REPRODUCTION IN PLANTS

Michael Nyaongo (Bsc[ed])

• In this topic were are going to deal with the following subtopics:

1. Events leading to fertilization in plants

• Concept of pollination

• Concept of double fertilization

• Changes which occur in the flowers after fertilization

2. fruit and seed development

• Formation of endospermic and non-endospermic seed

3. Life cycles of selected plants

• Alternation of generations

• Bryophytes, pteridophytes, angiosperms

INTRODUCTION

• Review Division coniferophyta and angiospermophyta

– Structure of the flower

– Draw the flower structure

– Life cycles

• Flowering plants reproduce by making seeds.

• Before a seed can grow into a new plant they have to be fertilized by
pollen.

• Pollen comes from another plant during a process called pollination.

• Pollination and fertilization take place in the flower of the plant.

The structure of a flower

• Petals: They are used to attract insects by their bright colour and scent.

• Sepals: Used to protect the flower while it is still in bud.

• Stamens: This is where pollen is made. It is the male part of the flower.
The stamen has two parts: the filament (a thin stalk) and the anther which
is where pollen is made

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 • Pistil is vase-like structure located at center of a flower; it contains one or
more carpels.
• Pistil may be simple or compound.
• Simple pistil contains one carpel; compound pistil contains multiple
carpels which are often fused
• Carpel: The carpel is the green stalk in the middle of the flower.
It is the female part of the flower

• Carpels are reproductive units of flowers and have three parts..

• Stigma is an enlarged sticky knob on end of a style; stigma
serves to receive pollen grains.
• Style is a slender stalk that connects stigma with the ovary.
• Ovary is enlarged base of a carpel that contains a number of
ovules.
•

Sexual Reproduction in flowering plants

• Sexual reproduction in a flowering plant involves the following stages:

– The formation of special reproductive cells(Gametogenesis)

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  The male gamete is the pollen grain that is a tough-walled single
cell with two nuclei: tube nucleus and Generative nucleus
 The female gamete is the egg cell that is a large cell present in
the embryo sac of the ovule

– Pollination: This is when pollen lands on a new flower

– Fertilization: This is when the pollen and the seed meet

– Fruit and seed development

– Seed and fruit dispersal: This is when the seed is spread around,
away from the plant that made it

– Seed Germination : This is when the seed, having reached the

ground, starts to grow into a new plant

1a. Formation of egg cell

• Female part of the plant consists of a carpel.

• Carpels consist of a stigma, usually mounted at the tip of a style with an

ovary at the base.

• Carpel/Pistil

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 • Often the entire whorl of carpels is fused into a single pistil.

• The megasporangia, called ovules, develop within the ovary.

• Ovules eventually develop into seeds

• Ovule is made up of parenchyma cells almost completely covered by

integuments.

• The inner layer of each ovule has a layer called the nucellus – which
nourishes the developing embryo sac

• At the apex of the ovule is a small opening through the integuments called
the micropyle. This is where the pollen tube will enter.

Structure of Ovule showing Embryo Sac

FORMATION AND DEVELOPMENT OF FEMALE GAMETOPHYTE

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 (EMBRYO SAC)

• One parenchyma cell enlarges to become a megasporocyte (megaspore

mother cell) which undergoes meiosis, producing four haploid megaspores.

• Three of these megaspore mother cells disintegrate, leaving one functional

embryo sac (megaspore).

• The nucleus of the functional megaspore undergoes 3 successive mitotic

divisions.

• The embryo sac (megaspore) enlarges and the haploid nucleus divides by
first round of mitosis to form 2 haploid nuclei

• The two haploid nuclei then undergo a second round of mitosis to form 4
haploid nuclei within the one embryo sac. A third and final round
of mitosis occurs to produce 8 haploid nuclei

• The 8 haploid nuclei move to various areas of the embryo sac

• Cell membranes and a thin cell wall form around 6 of the haploid nuclei
• The two remaining haploid nuclei remain free and are called polar nuclei
• The egg cell is present at the bottom of the embryo sac

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 Diagram showing the formation of female gametophyte (Embryo sac)

• The megagametophyte called the embryo sac consists of seven cells:

• One egg cell associated with

• Two synergid cells

• One central cell with two polar nuclei

• Three antipodal cells

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 • The Embryo Sac

• The egg cell will start the new sporophyte generation if it is fertilized

• It is surrounded by:

• 2 synergid cells which secrete an attractant that guides the

pollen tube through the micropyle into the embryo sac.

• The large central cell, which in most angiosperms contains 2

polar nuclei, will after its fertilization develop into the
endosperm of the seed.

• 3 antipodal cells.

1b. FORMATION OF MALE GAMETES

• Male gamete is the pollen grain

• The microgametophytes (pollen grain) are produced in the stemen.

• Each stamen consists of a

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 – lobed anther, containing the microsporangia(4 pollen sacs) and
supported by a thin

– filament

Pollen grain development

• Each pollen sac is filled with cells called microsporocyte (microspore

mother cells).

• Microspore mother cells divide by meiosis to produce four immature,

haploid microspores or pollen cells from which pollen grains develop.

• Immediately after meiosis, the four microspores are grouped together to

form a tetrad.

• The immature, haploid pollen grains (microspores) then divides mitotically

to form two cells enclosed by a finely sculptured wall.

• Mitosis of the haploid nucleus in each microspore also occurs during

maturation – this produces a pollen grain with two haploid nuclei:

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 • Tube nucleus: burrows into stigma and style

• Generative nucleus: fertilises egg

• This structure is the mature pollen grain.

Describe the formation and structure of Pollen grain and its adaptation

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2. Pollination

• Pollination is the transfer of pollen from anther to stigma of a flower of the

same species

– Pollen grains develop in the anthers.

– When mature, the anther wall splits open( a process called

dehiscence) and the pollen is shed.

There are two types:

1. Self-pollination: where a flower allows pollen to fertilise the egg cell within
the ovary of the same plant – disadvantageous to species as resulting seeds
less likely to form healthy plant

Self-Pollination

2. Cross-pollination: where a flower transfers pollen from anther to stigma of

different plant of same species – more advantageous as greater variation is
shown

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Cross-Pollination

Pollination Methods

• Wind: pollen is produced in very large amounts by the flower and is usually
small, light and smooth to allow easy transfer by wind, e.g., grasses

– wind-pollinated flowers are called anemophilous flowers

• Animal: pollen is produced in relatively small amounts grains are larger  and
stickier and they are usually transferred by insects (examples include
dandelions, daisies, tulips, roses), birds, bats,

– Insected pollinated flowers are called antomophilous flowers

• Assignment

1. Find out the adaptation of a flower to insect pollination

2. Summarize the differences between typical insect-polinated and wind-

pollinated flowers.

3. FERTILISATION

• Fertilization is the process of fusion of the female gamete, the ovum or egg
and the male gamete produced in the pollen tube by the pollen grain.

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 – The process of fertilization in plants occurs when gametes in haploid
conditions meet to create a zygote which is diploid.   

– The male gametes of the flower are transferred on to the female

reproductive organs through pollinators.

– The final product of this process is the formation of embryo in a

seed. 

• In plants, fertilization is after pollination of the carpel, there is

germination of the pollen grain and a pollen tube grows and travels to the
ovary.

• The pollen grain adheres to the stigma, a pollen tube grows and penetrates
the ovule and the pollen tube burst into the embryo sac.

Events leading to fertilization

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 • The pollen grain germinates after the carpel is pollinated. 

• The pollen grains attaches itself to the stigma of the female reproductive
structure, the pollen tube grows and enters the ovule making a tiny pore
called a micropyle and the pollen tube burst into the embryo sac.  

• Stigma secretes a sugary substance that stimulates the growth of the

pollen tube

• Two types of nucleus, the vegetative tube and generative nuclei of the
pollen grain pass into the pollen tube. 

• The tube/vegetative nucleus control the growth of the pollen tube

as it grows down the style and into the ovary of a flower.

• The generative nucleus enters the pollen tube and divides by mitosis
to form two haploid nuclei called sperm nuclei

• The sperm nuclei enter the embryo sac and ‘double fertilisation’
occurs:

• One fertilises the egg – diploid (2n) zygote results

• Other fuses with the two polar nuclei to form triploid (3n)
endosperm cell which goes on to absorb nutrients and
functions as a food store

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Mechanism of Double Fertilization

• The pollen grains attaches itself to the stigma of the female reproductive
structure, the pollen tube grows and enters the ovule making a tiny pore
called a micropyle. 

– Stigma secretes a sugary substance that stimulates the growth of the

pollen tube. 

• The pollen tube grows near the style and curls to the bottom of the ovary
and then near the receptacle. 

• The pollen tube then breaks into the ovule through the micropyle and then
the micropyle bursts into the embryo sac. 

• In the embryo sac, one of the male nucleus fuses with the nucleus of the
egg and forms a diploid zygote. This process is known as true fertilization or
syngamy

• The other male gamete or nucleus enters further into the embryo sac and it
fuses with two polar nuclei. This gives rise to a triploid nucleus called the
primary endosperm nucleus.

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 – The union of two sperm cells with different nuclei of the embryo sac
is called double fertilisation.

• Zygote divides mitotically to become the embryo; endosperm cell divides

mitotically to become endosperm.

– Embryo, in most plants, is a young sporophyte.

– Endosperm is tissue that will nourish embryo and seedling as they

undergo development.

Significance of double fertilization

• It gives stimulus to the plant due to which ovary develops into fruit and
ovules develop into seeds.

• It restores the diploid condition by fusion of haploid male and female

gametes.

• It results in the formation of diploid zygote, which develops into an embryo

and gives rise to a new plant.

• It results in the formation of triploid primary endosperm nucleus (PEN)

which develops into endosperm in the seed. It provides nourishment to the
developing embryo.

• It brings about recombination of characters resulting in variation among the

offspring

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Post Fertilization Changes in Flower

• After the process of fertilization, the zygote undergoes a number of mitotic

divisions to form multicellular embryo.

• The endopserm nucleus also goes through a series of divisions to form a

mass of endosperm cells. These endosperm cells provide nutrition to the
developing embryo. 

• After fertilization, the following changes are observed in a flower:

a. There is formation of a diploid zygote and it develops into an embryo,

which forms the future plant. 

b. The endosperm nucleus divides rapidly by mitosis to form a food

store.

 The endosperm cells serve as a source of nutrition for the

developing embryo.

c. The ovule becomes the seed.

 The outer and inner integuments of the ovule become the

testa or the seed coat of the seed.

d. The mature ovary becomes the fruit.

e. In most of the plants the antipodals and synnergids disintegrate

before, during or immediately after fertilization. 

f. Petals and sepals fall off. 

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 •

Seed Formation

• The fertilized ovule becomes the seed.

• The integuments become the wall of the seed called the testa. The
micropyle closes.

• The endosperm nucleus leads to the formation of triploid endosperm, a food

tissue.

• The diploid zygote, by mitosis, develops into a plant embryo. The

developing embryo draws nourishment from the endosperm. The embryo
ceases development and goes dormant.

• The ovule becomes a seed, which contains a dormant plant embryo, food
reserve, and the protective coat called the testa.

• The embryo is made up of the radicle or future root and the plumule or
future shoot.

• The endosperm cells divide many times and absorb the nucellus. This is the
nutrition (mainly fats, oils and starch) for the embryo

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Types of seeds

• Endospermic seeds: The endosperm is present in the mature seed and

serves as food storage organ.

• Testa and endosperm are the two covering layers of the embryo.

• In endospermic seeds the food reserve is the endosperm, which is outside

the plant embryo.

• Examples of this type of seed are maize and wheat

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 •

• Non-endospermic seeds: The cotyledons serve as sole food storage organs

as in the case of pea (Pisum sativum).

• During embryo development the cotyledons absorb the food reserves from
the endosperm.

• The endosperm is almost degraded in the mature seed and the embryo is
enclosed by the testa.

• Non-endospermic seeds have food reserve within the cotyledon(s) of the

plant embryo. This occurs in broad beans.

• E.g. pea (Pisum sativum), beans (Phaseolus vulgaris) and soyabean (Glycine
max).

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 •

Differences between endospermic and non-endospermic seed

Endospermic seed Non-endospermic seed

• Seed with endosperm • Seeds without endosperm

• Food storage of the seed is in • Food storage is in the

the endosperm cotyledons

• Seed of dicots or monocots only • Seed of dicots

• Eg maize • Eg beans

Class Assignment

1. Read on seed germination

• Conditions necessary for germination

• Events of germination

2. Different forms of seed germination

3. Discuss the concept of alternation of generation.

4. Describe the generalized Life cycles of bryophytes, pteridophytes and

angiosperms

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Life Cycles of Selected Plants

The Concept of Alternation of Generations

 Sexual reproduction involves the two alternating processes of meiosis

and fertilization.
o In meiosis, the chromosome number is reduced from the diploid
to the haploid number.
o In fertilization, the nuclei of two gametes fuse, raising the
chromosome number from haploid to diploid.
 All plants have a life-cycle with two distinct generations, the
sporophyte stage and the gametophyte stage.
 The sporophyte is diploid (2n), and produces haploid spores (n) through
meiosis.
 The gametophyte is haploid (n), and is formed from spores produced by
the sporophyte.
o Gametophyte release both male and female gametes, which join
to form a zygote (2n).
o The zygote then matures into a sporophyte through mitosis.
o Sporophytes produce spores via meiosis and therefore are
haploid. Spores grow into gametophytes and the cycle continues
again.
 Each generation producing offspring of the other
generation is called Alternation of Generations.
 Which generation is dominant depends on the type of plant.
o The life cycle of a Bryophyte is dominated by the gametophyte
stage, while
o Pteridophytes, Gymnosperms, and Angiosperms all have
dominant sporophyte generations. 
 In plant life cycles:
o Mitosis can occur in haploid cells as well as diploid ones.
o A haploid set of chromosomes, and hence a single set of genes
(one genome), is sufficient to control cell function in these
organisms (but not in most animals).

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 (a)

(b)

 The gametophyte is haploid and therefore produces haploid gametes

called sporophytes through mitosis.
 The sporophyte is diploid (because of mitosis, 2 sets of chromosomes)
and therefore produces diploid offspring called spores through meiosis.
 The spore is haploid (because of meiosis, 1 set of the
chromosomes) and can reproduce without fusing with another spore so
it then creates another gametophyte through mitosis.

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THE LIFE CYCLE OF A BRYOPHYTES

Bryophyta:

o Bryophytes include dependent on water to

mosses,liverworts and reproduce and survive.
hornworts. o This is because without
o Bryophytes are seedless, non vascular tissue, Bryophytes
vascular plants that lack true rely on Osmosis to transport
roots, leaves and stems.   nutrients throughout the
o The dominant generation of body.

Bryophytes is the o Rather than roots,

gametophyte. Bryophytes have rhizoids,

o Due to the lack of vascular which anchor the plant and

tissue and seeds, terrestrial help in taking in water and

bryophytes are completely nutrients. 

Life cycle of Bryophytes

 The male gametophyte is  The sporangium (at the tip of

called antheridia and the the stalk) is where meiosis
female is called archegonia. occurs, creating spores.
 The sperm swims through a  The tip pops off and spores
barrier of moisture to disperse.
fertilize the egg.  The spores germinate
 The diploid zygote divides (because they can reproduce
mitotically and develops into by themselves) by mitosis.
a sporophyte.  The spore eventually
 A long stalk evolves from the becomes mature and
embryonic sporophyte. becomes a fully grown moss
 The stalk remains connected land plant.
to the sporophyte, feeding it.

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THE LIFE CYCLE OF A PTERIDOPHYTE

Pteridophyta:

o Pteridophytes include ferns, o Unlike Bryophytes, the

horsetails, and club mosses. vascular tissue in
o Pteridophytes are seedless, Pteridophytes allows them to
vascular plants with true grow to a much greater
roots, leaves and stem. height.  
o In Pteridophytes, the
sporophyte generation is
dominant by a small margin.

Life cycle of Pteridophytes

 Ferns are homosporous,  The haploid spore produces a

producing one of the heart-shaped gametophyte,
same kind of spore.

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 maintaining itself by  The sporophyte develops
photosynthesis. from the fertilized egg and
 The male, antheridia, and grows out of the
female, archegonia, gametophyte.
gametophyte sex organs
mature at different times;  The spots under the leaves
therefore, cross-fertilization are called sori and sori are
occurs. clusters of sporangia.
 The flagellated sperm swim  The sporangia release
through the moisture layer spores, which will eventually
around the archegonia and continue the life cycle over
fertilize the egg. again.

THE LIFE CYCLE OF AN ANGIOSPERM

Angiosperms:

o Angiosperms include all

flowering plants.

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 o Angiosperms are seeded, o When cross fertilizied, the
o What makes
vascular themwith
plants, unique is
roots ovaries develop into fruit.
stemsreproductive
their and leaves. organ, the  This fruit protects the
flower. seeds, and helps to
o The flower contains pollen spread the seed over a
and seeds enclosed in much greater range of
ovaries. territory.

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Life cycle of Angiosperm

 The stamen, male (embryo sacs and

reproductive organ, and the pollen) together in the
carpel (or pistol), female ovary.
reproductive organ produces  Double fertilization, two
the micro-spores that form sperm with different nuclei of
the male gametophytes, the embryo sac, occurs and
pollen. zygotes become sporophyte
 The ovules produce mega- embryos that are protected
spores that form female and packaged into fruit
gametophytes, embryo sacs and/or seeds.
 Insects, such as bees, then  Animals then eat the fruit
feed off the nectar that the (which is actually
ovule produces and in turn, endosperm) and then dispose
unknowingly drops pollen of it and the seeds
attached to its fuzzy bottom are exposed.
into the pollen tube of the  The seeds then germinate to
flower. become the beautiful flower
o This brings the you see and the cycle
gametophytes continues.

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NECTA 2017 P1 QN 9b Use the above life cycle and the concept of meiosis
and mitosis to answer the question

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