Biology in essence is the story of life on earth. While individual
organisms die without fail, species continue to live through
millions of years unless threatened by natural or anthropogenic
extinction. Reproduction becomes a vital process without
which species cannot survive for long. Each individual leaves
its progeny by asexual or sexual means. Sexual mode of
reproduction enables creation of new variants, so that survival
advantage is enhanced. This unit examines the general
principles underlying reproductive processes in living organisms
and then explains the details of this process in flowering plants
and humans as easy to relate representative examples. A related
perspective on human reproductive health and how
reproductive ill health can be avoided is also presented to
complete our understanding of biology of reproduction.
Chapter 1
Reproduction in Organisms
Chapter 2
Sexual Reproduction in
flowering Plants
Chapter 3
Human Reproduction
Chapter 4
Reproductive Health
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PANCHANAN MAHESHWARI
(1904-1966)
Born in November 1904 in Jaipur (Rajasthan) Panchanan Maheshwari
rose to become one of the most distinguished botanists not only of India
but of the entire world. He moved to Allahabad for higher education
where he obtained his D.Sc. During his college days, he was inspired
by Dr W. Dudgeon, an American missionary teacher, to develop interest
in Botany and especially morphology. His teacher once expressed that
if his student progresses ahead of him, it will give him a great satisfaction.
These words encouraged Panchanan to enquire what he could do for
his teacher in return.
He worked on embryological aspects and popularised the use of
embryological characters in taxonomy. He established the Department
of Botany, University of Delhi as an important centre of research in
embryology and tissue culture. He also emphasised the need for initiation
of work on artificial culture of immature embryos. These days, tissue
culture has become a landmark in science. His work on test tube
fertilisation and intra-ovarian pollination won worldwide acclaim.
He was honoured with fellowship of Royal Society of London (FRS),
Indian National Science Academy and several other institutions of
excellence. He encouraged general education and made a significant
contribution to school education by his leadership in bringing out the
very first textbooks of Biology for Higher Secondary Schools published
by NCERT in 1964.
2020-21
Each and every organism can live only for a certain period
of time. The period from birth to the natural death of an
organism represents its life span. Life spans of a few
organisms are given in Figure 1.1. Several other organisms
are drawn for which you should find out their life spans
and write in the spaces provided. Examine the life spans
of organisms represented in the Figure 1.1. Isn’t it both
interesting and intriguing to note that it may be as short
as a few days or as long as a few thousand years? Between
these two extremes are the life spans of most other living
organisms. You may note that life spans of organisms are
not necessarily correlated with their sizes; the sizes of
crows and parrots are not very different yet their life spans
show a wide difference. Similarly, a mango tree has a much
shorter life span as compared to a peepal tree. Whatever
be the life span, death of every individual organism is a
certainty, i.e., no individual is immortal, except single-celled
organisms. Why do we say there is no natural death in
single-celled organisms? Given this reality, have you ever
wondered how vast number of plant and animal species
have existed on earth for several thousands of years? There
must be some processes in living organisms that ensure
this continuity. Yes, we are talking about reproduction,
something that we take for granted.
CHAPTER 1
REPRODUCTION IN ORGANISMS
1.1 Asexual
Reproduction
1.2 Sexual
Reproduction
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Figure 1.1 Approximate life spans of some organisms
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REPRODUCTION IN ORGANISMS
Reproduction is defined as a biological process in which an
organism gives rise to young ones (offspring) similar to itself. The offspring
grow, mature and in turn produce new offspring. Thus, there is a cycle
of birth, growth and death. Reproduction enables the continuity of the
species, generation after generation. You will study later in Chapter 5
(Principles of Inheritance and Variation) how genetic variation is created
and inherited during reproduction.
There is a large diversity in the biological world and each organism
has evolved its own mechanism to multiply and produce offspring.
The organism’s habitat, its internal physiology and several other factors
are collectively responsible for how it reproduces. Based on whether
there is participation of one organism or two in the process of
reproduction, it is of two types. When offspring is produced by a single
parent with or without the involvement of gamete formation, the
reproduction is asexual. When two parents (opposite sex) participate in
the reproductive process and also involve fusion of male and female
gametes, it is called sexual reproduction.
1.1 ASEXUAL REPRODUCTION
In this method, a single individual (parent) is capable of producing
offspring. As a result, the offspring that are produced are not only
identical to one another but are also exact copies of their parent.
Are these offspring likely to be genetically identical or different?
The term clone is used to describe such morphologically and
genetically similar individuals.
Figure 1.2 Cell division in unicellular organism: (a) Budding in
yeast; (b) Binary fission in Amoeba
(b)
(a)
Let us see how widespread asexual reproduction is, among different
groups of organisms. Asexual reproduction is common among
single-celled organisms, and in plants and animals with relatively simple
organisations. In Protists and Monerans, the organism or the parent
cell divides by mitosis into two to give rise to new individuals (Figure1.2).
Thus, in these organisms cell division is itself a mode of reproduction.
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Many single-celled organisms reproduce by binary fission, where a
cell divides into two halves and each rapidly grows into an adult (e.g.,
Amoeba, Paramecium). In yeast, the division is unequal and small
buds are produced that remain attached initially to the parent cell
which, eventually gets separated and mature into new yeast
organisms (cells). Under unfavourable condition the Amoeba withdraws
its pseudopodia and secretes a three-layered hard covering or cyst
around itself. This phenomenon is termed as encystation. When
favourable conditions return, the encysted Amoeba divides by multiple
fission and produces many minute amoeba or pseudopodiospores;
the cyst wall bursts out, and the spores are liberated in the surrounding
medium to grow up into many amoebae. This phenomenon is
known as sporulation.
Figure1.3 Asexual reproductive structures: (a) Zoospores of Chlamydomonas; (b) Conidia of
Penicillium; (c) Buds in Hydra; (d) Gemmules in sponge
(d)
(a)
(b)
(c)
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Members of the Kingdom Fungi and simple plants such as algae
reproduce through special asexual reproductive structures (Figure 1.3).
The most common of these structures are zoospores that usually are
microscopic motile structures. Other common asexual reproductive
structures are conidia (Penicillium), buds (Hydra) and gemmules (sponge).
You have learnt about vegetative reproduction in plants in Class XI.
What do you think – Is vegetative reproduction also a type of asexual
reproduction? Why do you say so? Is the term clone applicable to the
offspring formed by vegetative reproduction?
While in animals and other simple organisms the term asexual is used
unambiguously, in plants, the term vegetative reproduction is frequently
used. In plants, the units of vegetative propagation such as runner,
rhizome, sucker, tuber, offset, bulb are all capable of giving rise to new
offspring (Figure1.4). These structures are called vegetative propagules.
Figure 1.4 Vegetative propagules in angiosperms: (a) Eyes of potato; (b) Rhizome of ginger;
(c) Bulbil of Agave; (d) Leaf buds of Bryophyllum; (e) Offset of water hyacinth
Adventitious
Buds
(a)
(c)
(d)
(e)
(b)
Buds
Nodes
Adventitious
Root
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Obviously, since the formation of these structures does not involve two
parents, the process involved is asexual. In some organisms, if the body
breaks into distinct pieces (fragments) each fragment grows into an
adult capable of producing offspring (e.g., Hydra). This is also a mode
of asexual reproduction called fragmentation.
You must have he
ard about the scourge of the water bodies or about
the ‘terror of Bengal’. This is nothing but the aquatic plant ‘water hyacinth’
which is one of the most invasive weeds found growing wherever there is
standing water. It drains oxygen from the water, which leads to death of
fishes. You will learn more about it in Chapters 13 and 14. You may find
it interesting to know that this plant was introduced in India because of
its beautiful flowers and shape of leaves. Since it can propagate vegetatively
at a phenomenal rate and spread all over the water body in a short period
of time, it is very difficult to get rid off them.
Are you aware how plants like potato, sugarcane, banana, ginger,
dahlia are cultivated? Have you seen small plants emerging from the
buds (called eyes) of the potato tuber, from the rhizomes of banana and
ginger? When you carefully try to determine the site of origin of the new
plantlets in the plants listed above, you will notice that they invariably
arise from the nodes present in the modified stems of these plants. When
the nodes come in contact with damp soil or water, they produce roots
and new plants. Similarly, adventitious buds arise from the notches
present at margins of leaves of Bryophyllum. This ability is fully exploited
by gardeners and farmers for commercial propagation of such plants.
It is interesting to note that asexual reproduction is the common method
of reproduction in organisms that have a relatively simple organisation,
like algae and fungi and that they shift to sexual method of reproduction
just before the onset of adverse conditions. Find out how sexual
reproduction enables these organisms to survive during unfavourable
conditions? Why is sexual reproduction favoured under such conditions?
Asexual (vegetative) as well as sexual modes of reproduction are exhibited
by the higher plants. On the other hand, only sexual mode of reproduction
is present in most of the animals.
1.2 SEXUAL REPRODUCTION
Sexual reproduction involves formation of the male and female gametes,
either by the same individual or by different individuals of the opposite
sex. These gametes fuse to form the zygote which develops to form the
new organism. It is an elaborate, complex and slow process as compared
to asexual reproduction. Because of the fusion of male and female gametes,
sexual reproduction results in offspring that are not identical
to the parents
or amongst themselves.
A study of diverse organisms–plants, animals or fungi–show that
though they differ so greatly in external morphology, internal structure
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and physiology, when it comes to sexual mode of reproduction,
surprisingly, they share a similar pattern. Let us first discuss what features
are common to these diverse organisms.
All organisms have to reach a certain stage of growth and maturity in
their life, before they can reproduce sexually. That period of growth is
called the juvenile phase. It is known as vegetative phase in plants.
This phase is of variable durations in different organisms.
The end of juvenile/vegetative phase which marks the beginning of
the reproductive phase can be seen easily in the higher plants when they
come to flower. How long does it take for marigold/rice/wheat/coconut/
mango plants to come to flower? In some plants, where flowering occurs
more than once, what would you call the inter-flowering period – juvenile
or mature?
Observe a few trees in your area. Do they flower during the same
month year after year? Why do you think the availability of fruits like
mango, apple, jackfruit, etc., is seasonal? Are there some plants that flower
throughout the year and some others that show seasonal flowering?
Plants–the annual and biennial types, show clear cut vegetative,
reproductive and senescent phases, but in the perennial species it is very
difficult to clearly define these phases.
A few plants exhibit unusual
flowering phenomenon; some of them such as bamboo species flower only
once in their life time, generally after 50-100 years, produce large number
of fruits and die. Another plant, Strobilanthus kunthiana (neelakuranji),
flowers once in 12 years. As many of you would know, this plant flowered
during September-October 2006. Its mass flowering transformed large
tracks of hilly areas in Kerala, Karnataka and Tamil Nadu into blue
stretches and attracted a large number of tourists. In animals, the juvenile
phase is followed by morphological and physiological changes prior to
active reproductive behaviour. The reproductive phase is also of variable
duration in different organisms.
Can you list the changes seen in human beings that are indicative
of reproductive maturity?
Among animals, for example birds, do they lay eggs all through the
year? Or is it a seasonal phenomenon? What about other animals like
frogs and lizards? You will notice that, birds living in nature lay eggs only
seasonally. However, birds in captivity (as in poultry farms) can be made
to lay eggs throughout the year. In this case, laying eggs is not related to
reproduction but is a commercial exploitation for human welfare. The
females of placental mammals exhibit cyclical changes in the activities of
ovaries and accessory ducts as well as hormones during the reproductive
phase. In non-primate mammals like cows, sheep, rats, deers, dogs, tiger,
etc., such cyclical changes during reproduction are called oestrus cycle
where as in primates (monkeys, apes, and humans) it is called menstrual
cycle. Many mammals, especially those living in natural, wild conditions
exhibit such cycles only during favourable seasons in their reproductive
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(a)
(b)
(c)
Figure 1.5 Types of gametes: (a) Isogametes of Cladophora (an alga); (b) Heterogametes of
Fucus (an alga); (c) Heterogametes of Homo sapiens (Human beings)
phase and are therefore called seasonal breeders. Ma
ny other mammals
are reproductively active throughout their reproductive phase and hence
are called continuous breeders.
That we all grow old (if we live long enough), is something that we
recognise. But what is meant by growing old? The end of reproductive
phase can be considered as one of the parameters of senescence or old
age. There are concomitant changes in the body (like slowing of
metabolism, etc.) during this last phase of life span. Old age ultimately
leads to death.
In both plants and animals, hormones are responsible for the
transitions between the three phases. Interaction between hormones and
certain environmental factors regulate the reproductive processes and
the associated behavioural expressions of organisms.
Events in sexual reproduction : After attainment of maturity, all sexually
reproducing organisms exhibit events and processes that have remarkable
fundamental similarity, even though the structures associated with sexual
reproduction are indeed very different. The events of sexual reproduction
though elaborate and complex, follow a regular sequence. Sexual
reproduction is characterised by the fusion (or fertilisation) of the male and
female gametes, the formation of zygote and embryogenesis. For convenience
these sequential events may be grouped into three distinct stages namely,
the pre-fertilisation, fertilisation and the post-fertilisation events.
1.2.1 Pre-fertilisation Events
These include all the events of sexual reproduction prior to the fusion of
gametes. The two main pre-fertilisation events are gametogenesis and
gamete transfer.
1.2.1.1 Gametogenesis
As you are already aware, gametogenesis refers to the process of formation
of the two types of gametes – male and female. Gametes are haploid cells.
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In some algae the two ga
metes are so similar in appearance
that it is not possible to categorise them into male and female gametes.
They are hence called homogametes (isogametes) (Figure 1.5a).
However, in a majority of sexually reproducing organisms the gametes
produced are of two morphologically distinct types (heterogametes). In
such organisms the male gamete is called the antherozoid or sperm
and the female gamete is called the egg or ovum (Figure1.5 b, c).
Sexuality in organisms: Sexual reproduction in organisms generally
involves the fusion of gametes from two different individuals. But this
is not always true. From your recollection of examples studied in
Class XI, can you identify cases where self-fertilisation is observed? Of
course, citing such examples in plants is easy.
Plants may have both male and female reproductive structures in the
same plant (bisexual) (Figure 1.6 c, e) or on different plants (unisexual)
(Figure 1.6d). In several fungi and plants, terms such as homothallic
and monoecious are used to denote the bisexual condition and
heterothallic and dioecious are the terms used to describe unisexual
condition. In flowering plants, the unisexual male flower is staminate,
i.e., bearing stamens, while the female is pistillate or bearing pistils. In
some flowering plants, both male and female flowers may be present on
the same individual (monoecious) or on separate individuals (dioecious).
Some examples of monoecious plants are cucurbits and coconuts and of
dioecious plants are papaya and date palm. Name the type of gametes
that are formed in staminate and pistillate flowers.
But what about animals? Are individuals of all species either male or
female (unisexual)? Or are there species which possess both the
reproductive organs (bisexual)? You probably can make a list
of several unisexual animal species. Earthworms, (Figure 1.6a) sponge,
tapeworm and leech, typical examples of bisexual animals that possess
both male and female reproductive organs, are hermaphrodites.
Cockroach (Figure 1.6b) is an example of a unisexual species.
Cell division during gamete formation : Gametes in all heterogametic
species are of two types namely, male and female. Gametes are haploid
though the parent plant body from which they arise may be either haploid
or diploid. A haploid parent produces gametes by mitotic division. Does
this mean that meiosis never occurs in organisms that are haploid?
Carefully examine the flow charts of life cycles of algae that you have
studied in Class XI (Chapter 3) to get a suitable answer.
Several organisms belonging to monera, fungi, algae and bryophytes
have haploid plant body, but in organisms belonging to pteridophytes,
gymnosperms, angiosperms and most of the animals including human
beings, the parental body is diploid. It is obvious that meiosis, the reduction
division, has to occur if a diploid body has to produce haploid gametes.
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Figure 1.6 Diversity of sexuality in organisms (a) Bisexual animal (Earthworm); (b) Unisexual
animal (Cockroach); (c) Monoecious plant (Chara); (d) Dioecious plant (Marchantia);
(e) Bisexual flower (sweet potato)
(b)
Male
Female
Testis sac
with testis
Ovary
Clitellum
(a)
Testis
Ovary
Female thallus
Antheridiophor
e
Male thallus
Archegoniophore
(c)
(d)
(e)
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Table 1.1: Chromosome Numbers in Meiocytes (diploid, 2n) and Gametes
(haploid, n) of Some Organisms. Fill in the Blank Spaces.
Name of organism Chromosome number Chromosome number
in meiocyte (2n) in gamete (n)
Human beings 46 23
House fly 12 —
Rat — 21
Dog 78 —
Cat — 19
Fruit fly 8 —
Ophioglossum (a fern) — 630
Apple 34 —
Rice — 12
Maize 20 —
Potato — 24
Butterfly 380 —
Onion — 8
In diploid organisms, specialised cells called meiocytes (gamete mother
cell) undergo meiosis. At the end of meiosis, only one set of chromosomes
gets incorporated into each gamete. Carefully study Table 1.1 and fill in
the diploid and haploid chromosome numbers of organisms. Is there any
relationship in the number of chromosomes of meiocytes and gametes?
1.2.1.2 Gamete Transfer
After their formation, male and female gametes must be physically
brought together to facilitate fusion (fertilisation). Have you ever
wondered how the gametes meet? In a majority of organisms, male
gamete is motile and the female gamete is stationary. Exceptions are a
few fungi and algae in which both types of gametes are motile
(Figure1.7a). There is a need for a medium through which the male
gametes move. In several simple plants like algae, bryophytes and
pteridophytes, water is the medium through which this gamete transfer
takes place. A large number of the male gametes, however, fail to reach
the female gametes. To compensate this loss of male gametes during
transport, the number of male gametes produced is several thousand
times the number of female gametes produced.
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In seed plants, pollen grains are the carriers of male
gametes and ovule have the egg. Pollen grains
produced in anthers therefore, have to be transferred
to the stigma before it can lead to fertilisation (Figure
1.7b). In bisexual, self-fertilising plants, e.g., peas,
transfer of pollen grains to the stigma is relatively easy
as anthers and stigma are located close to each other;
pollen grains soon after they are shed, come in contact
with the stigma. But in cross pollinating plants
(including dioecious plants), a specialised event called
pollination facilitates transfer of pollen grains to the
stigma. Pollen grains germinate on the stigma and the
pollen tubes carrying the male gametes reach the ovule
and discharge male gametes near the egg. In dioecious
animals, since male and female gametes are formed in
different individuals, the organism must evolve a
special mechanism for gamete transfer. Successful
transfer and coming together of gametes is essential
for the most critical event in sexual reproduction, the
fertilisation.
1.2.2 Fertilisation
The most vital event of sexual reproduction is perhaps
the fusion of gametes. This process called syngamy
results in the formation of a diploid zygote. The term
fertilisation is also often used for this process. The
terms syngamy and fertilisation are frequently used
though , interchangeably.
What would happen if syngamy does not occur?
However, it has to be mentioned here that in some
organisms like rotifers, honeybees and even some lizards
and birds (turkey), the female gamete undergoes
development to form new organisms without fertilisation. This
phenomenon is called parthenogenesis.
Where does syngamy occur? In most aquatic organisms, such as a
majority of algae and fishes as well as amphibians, syngamy occurs in
the external medium (water), i.e., outside the body of the organism. This
type of gametic fusion is called external fertilisation. Organisms
exhibiting external fertilisation show great synchrony between the sexes
and release a large number of gametes into the surrounding medium
(water) in order to enhance the chances of syngamy. This happens in the
bony fishes and frogs where a large number of offspring are produced. A
major disadvantage is that the offspring are extremely vulnerable to
predators threatening their survival up to adulthood.
Figure 1.7 (a) Homogametic contact in
alga; (b) Germinating pollen
grains on the stigma of a flower
(a)
(
b)
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In many terrestrial organisms, belonging to fungi, higher animals such
as reptiles, birds, mammals and in a majority of plants (bryophytes,
pteridophytes, gymnosperms and angiosperms), syngamy occurs inside
the body of the organism, hence the process is called internal fertilisation.
In all these organisms, egg is formed inside the female body where they
fuse with the male gamete. In organisms exhibiting internal fertilisation,
the male gamete is motile and has to reach the egg in order to fuse with it.
In these even though the number of sperms produced is very large, there
is a significant reduction in the number of eggs produced. In seed plants,
however, the non-motile male gametes are carried to female gamete by
pollen tubes.
1.2.3 Post-fertilisation Events
Events in sexual reproduction after the formation of zygote are called
post-fertilisation events.
1.2.3.1 The Zygote
Formation of the diploid zygote is universal in all sexually reproducing
organisms. In organisms with external fertilisation, zygote is formed in
the external medium (usually water), whereas in those exhibiting internal
fertilisation, zygote is formed inside the body of the organism.
Further development of the zygote depends on the type of life cycle
the organism has and the environment it is exposed to. In organisms
belonging to fungi and algae, zygote develops a thick wall that is resistant
to dessication and damage. It undergoes a period of rest before
germination. In organisms with haplontic life cycle (As you have read
in Class XI), zygote divides by meiosis to form haploid spores that grow
into haploid individuals. Consult your Class XI book and find out what
kind of development takes place in the zygote in organisms with diplontic
and haplo-diplontic life cycles.
Zygote is the vital link that ensures continuity of species
between organisms of one generation and the next. Every sexually
reproducing organism, including human beings begin life as a single
cell–the zygote.
1.2.3.2 Embryogenesis
Embryogenesis refers to the process of development of embryo from the
zygote. During embryogenesis, zygote undergoes cell division (mitosis)
and cell differentiation. While cell divisions increase the number of cells
in the developing embryo; cell differentiation helps groups of cells to
undergo certain modifications to form specialised tissues and organs to
form an organism. You have studied about the process of cell division
and differentiation in the previous class.
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Animals are categorised into oviparous and viviparous based on
whether the development of the zygote takes place outside the body of
the female parent or inside, i.e., whether they lay fertilised/unfertilised
eggs or give birth to young ones. In oviparous animals like reptiles and
birds, the fertilised eggs covered by hard calcareous shell are laid in a
safe place in the environment; after a period of incubation young ones
hatch out. On the other hand, in viviparous animals (majority of mammals
including human beings), the zygote develops into a young one inside
the body of the female organism. After attaining a certain stage of growth,
the young ones are delivered out of the body of the female organism.
Because of proper embryonic care and protection, the chances of survival
of young ones is greater in viviparous organisms.
In flowering plants, the zygote is formed inside the ovule. After
fertilisation the sepals, petals and stamens of the flower wither and fall
off. Can you name a plant in which the sepals remain attached? The
pistil however, remains attached to the plant. The zygote develops into
the embryo and the ovules develop into the seed. The ovary develops into
the fruit which develops a thick wall called pericarp that is protective in
function (Figure 1.8). After dispersal, seeds germinate under favourable
conditions to produce new plants.
SUMMARY
Reproduction enables a species to live generation after generation.
Reproduction in organisms can be broadly classified into asexual and
sexual reproduction. Asexual reproduction does not involve the fusion
of gametes. It is common in organisms that have a relatively simple
organisation such as the fungi, algae and some invertebrate animals.
The offspring formed by asexual reproduction are identical and can be
referred to as clones. Zoospores, conidia, etc., are the most common
asexual structures formed in several algae and fungi. Budding
and gemmule formation are the common asexual methods seen in
lower animals.
Prokaryotes and unicellular organisms reproduce asexually by
cell division or binary fission of the parent cell. In several aquatic and
Figure 1.8 A few kinds of fruit showing seeds (S) and
protective pericarp (P)
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terrestrial species of angiosperms, structures such as runners,
rhizomes, suckers, tubers, offsets, etc., are capable of giving rise to
new offspring. This method of asexual reproduction is generally
referred to as vegetative propagation.
Sexual reproduction involves the formation and fusion of gametes.
It is a complex and slower process as compared to asexual reproduction.
Most of the higher animals reproduce almost entirely by sexual method.
Events of sexual reproduction may be categorised into pre-fertilisation,
fertilisation and post-fertilisation events. Pre-fertilisation events include
gametogenesis and gamete transfer while post-fertilisation events
include the formation of zygote and embryogenesis.
Organisms may be bisexual or unisexual. Sexuality in plants is
varied, particularly in angiosperms, due to the production of diverse
types of flowers. Plants are defined as monoecious and dioecious.
Flowers may be bisexual or unisexual flowers.
Gametes are haploid in nature and usually a direct product of meiotic
division except in haploid organisms where gametes are formed by mitosis.
Transfer of male gametes is an essential event in sexual reproduction.
It is relatively easy in bisexual organisms. In unisexual animals it occurs
by copulation or simultaneous release. In angiosperms, a special process
called pollination ensures transfer of pollen grains which carry the pollen
grains to the stigma.
Syngamy (fertilisation) occurs between the male and female gametes.
Syngamy may occur either externally, outside the body of organisms or
internally, inside the body. Syngamy leads to formation of a specialised
cell called zygote.
The process of development of embryo from the zygote is called
embryogenesis. In animals, the zygote starts developing soon after its
formation. Animals may be either oviparous or viviparous. Embryonal
protection and care are better in viviparous organisms.
In flowering plants, after fertilisation, ovary develops into fruit and
ovules mature into seeds. Inside the mature seed is the progenitor of
the next generation, the embryo.
EXERCISES
1. Why is reproduction essential for organisms?
2. Which is a better mode of reproduction: sexual or asexual? Why?
3. Why is the offspring formed by asexual reproduction referred to as clone?
4. Offspring formed due to sexual reproduction have better chances of
survival. Why? Is this statement always true?
5. How does the progeny formed from asexual reproduction differ from
those formed by sexual reproduction?
6. Distinguish between asexual and sexual reproduction. Why is vegetative
reproduction also considered as a type of asexual reproduction?
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7. What is vegetative propagation? Give two suitable examples.
8. Define
(a) Juvenile phase,
(b) Reproductive phase,
(c) Senescent phase.
9. Higher organisms have resorted to sexual reproduction in spite of its
complexity. Why?
10. Explain why meiosis and gametogenesis are always interlinked?
11. Identify each part in a flowering plant and write whether it is haploid
(n) or diploid (2n).
(a) Ovary ———————————
(b) Anther ———————————
(c) Egg ———————————
(d) Pollen ———————————
(e) Male gamete ———————————
(f) Zygote ———————————
12. Define external fertilisation. Mention its disadvantages.
13. Differentiate between a zoospore and a zygote.
14. Differentiate between gametogenesis from embryogenesis.
15. Describe the post-fertilisation changes in a flower.
16. What is a bisexual flower? Collect five bisexual flowers from your
neighbourhood and with the help of your teacher find out their common
and scientific names.
17. Examine a few flowers of any cucurbit plant and try to identify the
staminate and pistillate flowers. Do you know any other plant that
bears unisexual flowers?
18. Why are offspring of oviparous animals at a greater risk as compared
to offspring of viviparous animals?
2020-21
