2. SEXUAL REPRODUCTION IN FLOWERING PLANTS
- After entering the synergid, the pollen
tube releases 2 male gametes into the cytoplasm of the synergid. One male
gamete moves towards the egg cell and fuses with its nucleus (syngamy) to
form zygote (diploid).
- The other male gamete moves towards the
two polar nuclei located in the central cell and fuses with them to produce a triploid primary endosperm nucleus (PEN).
As it involves
fusion of 3 haploid nuclei, it is called triple
fusion.
- Since 2 types of fusions (syngamy &
triple fusion) take place in an embryo sac, it is called double
fertilisation.
It is an event unique to flowering plants.
- The central cell after triple fusion
becomes the primary endosperm cell (PEC) and develops into the endosperm
while the zygote develops into an embryo.
POST- FERTILISATION: STRUCTURES & EVENTS
Post-fertilisation
events: Endosperm & embryo
development, maturation of
ovule(s) into seed(s) & ovary into fruit.
Endosperm development
-
Primary endosperm cell (PEC)
divides repeatedly to form a triploid endosperm tissue.
-
Endosperm cells are filled with reserve
food materials. They are used for nutrition of the developing embryo.
-
In common endosperm development, PEN
undergoes successive nuclear divisions to give free nuclei (free-nuclear endosperm). Number of free
nuclei varies greatly.
- Endosperm
becomes cellular due to cell wall formation.
- Tender coconut
water is a free-nuclear endosperm (made up of thousands of
nuclei) and the surrounding white kernel is the cellular endosperm.
Embryo development
-
Embryo develops at the micropylar end of
the embryo sac where the zygote is situated.
-
Most zygotes divide only after the
formation of some endosperm. This provides nutrition to developing embryo.
- In monocots & dicots, seeds differ greatly but embryogeny
(early embryonic developments) is similar.
- Zygote → Pro-embryo
→ Globular → Heart-shaped → Mature embryo.
Dicotyledonous embryo
-
It has an embryonal axis and 2 cotyledons.
-
Portion of embryonal axis above the level
of cotyledons is the epicotyl, which terminates with plumule (stem
tip).
-
The cylindrical portion below the level of
cotyledons is hypocotyl that terminates with the radicle (root tip). The
root tip is covered with a root cap.
Monocotyledonous embryo
-
They possess only one cotyledon.
-
Cotyledon of the grass family is called scutellum.
-
It is situated lateral to the embryonal
axis. At its lower end, the embryonal axis has the radicle and root cap
enclosed in coleorrhiza (an undifferentiated sheath).
-
Portion of embryonal
axis above the level of attachment of scutellum is the epicotyl. It has a shoot
apex and a few leaf primordia enclosed in coleoptile (a hollow foliar
structure).
Seed from Ovule
-
Seed is the fertilized ovule formed inside
fruits. It is the final product of sexual reproduction.
- It consists of seed coat(s), cotyledon(s) & an
embryo axis.
-
The cotyledons are simple, generally thick
and swollen due to storage food (as in legumes).
-
Mature seeds are 2 types:
o
Non-albuminous (Ex-albuminous) seeds:
Have no residual endosperm as it is completely consumed during embryo
development. E.g. pea, groundnut, beans.
o
Albuminous seeds:
Retain a part of endosperm. E.g. wheat, maize, barley, castor, coconut.
- Occasionally, in some seeds (black pepper, beet etc.)
remnants of nucellus are also persistent. It is called perisperm.
-
Integuments of ovules harden as tough
protective seed coats. It has a small pore (micropyle) through which O2
& water enter into the seed during germination.
- As
the seed matures, it becomes dry by reducing water content (10-15 % moisture by
mass). The metabolic activity of the embryo slows down. It may enter a state of
inactivity (dormancy). Under favourable conditions (moisture, oxygen &
suitable temperature), they
germinate.
Structure of some seeds
Advantages of seeds:
· Since
pollination and fertilisation are independent of water, seed formation is more
dependable.
· Better
adaptive strategies for dispersal to new habitats. It helps the species to
colonize in other areas.
· They
have food reserves. So seedlings are nourished until they are capable of
photosynthesis.
·
The hard seed coat protects the young
embryo.
· Being
products of sexual reproduction, they generate new genetic combinations and
variations.
· Dehydration & dormancy helps to store seeds. It can be used as
food throughout year and to raise crop in next season.
Viability of seeds after dispersal:
-
In a few species, the seeds lose viability
within a few months. Seeds of many 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 the Dead Sea.
Fruit from Ovary
-
The ovary develops into a fruit. Transformation
of ovules into seeds and ovary into fruit proceeds simultaneously.
-
The wall of ovary develops into pericarp
(wall of fruit).
-
The fruits may be fleshy (e.g. guava,
orange, mango, etc.) or dry (e.g. groundnut, mustard etc.).
-
Fruits are 2 types:
o
True fruits: In
this, fruit develops only from the ovary. Other floral parts degenerate &
fall off. E.g. most plants.
o
False fruits: In
this, the thalamus also contributes to fruit formation. E.g. apple,
strawberry, cashew etc.
-
In some species, fruits develop without
fertilisation. Such fruits are called parthenocarpic fruits. E.g. Banana.
-
Parthenocarpy can be induced through the
application of growth hormones. Such fruits are seedless.
- Apomixis
is
the production of seeds without fertilisation. E.g. Some species of Asteraceae
and grasses.
- It
is a form of asexual reproduction that mimics sexual reproduction.
- In some species, diploid egg cell is formed without
reduction division and develops into the embryo without fertilisation.
- In many species (e.g. many Citrus & Mango varieties)
some nucellar cells surrounding the embryo sac divide, protrude into the
embryo sac to form embryos. Thus each ovule contains many embryos. Occurrence
of more than one embryo in a seed is called polyembryony.
Importance of apomixis in hybrid seed
industry
-
If the seeds collected from hybrids are
sown, plants in the progeny will segregate and lose hybrid characters.
-
Production of hybrid seeds is costly. So hybrid
seeds are also expensive. If the hybrids are made into apomicts, there is no
segregation in the hybrid progeny. So farmers can keep on using hybrid seeds to
raise new crop.
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