CH-2 Part-Ii

 Double Fertilisation -
Syngamy
One male gamete(n) + Egg(n) Zygote(2n) Embryo
Triple
2nd male gamete(2n) + Polar nuclei(2n) PEN(3n) Endosperm
Fusion
 Since two types of fusion, syngamy and triple fusion occurs

in an embryo sac, the process is called double fertilization.
• Post-fertilization events in a flowering plant
• Events which take place in a flowering plant after double fertilization are
termed as Post-fertilization events. Important post-fertilization events are:
• Development of Endosperm from PEC
• Development of Embryo from Zygote
• Development of Seed from Ovule
• Development of Fruit from Ovary
Endosperm
• Primary Endosperm Cell (PEC) formed as a result of
triple fusion undergoes repeated divisions and a triploid
endosperm tissue is formed.
• The main purpose of endosperm is to provide nutrition
to the developing embryo.
• Endosperm can be free-nuclear or cellular. Free-nuclear
endosperm is the most common type of endosperm
development.
• In a tender coconut, water present inside is the free-nuclear
endosperm.
• Multiple nuclei float in this highly nutritious clear fluid.
• Gradually cell walls develop enclosing several nuclei.
• These cells & nuclei start settling at the periphery, and layers of
cellular endosperm start appearing.
• Therefore, in a completely
mature coconut, water turns
milky surrounded by cellular
part, kernel & it does not
contain free nuclei.
• Endospermic seeds
• Seeds which have endosperm
• Albuminous seeds
• Example: wheat, rice
• Non-endospermic seeds
• Seeds which lack endosperm at maturity
• Non-albuminous seeds
• Example: pea, bean
Embryo Development
• Embryo is a diploid cell developing from zygote. It starts to
develop at the micropylar end.
• Most zygote divides to form embryo only after certain amount of
endosperm is formed. This is an adaptation to provide assured
nutrition to developing embryo.
• Early stages of embryo development are same in both monocot &
dicot embryo. Embryogeny refers to the stages of embryo
development.
• The embryo development starts with the fertilized egg (zygote),
then passes through the 2- cell stage, 8-cell stage, Globular, Heart
shaped stage and finally the mature embryo
structure of a dicotyledonous embryo
• Dicotyledonous embryo has the following important parts:
1) Embryonal axis
• Main axis of the embryo which divides it into different
regions.
2) Cotyledons
• 2 Seed leaves
• Helps in Food storage
• Provide nourishment to the
developing radicle & plumule.
 Epicotyl
• Part of embryonal axis above
the level of cotyledons
• Terminates at plumule
 Hypocotyl
• Part of embryonal axis below
the level of cotyledons
• Terminates at radicle
 Plumule : Stem tip
 Radicle : Root tip
 Root cap: Covering of root tip
Structure of a Monocotyledonous Embryo
1) Embryonal axis
• Main axis of the embryo which divides
it into different regions
2) Cotyledon
• Only one seed leaf exists
• Termed as Scutellum
• Located at one side of axis
 Plumule : Shoot tip
 Radicle : Root tip
 Root cap: Covering of root tip
 Coleorhiza : Sheath enclosing radicle &
root cap
 Coleoptile : Sheath enclosing plumule &
a few leaf primordia
Seed (Fertilised Ovule)
• Seed has 3 main parts-
1) Seed coat
2) Cotyledon
3) Embeyo
 On the basis of endosperm,

mature seeds are two types:
i) Non- albuminous or Ex-
alnuminous seed
ii) Albuminous seed
Perisperm- Residual, persistent nucellusin seed is called
perisperm. e.g Black pepper and Beet
Mycropyle remains as a small pore in seed coat and

facilitate entry of oxygen and water into the seed during
germination.
 Dormancy- it is a condition when embryo becomes in

active due to reduced water content (10-15%
moisture by mass)
Importance of Seed
1) Dispersal of plant to new habitat.
2) Nourishing young seedling by reserve food.
3) Protection of embryo by hard seed coat.
4) Leads to variation because it is produced by sexual
reproduction
VIABILITY OF SEEDS AFTER DISPERSAL
In a few species the seeds lose viability
within a few months. Seeds of a large
number of species live for several years.
 Some seeds can remain alive for hundreds
of years. The oldest is that of a lupine
(Lupinus arcticus) excavated from Arctic
Tundra. The seed germinated and
flowered after an estimated record of
10,000 years of dormancy.
2000 years old viable seed is of the date
palm (Phoenix dactylifera) discovered
during the archeological excavation at King
Herod’s palace near dead sea.
FRUIT FROM OVARY
 The Ovary develops into a fruit.

 The transformation of ovules into seeds and ovary into fruit
proceeds simultaneously.
 The wall of ovary develops into pericarp (wall of fruit).
 The fruit may be fleshy (e.g guava, orange, mango etc.) or may be
dry (e.g groundnut, mustard etc.)
FRUIT FROM OVARY
 Fruits are of two types:

 True fruits : In this, fruit develops only from the ovary. Other floral
parts degenerate and fall off. E.g most plants.
 False fruits : In this, the thalamus also contributes to fruit
formation. E.g apple, strawberry, cashew etc.
FRUIT FROM OVARY
 In some species fruits develop without fertilization. Such fruits are

called parthenocarpic fruits. E.g Banana.
 Parthenocarpy can be induced through the application of growth
hormones and such fruits are seedless.
APOMIXIS AND POLYEMBRYONY
 Apomixis is the production of
seeds without fertilization.
 E.g. In some species of Asteraceae
and grasses.
 It is a form of asexual
reproduction that mimics sexual
reproduction.
 Development of apomictic seeds:

In some species, diploid egg cell
is formed without reduction
division and develops into the
embryo without fertilization.
 In many species (e.g many Citrus and

Mango varieties) some of the nucellar
cells surrounding the embryo sac
starts dividing, protrude into the
embryo sac and develop into the
embryos. Thus each ovules contains
many embryos.
 Occurrence of more than one embryo
in a seed is referred to as
polyembryony.
IMPORTANCE OF APOMIXIS IN HYBRID SEED INDUSTRY
 If the seeds from hybrids are shown, the plants in the

progeny will segregate and lose hybrid characters.
 Production of hybrid seeds is costly, so hybrid seeds are
also expensive.
 If hybrid are made apomicts, there is no segregation of
characters in the hybrid progeny. So the farmers can
keep on using the hybrid seeds to raise new crop year
after year and he does not have to buy hybrid seeds
every year.

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CH-2 Part-Ii

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 Double Fertilisation -

 Since two types of fusion, syngamy and triple fusion occurs

 On the basis of endosperm,

Mycropyle remains as a small pore in seed coat and

 Dormancy- it is a condition when embryo becomes in

 The Ovary develops into a fruit.

 Fruits are of two types:

 In some species fruits develop without fertilization. Such fruits are

 Development of apomictic seeds:

 In many species (e.g many Citrus and

 If the seeds from hybrids are shown, the plants in the

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