Sexual Reproduction in Flowering Plants - Notes | Class 12 | Part 3: Fertilization and Post fertilization events

Double Fertilisation

• 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 a 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 include endosperm & embryo development, maturation of ovule(s) into seed(s), & ovary into fruit.

Endosperm Development

• The primary endosperm cell (PEC) divides repeatedly to form a triploid endosperm tissue.
• Endosperm cells are filled with reserve food materials, which are used for nutrition of the developing embryo.
• In common endosperm development, PEN undergoes successive nuclear divisions to give free nuclei (free-nuclear endosperm). The 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

• The 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, which provides nutrition to the developing embryo.
• In monocots & dicots, seeds differ greatly, but embryogeny (early embryonic developments) is similar.
• The zygote develops into a pro-embryo, then becomes globular, heart-shaped, and finally a mature embryo.

Dicotyledonous Embryo

• It has an embryonal axis and 2 cotyledons.
• The portion of the embryonal axis above the level of cotyledons is the epicotyl, which terminates with the plumule (stem tip).
• The cylindrical portion below the level of cotyledons is the hypocotyl, which terminates with the radicle (root tip). The root tip is covered with a root cap.

Monocotyledonous Embryo

• They possess only one cotyledon.
• The cotyledon of the grass family is called the scutellum.
• It is situated lateral to the embryonal axis. At its lower end, the embryonal axis has the radicle and root cap enclosed in the coleorrhiza (an undifferentiated sheath).
• The portion of the embryonal axis above the level of attachment of the scutellum is the epicotyl, which has a shoot apex and a few leaf primordia enclosed in the coleoptile (a hollow foliar structure).

Seed from Ovule

• The seed is the fertilized ovule formed inside fruits. It is the final product of sexual reproduction.
• It consists of seed coat(s), cotyledon(s), and an embryo axis.
• The cotyledons are simple, generally thick and swollen due to storage of food (as in legumes).
• Mature seeds are of 2 types:
• Non-albuminous (Ex-albuminous) seeds: Have no residual endosperm as it is completely consumed during embryo development. E.g., pea, groundnut, beans.
• Albuminous seeds: Retain a part of the endosperm. E.g., wheat, maize, barley, castor, coconut.
• Occasionally, in some seeds (e.g., black pepper, beet), remnants of the nucellus are also persistent, called the perisperm.
• The integuments of ovules harden as tough protective seed coats, with a small pore (micropyle) through which O₂ & water enter 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 and may enter a state of inactivity (dormancy). Under favourable conditions (moisture, oxygen, & suitable temperature), they germinate.

• Advantages of seeds:
• Since pollination and fertilisation are independent of water, seed formation is more dependable.
• Better adaptive strategies for dispersal to new habitats, helping 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 help to store seeds, which can be used as food throughout the year and to raise crops in the next season.
• Viability of seeds after dispersal:
• In a few species, seeds lose viability within a few months, while seeds of many species live for several years.
• Some seeds can remain alive for hundreds of years. The oldest recorded is a lupine (Lupinus arcticus) excavated from Arctic Tundra, which germinated and flowered after an estimated 10,000 years of dormancy.
• A 2000-year-old viable seed of the date palm (Phoenix dactylifera) was discovered during an archaeological excavation at King Herod’s palace near the Dead Sea.

Fruit from Ovary

• The ovary develops into a fruit, with the transformation of ovules into seeds and ovary into fruit proceeding simultaneously.
• The wall of the ovary develops into the pericarp (wall of fruit).
• Fruits may be fleshy (e.g., guava, orange, mango) or dry (e.g., groundnut, mustard).
• Fruits are of 2 types:
• True fruits: The fruit develops only from the ovary, with other floral parts degenerating and falling off. E.g., most plants.
• False fruits: The thalamus also contributes to fruit formation. E.g., apple, strawberry, cashew.

• In some species, fruits develop without fertilisation, called parthenocarpic fruits. E.g., banana.
• Parthenocarpy can be induced through the application of growth hormones, resulting in seedless fruits.

Apomixis and Polyembryony

• 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, a 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. The 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, making hybrid seeds expensive. If hybrids are made into apomicts, there is no segregation in the hybrid progeny, so farmers can continue using hybrid seeds to raise new crops.

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• Topic 1: Structure of Flower
• Topic 2: Pollination

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