 1 – 2 mm above the surface, it is in moving air, so when the sac
bursts the spores are carried off — which is precisely how the mould reached your
bread in the first place.

In-text Questions — Page 212
11.1 Asexual Reproduction

Q1 Do you observe thread-like structures with a round sac at the tip? Do you also see
tiny round structure (spores) inside the sac?

Yes to both. The thread-like structures are the hyphae, the round sac at the tip of an upright
hypha is the sporangium, and the tiny round bodies packed inside it are the spores.

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Class 9 Science Chapter 11 Reproduction: How Life Continues AglaSem · NCERT Solutions

STRUCTURE WHAT IT LOOKS LIKE WHAT IT DOES

Hyphae Fine branched threads Secrete enzymes onto the bread, digest the starch and
spreading over and into the absorb the products — this is how the fungus feeds
bread

Upright stalk A single hypha standing up Lifts the sac clear of the surface into moving air
from the mat

Sac Round, dark, pin-head sized Encloses and protects the spores until they are ready
(sporangium)

Spores Tiny round bodies, usually one Dispersal and reproduction — each can germinate into
cell each a whole new mould

Why this is asexual reproduction: a spore is made by mitosis from the parent
mould alone. It is not a gamete, it fuses with nothing, and the mould that grows
from it carries exactly the parent's chromosome set. One parent, mitosis, identical
offspring — the definition of asexual reproduction.

Q2 Where did the mould on your bread slice come from? It was not present when the
bread was fresh.

From spores that were already floating in the air of the room. They settled on the moist
bread, found warmth and nutrients there, and germinated. Nothing was added to the box
except bread, cotton, tissue and boiled water — so the air is the only possible source.

spores from one mould colony ≈ millions

each spore: usually single-celled and lightweight

→ carried far and wide by ordinary air currents

Why they were invisible and then suddenly obvious: a single spore is a few
micrometres across, far too small for the eye. It stays dormant while it is dry. The
moment it lands on something moist and nourishing it germinates, sends out a
hypha, and the hypha branches again and again. What you finally see after three
days is not one spore but a colony of millions of cells built from it — which is why the
growth seems to appear "out of nowhere".

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Class 9 Science Chapter 11 Reproduction: How Life Continues AglaSem · NCERT Solutions

Did you know? The same thing happens on rotting fruit and in a manure heap, as
the chapter notes. And it is a mixed blessing: fungi spoil our food, but the same fast
spore-based multiplication lets them break down organic waste and pollutants,
remove heavy metals from industrial waste, and give us antibiotics such as penicillin
and amoxicillin.

Bridging Science and Society — Page 212
Fungi and society

BRIDGING SCIENCE AND SOCIETY

Q1 Do you know any fungus that can degrade plastic?

Yes — several are known. The best-studied are:

Aspergillus tubingensis — a relative of the very Aspergillus in Fig. 11.8, isolated in 2017 from
a rubbish dump in Islamabad. It grows on polyester polyurethane and breaks it down in a
matter of weeks.
Pestalotiopsis microspora — found in the Amazon rainforest. It can live on polyurethane as
its only food, and remarkably it can do so even without oxygen, which is the condition deep
inside a landfill.
Aspergillus flavus and several Penicillium species — these attack polythene film, though
slowly.

Why a fungus can do this at all: a fungus feeds by secreting enzymes outside its
body and absorbing the small molecules that result — exactly what the hyphae did
to the starch of your bread. Plastics such as polyurethane and polyester are long
chains held together by ester and urethane linkages, which are chemically not so
different from the linkages in natural polymers. Fungal enzymes (esterases,
cutinases, laccases) can cut those linkages, chopping the chain into fragments small
enough for the hyphae to take in and respire. The plastic literally becomes the
fungus's food.

Tip: do not treat this as a solution to the plastic problem. It works on some plastics
and not others, it is slow, and it needs controlled warmth and moisture. Reducing,
reusing and segregating plastic is still far more effective than hoping a fungus will
eat it.

Page 18 of 75

as e
a
Class 9 Science Chapter 11 Reproduction: How Life Continues
g l AglaSem · NCERT Solutions

co m
m.
In-text Questions — Page 213
m as e
.co
11.2.1 How does meiosis help create variations in sexual reproduction?
a g l
a s em
aQ1gl How many combinations of characters can gametes carry?

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. ag

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A very large number — because each chromosome pair separates independently of every

agare n pairs, the number of different gametes is 2n.
other pair during meiosis. If there

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number of gamete types = 2n, where n = number of chromosome pairs
m as e
.co
n = 3 (Activity 11.4) → 23 = 8
a g l
s m
e23
g l a (human) → 223 = 8,388,608 ≈ 84 lakh kinds of gamete
a
n =

zygotes possible = 223 × 223 = 246 = 70,368,744,177,664 ≈ 7.0 × 1013
m a s
m .co agl
l a se
a g
Why the powers of 2 appear: take one pair at a time. A gamete gets either the
chromosome that came from the mother or the one that came from the father —
two choices. The next pair is decided quite separately, giving two more choices, and
co m
so on. Three independent two-way choices give 2 × 2 × 2 = 8 outcomes; twenty-three
m .
m as e
.co g l
of them give 223. And since a zygote is formed by one gamete from each parent, the
a
a s em counts multiply again.

agl
Tip: notice what this does not require. There is no need for anything unusual to
se m
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happen — no damage, no accident.oOrdinary l a
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meiosis, working normally, guarantees

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that no two of your gametes are
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in feature of sexual reproduction and not a rare event.

co m
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m 11.4 — Page 213 as e
Activity
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a c
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m a s e
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g l as
a

co m
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.co


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Class 9 Science Chapter 11 Reproduction: How Life Continues AglaSem · NCERT Solutions

Let us explore — 11.2.1 How does meiosis help create variations in sexual reproduction?

ACTIVITY 11.4

Q1 How many combinations can you make with just these three pairs of characters?

Eight. One bead is picked from each of the three pairs, and each pair offers two choices.

combinations = 2 × 2 × 2 = 23 = 8

NO. PAIR 1 (GREEN) — HAIR PAIR 2 (BLUE) — HAIR PAIR 3 (RED) — EYE
COLOUR TYPE COLOUR

1 light green — blonde light blue — straight light red — brown

2 light green — blonde light blue — straight dark red — black

3 light green — blonde dark blue — curly light red — brown

4 light green — blonde dark blue — curly dark red — black

5 dark green — black light blue — straight light red — brown

6 dark green — black light blue — straight dark red — black

7 dark green — black dark blue — curly light red — brown

8 dark green — black dark blue — curly dark red — black

Combination 1 in the list is the one drawn in Fig. 11.9 — blonde and straight hair with brown
eyes.

Why the beads model meiosis correctly: the three pairs sit on three different
chromosomes, and when the chromosome pairs line up in meiosis, the way one pair
happens to be oriented has no effect on how the next pair is oriented. Picking a
bead at random from each pair, independently, is exactly that. This independent
separation of pairs is what the chapter means when it says "the chromosomes of
each pair separate so that each gamete receives only one chromosome from each
pair".

Page 20 of 75

Class 9 Science Chapter 11 Reproduction: How Life Continues AglaSem · NCERT Solutions

Q2 Imagine how many combinations are possible with 23 pairs of chromosomes, each
carrying genetic information for many characters.

223 = 8,388,608 — about 84 lakh different gametes from a single person. And that is only one
parent.

gametes from the mother = 223 = 8,388,608

gametes from the father = 223 = 8,388,608

zygotes = 223 × 223 = 246

= 70,368,744,177,664 ≈ 7.0 × 1013 combinations

About seventy lakh crore — roughly ten thousand times the whole human population of the
Earth. The chance that two children of the same parents (other than identical twins) receive the
same chromosome combination is about 1 in 7 × 1013.

Why this matters for a species: each of those combinations is a slightly different
individual. When conditions change, some of these individuals happen to cope
better — people who tolerate low oxygen at high altitude, or who can still digest milk
as adults, are the two examples the chapter gives. Variation is not a defect in
copying; it is the raw material the species draws on to survive change, and over long
periods it is what drives evolution.

Tip: the real figure is even larger. The chapter's model counts only the independent
separation of whole chromosomes. Chromosomes also exchange pieces with their
partners before they separate, so the number of genuinely different gametes is far
beyond 223.

In-text Questions — Page 214

Page 21 of 75

Class 9 Science Chapter 11 Reproduction: How Life Continues AglaSem · NCERT Solutions

11.2.2 Sexual reproduction in flowering plants

Q1 Flowers also enhance the aesthetics of the plants as they are coloured and/or
fragrant. How are these features of flowers useful for reproduction?

Colour and fragrance are not decoration — they are advertising. They bring in the animals
that carry pollen from one flower to another, and without that carriage the pollen would
stay where it was made.

Colour is visible from a distance and tells a bee, butterfly or sunbird that food is available
here.
Fragrance works where colour cannot — at night, and inside dense foliage. Scent molecules
travel on air and can be followed by a moth from many metres away.
Nectar at the base of the petals is the reward. The insect comes for the nectar; it leaves with
pollen stuck to its body.

Why the plant must pay for a carrier at all: a plant is rooted. Its male gametes are
inside pollen grains sitting in an anther, and the female gametes are inside ovules in
an ovary, often on a different plant. Something has to move the pollen. Either the
wind does it free of charge — and then the plant must make five to ten lakh pollen
grains per flower to make up for how wasteful wind delivery is — or an animal does
it accurately, and the plant pays it in nectar. Colour and scent are the price of that
accuracy.

Tip: the argument runs in reverse too. Wheat, maize and rice flowers are small,
green and scentless — because they are wind-pollinated, so spending on petals and
nectar would buy them nothing.

Q2 Explore flowers and their buds on some plants. Record your observations in your
notebook. Do you observe thin, flat, green covering, and coloured projections
present in both?

Yes — both are present in the bud and in the open flower, but they are arranged
differently.

Page 22 of 75

Class 9 Science Chapter 11 Reproduction: How Life Continues AglaSem · NCERT Solutions

PART IN THE BUD IN THE OPEN FLOWER

Thin, flat, green covering Outermost, wrapped tightly over Spread out below the petals, still
— the sepals everything else the outermost whorl

Coloured projections — Folded and hidden under the sepals; often Fully expanded and displayed
the petals only a streak of colour shows

Why the order never changes: a flower is built as a set of whorls, one inside the
next — sepals outside, then petals, then stamens, then the pistil in the centre. The
bud is simply that same flower before the whorls have expanded. So a green
covering on the outside and colour hidden underneath is what you should expect in
every bud you open.

Try this: open buds of different ages on the same hibiscus or rose plant and lay
them in a row. You get a picture of the flower unfolding in stages, without waiting
for days.

Q3 Do you think that sepals protect flowers in the bud stage, along with other parts of
the flower when it blooms?

Yes. The sepals are the outermost whorl, and in the bud they completely enclose the petals,
stamens and pistil.

Against drying — the delicate parts inside are shielded from sun and wind until they are
ready.
Against rain — water would spoil pollen inside the anther before it can be shed.
Against insects — chewing insects would reach the developing ovules if the bud were open.

After the flower blooms the sepals do not fall off in most flowers. They stay below the petals and
hold the whole flower together on its stalk, supporting the parts from beneath.

Why sepals are green and petals are not: sepals contain chlorophyll and carry out
some photosynthesis, which is useful for a protective covering that must be tough
and self-supporting. Petals contain other pigments instead, which is what makes
them visible to pollinators but useless for making food. The colour difference follows
straight from the difference in job.

Page 23 of 75

as e
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Class 9 Science Chapter 11 Reproduction: How Life Continues
g l AglaSem · NCERT Solutions

co m
m.
Did you know? In a tomato or a brinjal you can still see the sepals sitting on the fruit

m attached at its base. as e
l
as the green "cap". The fruit is the ripened ovary, and the sepals that once guarded
. costill a g
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the bud are
a s
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ag
Do all flowers have these two parts? What are the other parts of a flower?
.
Q4

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No, not all flowers have both. A complete flower has four parts — sepals, petals, stamens and

co m
m.
pistil — but many flowers are missing one or more of them.

o m l a se
Grasses such as wheat, maize and rice have no showy petals at all; being wind-pollinated,
.c gain nothing from them. a g
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sesome flowers the sepals and petals are fused, or look alike and cannot be told apart.
they would

g l aIn
a A papaya tree bears male flowers on one plant and female flowers on another — the male

om
flowers have no pistil and the female flowers have no stamens.
a s
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The other two parts are the reproductive. cones: agl
a s
l WHAT IT DOES
PART SUB-PARTS ag

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Stamen — the Filament and The anther produces pollen grains, which contain the male gametes
male part anther
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Stigma, style and The stigma receives pollen; the style

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ovary ovary holds the ovules, each with an egg cell

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co m
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.co


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Class 9 Science Chapter 11 Reproduction: How Life Continues AglaSem · NCERT Solutions

petal

anther stigma

style

filament
ovary

ovules
sepal

The four whorls of a complete flower, following Fig. 11.10 and Fig. 11.11: sepals outermost, then
petals, then the stamens (filament + anther), and the pistil (stigma, style, ovary with ovules) in the
centre.

Activity 11.5 — Pages 214 – 215

Page 25 of 75

Class 9 Science Chapter 11 Reproduction: How Life Continues AglaSem · NCERT Solutions

Let us explore — 11.2.2 Sexual reproduction in flowering plants

ACTIVITY 11.5

Q1 Record the presence of various floral parts in the different flowers that you
collected in Table 11.1.
Table 11.1: Observation table to study the parts of a flower

S. FLOWER PRESENCE OF FLORAL OTHER GUESS THE
NO. PARTS PARTS IN DIFFERENT FEATURE(S) FUNCTION OF
FLOWERS FLOWER PART

A B C D

1. Sepal Yes

2. Petal Yes

3. Stamen No

4. Pistil Yes

Work outward to inward, one whorl at a time, and put a Yes or No against each part. The
book has already filled in column A for you — Sepal Yes, Petal Yes, Stamen No, Pistil Yes —
which tells you at once that flower A is a female (pistillate) flower, such as a papaya or a
pumpkin female flower.

S. FLOWER A (AS PRINTED — B— C— D — MAIZE
NO. PARTS A FEMALE HIBISCUS MUSTARD MALE FLOWER
FLOWER)

1. Sepal Yes Yes Yes No

2. Petal Yes Yes Yes No

3. Stamen No Yes Yes Yes

4. Pistil Yes Yes Yes No

Page 26 of 75

Class 9 Science Chapter 11 Reproduction: How Life Continues AglaSem · NCERT Solutions

Why a flower can be missing a whorl: the four whorls develop independently, so a
plant can leave one out. Where the male and female parts are in separate flowers —
as in maize, papaya and pumpkin — self-pollination becomes impossible or difficult,
and the plant is pushed towards cross-pollination and therefore towards more
variation. Where petals are missing, as in the grasses, it is because the flower is
wind-pollinated and has no use for them.

Tip: record the "Other feature(s)" column too — is the stigma sticky or feathery? are
the petals fused into a tube? is the anther powdery? Those details are what let you
predict the pollinating agent in Question 11 of the exercises.

Q2 Analyse the function of each part of the flower based on visible characters.

Every visible character of a flower part points to the job it does. Read the structure and the
function follows.

PART WHAT YOU CAN SEE FUNCTION YOU CAN INFER

Sepal Green, thin, flat, tough; outermost; Protects the bud; being green, it also
wraps the bud photosynthesises

Petal Coloured, large, often scented, nectar at Attracts pollinators and rewards them with
the base nectar

Stamen — Thin stalk holding the anther out and up Places the anther where a visiting insect or the
filament wind will reach it

Stamen — Swollen, powdery, yellow; powder rubs Produces the pollen grains, which carry the
anther off on the finger male gametes

Pistil — At the very top; flat, sticky and/or Catches and holds pollen; stickiness makes the
stigma feathery pollen stay put

Pistil — style Long thin tube connecting stigma to The path along which the pollen tube grows
ovary down to the ovule

Pistil — ovary Swollen base; cut it open and small Holds the ovules, each containing an egg; later
white ovules are inside becomes the fruit

Page 27 of 75

Class 9 Science Chapter 11 Reproduction: How Life Continues AglaSem · NCERT Solutions

Why the stigma sits at the top of a long style: two reasons at once. Raised above
the flower it is the first thing an arriving insect or a gust of wind touches, so it
intercepts pollen efficiently. And the length of the style acts as a test — only a pollen
grain of the right species can grow a tube all the way down it, so unsuitable pollen
never reaches the ovule.

Q3 Cut a transverse and a longitudinal section of the ovary (swollen base of the pistil)
and observe it under a dissecting microscope. Draw a diagram of the structure you
observed under the microscope.

Both cuts show the same thing from two directions — a hollow chamber with small white
ovules attached to the inner wall by short stalks.

Longitudinal section (L.S.) — cut from the stigma down through the ovary. You see the
ovary wall on both sides and a row of ovules standing along the inside, each on a short stalk.
In a hibiscus or a pea the ovules are lined up one above the other.
Transverse section (T.S.) — cut across the ovary. You see the ovary in cross-section, divided
by partitions into chambers (locules), with ovules in each chamber. A tomato ovary shows
two to four chambers; a mustard ovary shows two; a pea pod shows one.

Why you should cut a fruit of the same plant next: the fruit is the ripened ovary
and the seeds are the ripened ovules. So the number of chambers and the way the
seeds are attached in a tomato fruit is exactly the number of chambers and the way
the ovules are attached in a tomato ovary — just larger. Comparing the two, as the
chapter's project work asks you to do, is a direct demonstration that the ovary
becomes the fruit and the ovule becomes the seed.

Tip: the ovary of a small flower is only a few millimetres across. Use a fresh razor
blade, cut with a single stroke rather than sawing, and put a drop of water on the
section before looking at it. A jagged, dried-out cut shows nothing.

In-text Questions — Page 215

Page 28 of 75

as e
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Class 9 Science Chapter 11 Reproduction: How Life Continues
g l AglaSem · NCERT Solutions

11.2.2 Sexual reproduction in flowering plants
co m
e m.
m l as
Q1
.co a g
Can you guess the function of the most attractive part of a flower — the coloured
m
l a se
petals?

a g

. com
The petals attract the pollinator. They make the flower conspicuous from a distance, and in
ag
a s em and hold the nectar at their base.
many flowers they also carry the scent glands

agl
Why a plant invests so much in petals: the plant cannot move, so its pollen must

c om
be carried to another flower's stigma. Wind will do it, but wastefully — a wind-
.
s e
pollinated grass has to release five to ten lakh pollen grains per flower mto get 50 –
.
200 seeds.
gla kind to another,
comAn insect, by contrast, flies from one flower of theasame
m pollen it carries has a good chance of arriving somewhere useful. An insect-
soethe
s
a
gl pollinated sunflower gets 800 – 1,000 seeds from only 20,000 – 40,000 grains. The
a
petals are what buy that accuracy.

om a s
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m platform for the insect, many carry nectar guides agl
s
Petals also do other jobs: they form a landing

a g la to the nectar and, in passing, to the anthers and stigma
— lines or patches that point the visitor
— and at night they close in some plants and protect the inner parts.

co m
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Did you know? Bees see ultraviolet light, which we cannot. Many flowers that look

c o m g l as
plain yellow to us carry bold ultraviolet patterns that a bee sees as a bull's-eye
.
m the nectar. a
a s earound

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Q2
.
You have learnt that the transfer of pollen from stamen to stigma is essential for
m a
ase
the formation of fruits. How can you investigate it?

agl

. c om
By an experiment that takes away the pollen, or the visitors that carry it, and sees

m a s emon a pea plant.
whether fruits still form — which is exactly what Activity 11.6 does

. codesign has three moves: a gl
em
The

as
agl
1. Remove the stamens (emasculation) from some flowers, so that flower can supply no pollen
of its own.
.c
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m a
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2. Bag with muslin, so no pollen can arrive from outside. Muslin is used rather than polythene
. co
m
because air and light must still pass, otherwise you would be testing suffocation rather than

as e
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pollination.
a g

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m .
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Class 9 Science Chapter 11 Reproduction: How Life Continues AglaSem · NCERT Solutions

3. Keep controls — a flower bagged with its stamens intact, and a flower left completely open.
Without these you could not tell whether a failure was caused by the missing pollen or by the
bag itself.

Why only one thing may be changed at a time: a fair test needs every other
condition — same plant, same age of flower, same water, same sunlight, same day of
counting — to be identical across the treatments. Then any difference in fruit
formation can only be due to the one factor you altered, the availability of pollen.
This is the logic that will reappear in exercise Questions 10 and 12.

Activity 11.6 — Pages 215 – 216
Let us investigate — 11.2.2 Sexual reproduction in flowering plants

ACTIVITY 11.6

Q1 In which treatment(s) do you find the flowers are replaced by fruits?

Fruits form in every treatment except one — the bud whose stamens were removed and
which was then bagged.

TREATMENT COULD POLLEN REACH THE FRUIT FORMATION
STIGMA? (YES/NO)

Flower bud (bagged, stamens Yes — its own pollen, inside the closed Yes
intact) bud

Flower bud with stamens removed No — no pollen inside, none can enter No
(bagged)

Flower with stamens removed Yes — it had already pollinated itself Yes
(bagged) while still a bud

Flower (bagged, stamens intact) Yes — its own pollen Yes

Flower without bag Yes — its own pollen, and visitors Yes
could also come

Page 30 of 75

Class 9 Science Chapter 11 Reproduction: How Life Continues AglaSem · NCERT Solutions

Why removing the stamens from an open flower is too late: in pea, the petals
form a closed keel and the anthers shed their pollen straight onto the stigma while
the flower is still a bud. By the time the flower blooms, it has already been
pollinated. So the third treatment still gives a pod. Only in a juvenile bud, before the
anthers have opened, does removing the stamens actually take the pollen away —
and only that treatment fails to give a fruit. The pattern of results is not an accident
of the experiment; it is a direct fingerprint of how a pea flower pollinates itself.

Tip: this is why plant breeders emasculate flowers in the bud stage when they want to
make a controlled cross. Wait for the flower to open and the plant has already done
the job for itself.

Q2 What can we infer from this activity?

That the transfer of pollen grains from the anther to the stigma — pollination — is
necessary before a fruit can form. Take the pollen away and no fruit appears; leave it, whether
the flower is bagged or open, and a fruit does.
Two further conclusions follow from the same table:

The bag itself does not stop fruiting. The bagged flower with stamens intact fruited just as
well as the open one, so muslin bagging is not what caused the failure in the emasculated
bud.
Pea is normally self-pollinated, and self-pollination happens in the bud. That is the only
way an emasculated open flower could still make a pod.

Why pollination alone is not the whole story: pollination only delivers the pollen
to the stigma. What actually starts the fruit is what follows — the grain germinates,
grows a pollen tube down the style, and the male gamete fuses with the egg cell in
the ovule. That fusion is fertilisation. Only then does the ovary begin to enlarge into
a fruit and the ovules into seeds. Pollination is the necessary first step, not the
finished process.

In-text Questions — Page 216

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Class 9 Science Chapter 11 Reproduction: How Life Continues AglaSem · NCERT Solutions

11.2.3 How does the process of pollination occur in flowers?

Q1 The transfer of the pollen grains to the stigma is important for the formation of
fruits and seeds. Nature has provided plants with various strategies for pollination
to occur. What do you think will happen if pollination does not occur?

The whole chain that leads to a fruit stops at its first link. Follow it through:

no pollen on the stigma → no pollen tube down the style

→ the male gamete never reaches the ovule

→ no fertilisation → no zygote → no embryo

→ the ovules do not become seeds

→ the ovary does not enlarge → no fruit

The unpollinated flower simply withers and drops off the plant.
What this means beyond the single flower:

For the crop — no grain, no pulses, no fruit. The plant may look healthy and flower well and
still yield nothing.
For the next season — no seed, so nothing to sow.
For everything that eats those seeds and fruits — birds, animals and people lose the
food.
For the species — no new individuals from sexual reproduction, so no new combinations of
characters and no variation. A plant that can still spread vegetatively would survive, but only
as clones.

Why this is a live problem, not a hypothetical one: the exercise Question 12
describes exactly this happening — apple yields in the lower Himalayan region
falling because the population of natural pollinators has declined. The flowers open,
but too few of them are pollinated, so too few set fruit. Bringing in a bee colony
restores the missing link.

In-text Questions — Page 217

Page 32 of 75

Class 9 Science Chapter 11 Reproduction: How Life Continues AglaSem · NCERT Solutions

11.2.4 Pollination strategies and reproductive success

Q1 What is the approximate success rate of the common strategies — wind pollination
and insect pollination — with respect to the seed formation?

Measured as seeds formed per pollen grain released, wind pollination succeeds about
once in 6,000 grains and insect pollination about once in 33 grains — insect pollination is
roughly 180 times more efficient per grain. The figures come straight from Table 11.3.

success rate = seeds formed ÷ pollen grains released

Wind (maize, wheat): mid-values → pollen = (5,00,000 + 10,00,000) ÷ 2 = 7,50,000; seeds
= (50 + 200) ÷ 2 = 125

success rate = 125 ÷ 7,50,000 = 1.7 × 10−4 = 0.017%, i.e. about 1 seed per 6,000 grains

Insect (sunflower): mid-values → pollen = (20,000 + 40,000) ÷ 2 = 30,000; seeds = (800 +

1,000) ÷ 2 = 900

success rate = 900 ÷ 30,000 = 3.0 × 10−2 = 3.0%, i.e. about 1 seed per 33 grains

ratio of efficiencies = 3.0% ÷ 0.017% ≈ 180 times

Why wind is so wasteful: the wind does not know where it is going. A grain
released from a wheat anther travels wherever the air happens to move; the chance
that it lands on a stigma of its own species is tiny. An insect, in contrast, visits one
kind of flower after another of the same kind, because that is where it has learnt the
nectar is — so the pollen stuck to its body is delivered almost to the right address.
Accuracy is what the plant is buying when it spends on petals, scent and nectar.

Activity 11.7 — Page 217

Page 33 of 75

as e
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Class 9 Science Chapter 11 Reproduction: How Life Continues
g l AglaSem · NCERT Solutions

Let us find out — 11.2.4 Pollination strategies and reproductive success
co m
e m.
m as
ACTIVITY 11.7

.co a g l
a s em and analyse the two strategies in terms of (Table 11.3) — • Pollen to seed
gl
Compare
a
Q1
ratio • Efficiency of pollination and seed formation

m
Table 11.3: Pollen production and seed formation data

. co ag
se m
POLLINATION
l a
APPROXIMATE POLLEN
g RELEASED PER
ESTIMATED AVERAGE
STRATEGY
aGRAINS NUMBER OF SEEDS
FLOWER FORMED

co m
em.
Wind-pollinated grasses 5,00,000 – 10,00,000 50 – 200

m l as
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(e.g., maize, wheat)

a g
a s emInsect-pollinated plants
gl
20,000 – 40,000 800 – 1,000

a
(e.g., sunflower)

m a s
em
.co agl
s

a
gl about 6,000 pollen grains for every seed it sets; the
a
The wind-pollinated grass makes
insect-pollinated sunflower makes only about 33. Insect pollination is far more efficient
per grain, but wind pollination is far cheaper per grain.
co m
m .
a se
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SEED (MID-ag OF THE
POLLINATION POLLEN SEEDS POLLEN : RANGE SEEDS PER

m .
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STRATEGY GRAINS FORMED 1,00,000

agl
PER VALUES) RATIO GRAINS
FLOWER

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5,00,000 –
com
50 – 200
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.
Wind-pollinated 7,50,000 : 125 2,500 : 1 to about 17
grasses (maize, wheat) 10,00,000
e m
= 6,000 : 1 20,000 : 1 a
a s
Insect-pollinated
a gl
20,000 – 800 – 1,000 30,000 : 900 = 20 : 1 to 50 : about 3,000
plants (sunflower) 40,000 33 : 1 1

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m as e
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best case for wind = 5,00,000 ÷ 200 = 2,500 : 1 worst case = 10,00,000 ÷ 50 = 20,000 : 1
a
se m best case for insects = 20,000 ÷ 1,000 = 20 : 1 worst case = 40,000 ÷ 800 = 50 : 1
g l a
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efficiency ratio = 6,000 ÷ 33 ≈ 180

s e m
m a
Reading the comparison:
e m . co agl
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a

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m .
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.co


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Class 9 Science Chapter 11 Reproduction: How Life Continues AglaSem · NCERT Solutions

Pollen to seed ratio. Wind pollination needs about 180 times more pollen for each seed
produced. Even in the wind's best case (2,500 : 1) it is fifty times worse than the insect's worst
case (50 : 1).
Efficiency of pollination. Very low for wind — most grains land on soil, water, leaves or the
wrong species. High for insects, because a pollinator flies from one flower to another of the
same kind.
Efficiency of seed formation. The insect-pollinated sunflower forms 800 – 1,000 seeds from
a single flower head against 50 – 200 for a grass, so it converts a far larger share of its ovules
into seeds.

Why the structures match the strategies: a wind-pollinated grass has small, light,
smooth, dry pollen that stays airborne, and a long feathery stigma that acts like a
net across the air stream. An insect-pollinated flower has large, sticky or spiny pollen
that clings to a body, and a small sticky stigma that only needs to intercept what the
insect brings. Each design is exactly what its delivery system requires.

Q2 Explain why producing a very large number of pollen grains can still be an effective
pollination strategy.

Because what matters to the plant is not the success rate of one grain but the number of
seeds at the end — and a very small probability multiplied by a very large number still
gives a useful result.

expected seeds = N × p (N = grains released, p = chance one grain reaches a stigma of its

own species)

for a wheat flower: N = 7,50,000, p ≈ 1.7 × 10−4

N × p = 7,50,000 × 1.7 × 10−4 ≈ 125 seeds

The plant cannot raise p, because it cannot steer the wind. So it raises N instead. Four further
reasons make this a sound bargain:

A pollen grain is a cheap thing to make. It is microscopic, dry and light. Petals, scent
glands and nectar cost the plant far more per flower than a few lakh grains do — so a grass
spends on pollen and a sunflower spends on advertising, and both get seeds.
No partner species is needed. Wind blows everywhere, all the time. An insect-pollinated
plant fails when its pollinators disappear — exactly the apple orchard problem of exercise
Question 12 — while a wind-pollinated one is never left without a carrier.

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Class 9 Science Chapter 11 Reproduction: How Life Continues AglaSem · NCERT Solutions

The odds are improved by how these plants grow. Wheat, maize and rice stand in dense
stands of the same species, so a grain does not have to travel far to meet a stigma of its own
kind. In a crowded field the effective value of p is much better than it would be for a lone
plant.
The stigma is built to catch. A long, feathery stigma presents a large surface across the
moving air, which raises the capture probability without the plant having to attract anything.

Why "efficient" and "effective" are not the same word here: efficiency asks how
much is achieved per unit spent; effectiveness asks whether the job gets done. Wind
pollination is inefficient per grain and yet completely effective, because the grains
are cheap enough to be produced in the numbers the low odds demand. Wheat,
maize and rice — the three crops that feed most of the world — all use it.

Pause and Ponder — Page 217
11.2.5 Fertilisation and seed formation

PAUSE AND PONDER

Q1 In a china-rose (hibiscus or gudhal) plant, a pollen tube grows and continues
through the style after pollen lands on the stigma. Which process is about to
happen next?

Fertilisation. The pollen tube is on its way to the ovule, and the moment the male gamete
inside it fuses with the egg cell, fertilisation has taken place and a zygote is formed.

pollination → pollen germinates on the stigma → pollen tube grows down the style

→ tube enters the ovary and reaches an ovule

→ male gamete fuses with the egg cell = FERTILISATION

→ zygote → embryo; ovule → seed; ovary → fruit

Why a tube is needed at all: the male gamete has no tail and cannot swim, and the
egg cell is locked inside an ovule at the bottom of the ovary, several millimetres
below the stigma. The pollen grain solves this by growing a tube — a living extension
of itself — that pushes down through the tissue of the style and delivers the gamete
right to the ovule. In a plant, the male gamete does not travel; the tube carries it.

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Class 9 Science Chapter 11 Reproduction: How Life Continues AglaSem · NCERT Solutions

Tip: keep the two words apart. Pollination is only the transfer of pollen from anther
to stigma. Fertilisation is the fusion of the two gametes. Pollination can happen
without fertilisation following it — for instance if the pollen is of the wrong species
and cannot grow a tube down that style.

Q2 Look at the pictures (Fig. 11.16) of calotropis (madar) seeds and dandelion seeds
given below. Can you guess what kind of seed dispersal these seeds are adapted
for?

(a) Madar seeds (b) Dandelion seeds

Fig. 11.16, page 217 — redrawn sketch: (a) calotropis (madar) seeds spilling from a split
pod, and (b) a dandelion seed head on its stalk.

Both are adapted for dispersal by wind. Look at what Fig. 11.16 actually shows.

(a) Madar (calotropis) seeds — flat brown seeds, each carrying a dense tuft of long, fine,
silky white hairs at one end.
(b) Dandelion seeds — a whole spherical white head; each seed hangs below a stalk that
opens out at the top into a ring of feathery hairs, like a tiny parachute. In the picture some
have already broken free and are floating away.

Why hairs make a seed travel: a falling seed speeds up until the upward drag of
the air balances its weight. At that steady speed,

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Class 9 Science Chapter 11 Reproduction: How Life Continues AglaSem · NCERT Solutions

weight = drag → mg = ½ ρ Cd A vt2

so vt = √(2mg ÷ ρ Cd A) → vt ∝ √(m / A)

The hairs add a great deal of area A while adding almost no mass m. So the terminal speed vt
drops sharply — a dandelion seed drifts down at a few centimetres per second instead of falling.
Staying airborne for minutes rather than a fraction of a second, even a gentle breeze carries it
hundreds of metres.

Check it yourself: drop a dandelion or madar seed with its hairs intact, and another
with the hairs pinched off, from the same height. The stripped one hits the ground
first every time — the mass barely changed, so the difference must come from the
area.

Did you know? Dispersal matters because a seed that germinates directly under its
parent competes with the parent for light, water and minerals. Wind dispersal, along
with dispersal by water and by animals, is how a plant sends its offspring to ground
it does not already occupy.

Q3 A farmer plants two varieties of maize side by side, but notices that seeds form only
when pollen from one variety reaches the stigma of the other. What type of
pollination is this?

Cross-pollination. The pollen is moving from the anther of a flower on one plant to the stigma
of a flower on a different plant of the same kind — which is the chapter's definition of cross-
pollination.

Page 38 of 75

as e
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Class 9 Science Chapter 11 Reproduction: How Life Continues
g l AglaSem · NCERT Solutions

. com
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Cross-pollination — two different
a
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Self-pollination — same flower or
plants
same plant
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one plant variety 1
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Self- and cross-pollination as in Fig. 11.13. In the farmer's field the pollen must leave one variety and

m a s
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land on the stigma of the other, so only the right-hand route sets seed.

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Why the seeds set only in this direction: the farmer's observation says that pollen
landing on a stigma of the same variety produces nothing. That is a self-

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incompatibility — the plant recognises its own pollen and does not let it grow a tube

m a s em
down the style. Only pollen from a genetically different plant is accepted. Nature
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cothis device widely to force cross-pollination, becauseacross-pollination
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s characters of two parents and so keeps producing variation.
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Tip: maize is a natural cross-pollinator anyway. Its male flowers are in the tassel at
g l a
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the top of the plant and its female flowers are the cobs lower down, with long silky
as from the tassel is carried by wind, and much of it
styles hanging out. Pollenlshed
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lands on the silks of neighbouring plants — which is exactly why the farmer's two

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varieties, planted side by side, pollinate each other.
. co
e m
m l as
.co a g
a s em and Ponder — Page 218
gl
Pause
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co m
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Class 9 Science Chapter 11 Reproduction: How Life Continues AglaSem · NCERT Solutions

11.3 Sexual Reproduction in Animals

PAUSE AND PONDER

Q4 Why do animals with external fertilisation generally produce more eggs than
animals with internal fertilisation?

Because in external fertilisation the chance that any one egg is fertilised and grows up is
very small, so the number of eggs must be large enough to make up for it.

expected number of surviving young = N × p

N = number of eggs laid, p = chance one egg is fertilised and survives

external (frog): N = 5,000 – 50,000 at a time, survival low

internal (bird): N = 1 – 15 at a time, survival moderate to high (Table 11.4)

Three separate losses drive p down when fertilisation is external:

The gametes are released into open water. Sperm are diluted the instant they leave the
male, and currents sweep both sperm and eggs apart. Many eggs are never reached by a
sperm at all.
The eggs are exposed. As the chapter says, many are destroyed by water currents or eaten
by other animals. Nothing shelters them.
The young are on their own. There is no shell, no womb, no parent guarding them; the
larva must find its own food from the moment it hatches.

Why the mother cannot simply give each egg more: there is a fixed budget. The
chapter puts it plainly — the mother's body cannot provide large quantities of yolk
for so many eggs. So the strategy is to put in just enough yolk to produce a larva,
which then hatches and feeds itself on organic waste, grows, and finally transforms
into the adult, as in the butterfly of Fig. 11.17 and in the frog. Internal fertilisation
reverses the choice: very few eggs, but each with enough yolk (in a reptile or bird) or
a direct supply from the mother's body (in a mammal) to reach a well-developed
young one.

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Class 9 Science Chapter 11 Reproduction: How Life Continues AglaSem · NCERT Solutions

Tip: read it as two ways of spending the same amount. External fertilisers spend it
on many cheap eggs; internal fertilisers spend it on few expensive ones. Neither is
better — each suits its habitat.

Q5 In animals, which fertilisation method the gametes are more protected?

Internal fertilisation. The gametes meet inside the female's body, where they are shielded
from everything that destroys them in open water.

RISK TO EXTERNAL FERTILISATION INTERNAL FERTILISATION
GAMETES

Being swept apart High — currents carry sperm and None — sperm are delivered directly into
eggs away from each other the female tract

Dilution of sperm Very high — sperm spread through a None — sperm stay concentrated in a
large volume of water narrow tract

Being eaten High — eggs and larvae are food for Very low — the embryo is inside the mother
many animals or a shell

Drying and Not a problem in water, but it None — body fluids stay warm and moist,
temperature change confines the animal to water so the animal can live on land

Number of eggs Thousands (Table 11.4) A few — lizard 2 – 20, bird 1 – 15
needed

Why the protection changes everything downstream: because fertilisation is
almost certain, the animal does not need to make thousands of eggs. Freed of that,
it can put a large investment into each one — a big yolk and a shell in reptiles and
birds, or development inside the uterus in mammals. That is why the survival
column in Table 11.4 rises from "low" for fish and frog to "moderate" for lizard and
"moderate to high" for bird. Internal fertilisation is also what allowed vertebrates to
leave the water altogether: a frog must return to a pond to breed, a lizard need not.

Threads of Curiosity — Page 221

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Class 9 Science Chapter 11 Reproduction: How Life Continues AglaSem · NCERT Solutions

11.5.6 What happens when an egg is not fertilised?

THREADS OF CURIOSITY

Q1 What determines a baby's biological sex? So far, you have learnt that every person
has two sex chromosomes. Females have XX and males have XY chromosomes. The
mother always contributes an X chromosome to a baby and the father contributes
either an X (Female: XX) or a Y (Male: XY) chromosome. Now, from the above
information, can you predict who determines the sex of a baby?

The father — more precisely, which of his two kinds of sperm happens to fertilise the egg.
The mother cannot influence it at all.

mother is XX → meiosis gives eggs that all carry X

father is XY → meiosis gives two kinds of sperm: half carry X, half carry Y

SPERM CARRIES X SPERM CARRIES Y

Egg carries X XX → girl XY → boy

Since the two kinds of sperm are made in equal numbers, the expected chance is 50% girl and
50% boy at every conception — and each conception is independent of the last, exactly like a
fresh toss of a coin.

Why the mother cannot decide: she has no Y chromosome to give. Every egg she
makes carries an X, because meiosis in an XX cell can only put an X into each
gamete. The sex of the baby is therefore settled entirely by which sperm arrives first
— and that is a matter of chance, not of anyone's wish, diet or effort.

Why this matters: in many families a woman is still blamed for the sex of her child.
That belief is simply biologically wrong. The chapter's later section on the same
theme is worth reading beside this box: self-selective abortion driven by a
preference for one sex distorts the sex ratio of society, which is why prenatal sex
determination is strictly prohibited by law in India.

Pause and Ponder — Page 222

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Class 9 Science Chapter 11 Reproduction: How Life Continues AglaSem · NCERT Solutions

11.5.7 Pregnancy and childbirth

PAUSE AND PONDER

Q6 Ravi suddenly notices that he is growing taller rapidly, his shoulders are
broadening, and his voice cracks. What stage of life is he entering?

Ravi is entering puberty — the stage of adolescence at which the body becomes
reproductively mature. All three changes he has noticed are classic signs of it.

WHAT RAVI WHAT IS HAPPENING INSIDE
NOTICES

Rapid gain in height The growth spurt — the long bones lengthen quickly under the influence of hormones

Shoulders broadening Bone and muscle build up in the shoulder girdle, one of the physical changes the chapter
says the testes' hormone brings about in boys

Voice cracking The larynx (voice box) enlarges and the vocal cords lengthen and thicken, so the pitch of
the voice drops

Why the voice "cracks" rather than simply deepening: the vocal cords grow
longer over a few months, but the muscles that control their tension take time to
relearn how to work with the new length. During that period the pitch slips
unpredictably between the old high register and the new low one. Once the growth
is complete and the control is re-learned, the cracking stops.

What is driving all of it: the testes now produce a hormone — a chemical that
regulates functions elsewhere in the body. The chapter says it controls sperm
production and causes the physical changes seen in boys during puberty. So the
same signal that starts sperm formation is producing the height, the shoulders and
the voice.

Tip: the chapter is careful to add a point Ravi should hear. Sexual maturity is not the
same as emotional maturity. The body becomes capable of reproduction gradually,
but handling feelings, communicating clearly and making thoughtful decisions takes
longer to develop.

Page 43 of 75

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Class 9 Science Chapter 11 Reproduction: How Life Continues
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co m
m.
Rina’s period occurs every 28 days. Her last period was on the 5th of March. On
e
Q7

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which day is she most likely to get her next period?

a g
se m
g l a
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On 2 April.

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Day 1 of the cycle = 5 March
l a
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next period begins on day 1 of the

m
days left in March after the 5th = 31 − 5 = 26
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28 − 26 = 2 → the count runs 2 days into April

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next period

a s
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omMarch a s
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= 18
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eovulation, day 14
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days 1–5 days 6–14 rebuild m
days 15–28 lining thickens
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ase
2 April

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Rina's 28-day cycle laid out on a calendar. Day 1 is the first day of bleeding, so the next period starts

a se
com l
on day 1 of the following cycle — 28 days later.

. a g
m
ase
agl
Why the count starts on the first day of bleeding: a cycle is counted from day 1 of
one period to day 1 of the next, not from the day the bleeding stops. So the 28 days
are added to 5 March, giving 2 April as the next day 1.
co m
m .
o m l a se
m agchapter says the cycle
.cTip: "most likely" is doing real work in this question. The
l a se
g
repeats typically every 21 – 35 days, and even in the same person it varies with
a c
illness, stress, travel and weight change. 2 April is the best estimate, not a guarantee
m .
— a point that returns in exercise Question 13.
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e m . co agl
g l as
a

co m
m .
m ase
.co


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Class 9 Science Chapter 11 Reproduction: How Life Continues AglaSem · NCERT Solutions

Q8 A human zygote has just formed. How many chromosomes does it have?

46 — the full human number, restored the instant the two gametes fuse.

sperm (haploid) = 23 chromosomes

egg (haploid) = 23 chromosomes

zygote = 23 + 23 = 46 chromosomes = 23 pairs

meiosis halves the number
23
46 23 pairs —
46
one of each pair
body cell 23 from each parent
fertilisation
zygote
sperm / egg

Meiosis halves the chromosome number, fertilisation restores it. This is why the number stays at 46
generation after generation instead of doubling.

Why the halving must come first: if each gamete carried the full 46, the zygote
would have 92, the next generation 184, and so on. Meiosis prevents this by
separating the members of each pair so that a gamete receives only one
chromosome from each pair. The chapter states the outcome exactly: in humans,
cells have 46 chromosomes but sperm and eggs have only 23, so that when they
combine the zygote has the same number as the parents.

Tip: the zygote's 46 are not a random 46. They are 23 matched pairs — one member
of each pair from the father and the other from the mother. That pairing is the whole
basis of the variation counted in Activity 11.4.

Pause and Ponder — Page 223

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Class 9 Science Chapter 11 Reproduction: How Life Continues AglaSem · NCERT Solutions

11.5.10 How can unwanted pregnancies and infections be prevented?

PAUSE AND PONDER

Q9 What protective devices can be used during sexual activity to reduce the spread of
STIs?

Barrier devices — condoms, and vaginal covers (female condoms). These are the only
contraceptive methods that also protect against sexually transmitted infections.

Why a barrier works against infection while a pill does not: the organisms that
cause gonorrhoea, syphilis, herpes, genital warts and HIV are carried in body fluids
and passed on during close physical contact. A condom is a physical sheet between
the two people, so those fluids are not exchanged and the organisms have no route
across. A pill changes hormone levels inside one person's body; it does nothing
whatever to the exchange of fluids, so it cannot block transmission.

The chapter names the infections concerned: gonorrhoea, herpes, syphilis, genital warts and
HIV (which can eventually lead to AIDS). It also makes the point that some of these are not
curable yet — which is exactly why prevention matters so much more here than treatment.

Tip: a barrier reduces risk sharply but does not reduce it to zero. Some infections
can spread from skin the device does not cover. The complete answer is: use a
barrier device, avoid unsafe contact, and if there is any doubt see a doctor early —
several STIs are curable if treated promptly and are far harder to deal with later.

Q10 If a couple uses oral contraceptive pills but not condoms, which risks remain and
why?

The risk of every sexually transmitted infection remains, in full. The pill guards against
pregnancy; it does not guard against infection.

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Class 9 Science Chapter 11 Reproduction: How Life Continues AglaSem · NCERT Solutions

RISK ORAL PILL WHY
ALONE

Pregnancy Largely The pill alters hormone levels so that the release of eggs is
prevented changed — with no egg released, fertilisation cannot occur

Gonorrhoea, syphilis, Not reduced at These pass in body fluids during contact. A pill acts inside
herpes, genital warts, HIV all the body's hormone system and puts nothing between the
two people

Side effects Present The chapter notes that hormonal pills may have some side
effects

Why the distinction is worth being precise about: a contraceptive is judged on
one thing — does it stop a sperm meeting an egg. There are three quite different
ways of doing that: a barrier (condom, vaginal cover) that physically stops the sperm;
a hormonal method (the pill) that stops the egg being released; and a device or
surgical method (Copper-T, or blocking the vas deferens or the fallopian tubes). Only
the first of these happens to also block the passage of infection, because only the
first works by putting something in the way.

Tip: this is why health workers advise using a condom as well as another method
when protection against both pregnancy and infection is wanted. India's ASHA
workers, described in the chapter, advise on exactly these choices.

Pause and Ponder — Page 224
11.5.10 How can unwanted pregnancies and infections be prevented?

PAUSE AND PONDER

Q11 In many animals, the young ones can walk or find food soon after birth but human
babies are completely dependent on adults for a long time. What might be some
advantages and disadvantages of this for humans as a species?

The long dependence is the price humans pay for a large, slowly-built brain — and the
learning that this makes possible is precisely what has made the species successful.

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Class 9 Science Chapter 11 Reproduction: How Life Continues AglaSem · NCERT Solutions

ADVANTAGES DISADVANTAGES

The brain can keep growing and wiring itself after birth, so it need The child is helpless for years and needs
not be finished before the head passes the birth canal constant care and protection

A long childhood is a long period of learning — language, tool use, Enormous cost in food, time and energy to the
skills, customs and values are passed on by teaching, not only by parents and the family
inheritance

Breastfeeding gives complete nutrition and protection against many Few offspring in a lifetime; the mother's next
diseases, as the chapter notes child is delayed for years

The need for care builds strong family and social bonds, and brings If the parents fall ill or the family lacks
grandparents and the community into raising the child support, the child's survival is directly at risk

Behaviour can be adapted to a new place or a new problem within Population grows slowly, so recovery from a
one generation, without waiting for inherited change disaster takes a long time

Why the trade-off is the same one seen throughout this chapter: Table 11.4
shows animals choosing between many offspring with little investment and few
offspring with heavy investment. A frog lays 5,000 – 50,000 eggs and leaves them; a
bird lays 1 – 15 and feeds them. Humans sit at the far end of that same line —
usually one child at a time, cared for over many years, with survival correspondingly
high. The helplessness of a human baby is not a flaw in the design; it is what a very
heavy investment strategy looks like.

Tip: a horse foal walks within an hour of birth because a foal that cannot run is
eaten. A human infant does not walk for a year because it is protected by adults
during that year. Each pattern fits the way that species lives, and neither would work
for the other.

Revise, Reflect, Refine — Pages 225 – 226

Page 48 of 75

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Class 9 Science Chapter 11 Reproduction: How Life Continues
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End-of-chapter questions
co m
e m.
m as
REVISE, REFLECT, REFINE

.co a g l
s m anthers are removed before it matures. Later, pollen from another plant
eflower’s
gl aA
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Q1
of the same species is dusted onto its stigma and seeds are produced. Which
process has been ensured here? (i) Self-pollination (ii) Cross-pollination (iii)

co m
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Fertilisation (iv) Tissue culture
em
g l as
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(ii) Cross-pollination.

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se m.
m l a
Why: the two steps of the treatment do exactly the two things needed to guarantee
.c o a g
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a cross.
e
ag las Removing the anthers before the flower matures takes away the flower's own
pollen before it can be shed — so self-pollination is now impossible.

a s
comof cross-pollination: pollen transferred agl
Dusting pollen from another plant of the same species onto the stigma is,
.
em
word for word, the chapter's definition
from the anther of a flowerlaofs one plant to the stigma of a flower of another plant
of the same type. ag

co m
m .
o m l a se
Why (iii) is the tempting wrong answer: fertilisation certainly did happen — seeds
g
.c produced, and seeds cannot form without it. But fertilisation
a
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were is not something
as the experimenter ensured; it followed on its own once the pollen was there. What the
agl treatment ensured, by design, is that the pollen came from a different plant. (i) is

se m
cominvolves no pollen at all. a
ruled out because the anthers were removed, and (iv) is unrelated — tissue culture is
a method of asexual propagation.and a g l
a s em
a gl
Tip: this procedure has a name — artificial hybridisation. The chapter describes it in
the Bridging Science and Society box on page 217: remove the stamens, bag the
co m
m .
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flower to keep out stray pollen, then hand-transfer pollen carrying the desired
m l as
.co g
characters. It is how plant breeders create new high-yielding and disease-resistant

e m varieties. a
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m a s e
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com
m .
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.co


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Class 9 Science Chapter 11 Reproduction: How Life Continues AglaSem · NCERT Solutions

Q2 Arrange the following stages of sexual reproduction in plants in the correct order:
(i) Pollen germination on stigma (ii) Fertilisation (iii) Pollination (iv) Formation of
zygote

(iii) Pollination → (i) Pollen germination on stigma → (ii) Fertilisation → (iv) Formation of
zygote.

anther → stigma gametes fuse in the ovule

(i)
(iii) (ii) (iv)
Pollen tube
Pollination Fertilisation Zygote
down the style

then: zygote → embryo | ovule → seed | ovary → fruit

The order is fixed by physical necessity — each stage supplies the condition the next one needs.

Why no other order is possible:

The pollen must first be on the stigma before it can germinate there — so (iii)
precedes (i).
The male gamete cannot reach the ovule until the tube has grown down the style
— so (i) precedes (ii).
The zygote is the product of that fusion. It cannot exist before the fusion — so (ii)
precedes (iv).

Tip: (ii) and (iv) are almost the same instant, which is what makes this pair the trap.
Fertilisation is the event — two gametes fusing. The zygote is the result of that event.
Cause must be written before effect.

Page 50 of 75

Class 9 Science Chapter 11 Reproduction: How Life Continues AglaSem · NCERT Solutions

Q3 Assertion (A): The zygote formed after fertilisation immediately attaches to the
uterus wall. Reason (R): The uterus wall is always prepared to receive the zygote. (i)
Both A and R are true, and R is the correct explanation of A. (ii) Both A and R are
true, but R is not the correct explanation of A. (iii) A is true, but R is false. (iv) A is
false, but R is true.

(iv) A is false, but R is true.
Why A is false. The zygote does not attach immediately. Fertilisation happens in the oviduct,
not in the uterus. The chapter is explicit: the zygote undergoes a series of mitotic divisions while
travelling to the uterus, and only then implants into the inner lining. Several days pass between
fertilisation and implantation, and the structure that implants is no longer a single-celled zygote
but a ball of cells.

fertilisation in the oviduct → repeated mitotic divisions during the journey

→ the dividing mass reaches the uterus

→ implantation in the thickened inner lining = beginning of pregnancy

Why R is taken as true. The uterus does not wait for a zygote to arrive before getting ready —
it prepares in advance, in every single cycle. As the chapter says, before ovulation the uterus
starts to prepare itself and the inner lining becomes thick, and after ovulation it becomes
thicker still and richer in blood vessels. So a lining is made ready each month whether or not a
zygote is ever formed.

Why R still does not explain A: even a fully prepared lining cannot make the
attachment happen sooner, because the zygote is not in the uterus yet. The delay is
set by the journey down the oviduct, not by the readiness of the lining. So the
reason, while true in itself, is not the reason for the assertion — and in this case the
assertion is false anyway.

A note on the wording: the word "always" in R deserves care. The lining is prepared
in every cycle, which is the sense in which R is true. It is not literally present at all
times — during menstruation (days 1 – 5) that very lining is being shed, and it is
rebuilt over days 6 – 14. Read R as "in every cycle the wall is prepared in advance",
and option (iv) is the correct choice.

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Class 9 Science Chapter 11 Reproduction: How Life Continues AglaSem · NCERT Solutions

Q4 Why does asexual reproduction produce offsprings that are genetically identical to
the parent?

Because there is only one parent and only one kind of cell division involved — mitosis,
which copies the chromosome set exactly instead of reshuffling or halving it.

parent cell (chromosome number 2n)

↓ DNA replicated: every chromosome copied once

↓ mitosis: the two copies of each chromosome separated into different daughter cells

two daughter cells, each with 2n chromosomes carrying the same information

Three things that would create differences are all absent:

No meiosis, so chromosome pairs never separate independently and gametes are never
formed — the 2n reshuffling of Activity 11.4 simply does not occur.
No second parent, so no chromosome from anywhere else is ever brought in.
No fertilisation, so no two different sets are ever combined.

The result is a group of individuals with identical genetic information — what the chapter calls
clones. It applies equally to a budding yeast cell, a hydra bud, a spore from Rhizopus, a potato
tuber and a grafted rose.

Why "identical" is not quite absolute: DNA copying is extremely accurate but not
perfect. A rare copying error — a mutation — can slip in and be passed to that
daughter cell and all of its descendants. This is the only source of variation available
to a purely asexual line, and it is far too slow and rare to compare with the
reshuffling of sexual reproduction. That difference is exactly what makes clonal
crops vulnerable, as Question 7 explores.

Q5 Explain why the menstrual cycle stops during pregnancy.

Because menstruation is the shedding of a uterine lining that is no longer needed — and
during pregnancy that lining is very much needed, so it is kept instead of being shed.
Compare the two situations side by side:

Page 52 of 75

Class 9 Science Chapter 11 Reproduction: How Life Continues AglaSem · NCERT Solutions

STAGE NO FERTILISATION PREGNANCY

The egg Stays viable about a day, then Fused with a sperm — a zygote forms
degenerates

The uterine Thick and rich in blood vessels, but with Thick and rich in blood vessels, and the
lining nothing to nourish embryo is implanted in it

What happens The lining sheds with some blood — The lining is maintained and thickened
next menstruation further to nourish the embryo

Ovulation Occurs again in the next cycle Stops for the whole pregnancy — no new egg
is released

Why the body knows the difference: once the embryo implants, it and the tissues
that grow around it release hormones into the mother's blood. Hormones, as the
chapter defines them, are chemicals that regulate different functions elsewhere in
the body. These particular hormones do two things at once — they keep the uterine
lining intact instead of letting it break down, and they stop the ovary from releasing
another egg. So both halves of the cycle, ovulation and shedding, are switched off
together and stay off for the whole nine months.

Tip: this is why a missed period is usually the first sign of pregnancy. It is not a
coincidence — it is the direct consequence of the lining being retained. The cycle
resumes some time after childbirth, often later in mothers who are breastfeeding.

Q6 Why are flowers that bloom at night white or light in colour as compared to flowers
that bloom during the day?

Because their pollinators fly at night, and in the dark a white surface is the only one that
can still be seen. A brightly coloured petal would be invisible.

Why white works and colour does not, in the dark: a coloured petal looks
coloured because its pigment absorbs most wavelengths and reflects only a narrow
band. In daylight that is fine — the reflected band is still bright. At night there is very
little light to start with, so after most of it has been absorbed almost nothing comes
back to the moth's eye. A white petal has no such pigment and reflects nearly all of
the little light there is, so it appears as a pale patch against dark leaves. In moonlight
a white flower is often the brightest object in the plant.

Page 53 of 75

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Class 9 Science Chapter 11 Reproduction: How Life Continues
g l AglaSem · NCERT Solutions

co m
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sunset. Scent molecules travel on air and can

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a good reward.
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Why do vegetatively propagated plants tend to be more vulnerable to diseases than

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Class 9 Science Chapter 11 Reproduction: How Life Continues AglaSem · NCERT Solutions

Did you know? This is not a theoretical worry. The Gros Michel banana, once the
world's main commercial variety, was propagated clonally and was wiped out by
Panama disease. Its replacement, the Cavendish, is also a clone and is now under
attack from a new strain of the same fungus. It is exactly to break this cycle that the
chapter's example of virus-free tissue-culture banana plantlets raised from the shoot
tip matters — the apical meristem is usually free of virus, so the clones at least start
clean.

Q8 If all flowers in a type of plant were only capable of self-pollination, how would it
affect the genetic diversity over several generations? Explain.

Genetic diversity would fall sharply, and after a few generations the population would be
almost uniform.

Why: in self-pollination both gametes come from the same plant, so a gene pair can
only be matched with copies of itself. Any pair that was mixed (one form from each
parent) has a one-in-two chance of staying mixed in the next generation, and a one-
in-two chance of becoming a matched pair — and once matched, it can never
become mixed again by selfing.

fraction of mixed gene pairs left after n generations of selfing = (½)n

after 1 generation = 50%

after 3 generations = (½)3 = 12.5%

after 5 generations = (½)5 = 1/32 = 3.1%

after 10 generations = (½)10 ≈ 0.1%

So within about five to ten generations the plants become almost entirely uniform — what
breeders call a pure line.
What follows from that loss:

Hidden harmful characters get exposed. A harmful form of a gene that was masked while
the pair was mixed shows up once the pair is matched. Plants become weaker, smaller and
less fertile — this is inbreeding depression.
No new combinations. The reshuffling of Activity 11.4 still happens inside the plant, but
since both gametes come from the same plant there is no new material to shuffle with. The
population stops generating novelty.

Page 55 of 75

Class 9 Science Chapter 11 Reproduction: How Life Continues AglaSem · NCERT Solutions

No ability to adapt. When a new pest, a new disease or a change in rainfall arrives, there is
no variant plant that happens to cope. A single epidemic can take the whole population,
exactly as in the clonal crops of Question 7.

Tip: uniformity is not always a disadvantage, which is why some crops do it
deliberately. Wheat, rice and pea are largely self-pollinated: every plant in the field
ripens together and yields alike, and no pollinator is needed at all, so seed set is
assured. Breeders even use forced selfing on purpose — first to fix a pure line, and
then to cross two such lines to produce a vigorous hybrid.

Q9 A farmer wants to produce a large number of genetically identical plants quickly.
Suggest suitable reproduction methods and explain why they are effective.

He should use vegetative propagation — above all tissue culture from the shoot tip, and,
depending on the crop, cutting, grafting or layering. All of these are asexual, so all of them
give clones.

METHOD BEST FOR WHY IT SUITS THIS FARMER

Tissue culture from Banana, sugarcane, Thousands of identical plantlets from one small piece of
the shoot tip (apical orchids, potato tissue, in a sterile nutrient medium, in any season and in a
meristem) small space; the apical meristem is usually free of virus, so the
plantlets are healthy

Cutting Sugarcane, money Simplest and cheapest; a cutting with 3 – 5 nodes roots in two
plant, rose, hibiscus to three weeks

Grafting Mango, citrus, rose, Combines a chosen variety's shoot with a hardy, disease-
apple resistant root system

Layering Lemon, guava, Very reliable, because the twig stays attached to the parent
jasmine, and cannot wilt while rooting
pomegranate

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Class 9 Science Chapter 11 Reproduction: How Life Continues AglaSem · NCERT Solutions

Why these methods are effective for what he wants:

Genetically identical — only mitosis is involved, from a single parent, so every
plant carries exactly the parent's characters: the same yield, the same fruit size,
the same sweetness, the same ripening date. A whole orchard behaves as one
plant, which makes spraying, harvesting and marketing far simpler.
Quick — the plant skips flowering, pollination, seed set, seed dormancy and
germination. A cutting already has nodes and stored food and starts growing at
once, so the crop is ready months or years sooner than one raised from seed.
Large numbers — tissue culture in particular multiplies one shoot tip into
thousands of plantlets. The chapter's own example is banana farming, where
mass-produced healthy plantlets have revolutionised the practice and ensured
high yields.
The only option for some crops — banana, seedless grape and sugarcane
produce no usable seed, so they cannot be raised any other way.

Tip to give the farmer: uniformity is also a risk. A field of clones has no plant that
can resist a new disease. Advise him to plant more than one variety, to buy certified
virus-free planting material, and to rotate his crop.

Q10 Suresh prepares slides with pollen grains in different sugar concentrations (0%,
2.5%, 5%, 7.5%, 10%) to study the germination of pollen. (i) What are the different
hypotheses which can be tested using this set-up? (ii) What parameters should be
kept the same in this set-up?

(i) Hypotheses that this set-up can test. Each must be a statement that the experiment could
prove wrong.

1. Pollen grains need an external sugar supply to germinate. Prediction: little or no
germination at 0% (plain water), germination at the higher concentrations.
2. The percentage of grains that germinate depends on the sugar concentration.
Prediction: the count of germinated grains changes from slide to slide.
3. The length of the pollen tube depends on the sugar concentration. Prediction: tubes are
measurably longer on some slides than others.
4. There is an optimum concentration, with germination falling on both sides of it.
Prediction: the graph of germination against concentration rises, peaks and then falls.

Page 57 of 75

Class 9 Science Chapter 11 Reproduction: How Life Continues AglaSem · NCERT Solutions

Why an optimum is expected rather than "more sugar, more growth": sugar
plays two roles at once. It is the respiratory substrate that supplies energy for
building the pollen tube, so too little of it starves the grain. It is also an osmotic
agent. In plain water the solution outside is more dilute than the contents of the
grain, so water rushes in and the grain may burst. In a very concentrated solution
the reverse happens — water is drawn out of the grain, it shrinks, and germination
stops. Somewhere between the two lies the concentration that both feeds the grain
and balances its water, and that is where germination peaks.

(ii) Parameters that must be kept the same — everything except the one factor being tested.

TYPE OF WHAT IT IS HERE
VARIABLE

Independent Sugar concentration: 0%, 2.5%, 5%, 7.5%, 10%
variable (the only
thing changed)

Dependent Percentage of grains germinated; length of the pollen tube
variables (what is
measured)

Controlled Same plant species and preferably the same flower; pollen of the same age and freshness;
variables (kept roughly the same number of grains per slide; same volume and size of drop; same slide and
identical) coverslip; same temperature; same humidity; same light; same waiting time before
counting; same microscope magnification; same definition of "germinated" (say, a tube at
least as long as the grain is wide); same person counting

Tip: the 0% slide is not a wasted slide — it is the control. Without it Suresh could not
claim that sugar is what caused germination, because he would have nothing to
compare the other four against. Also, he should count several fields of view on each
slide and take the average; a single field can mislead.

Page 58 of 75

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Class 9 Science Chapter 11 Reproduction: How Life Continues
g l AglaSem · NCERT Solutions

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Look at the picture given below and think in line with the given prompts and find
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Q11

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out which type(s) of pollination might have been followed in these flowers —
gpollination. Papaya:
m .c Stamens cover the stigma. Wheat: Flowers open after
Tomato: a
e and female flowers are often borne on different papaya trees.
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Question 11, page 226 — redrawn sketches of the three pictures printed with the

om a
and papaya flowers — small ones.cborne in a branched hanging spray, and larger ones agl
question: an open tomato flower and the same flower in side view; a wheat spikelet;

s m prompt printed under each picture is given in the
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sitting close to the stem.
question above.

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Tomato and wheat are self-pollinated; papaya must be cross-pollinated. Each prompt is a

agl structural clue that settles the question by itself.

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