Post-Fertilisation : Structures and Events

Post-fertilisation events in flowering plants turn each fertilised ovule into a seed and the ovary into a fruit. This page covers endosperm development, the stages of embryo development, dicot and grass embryos, seed structure, albuminous and non-albuminous seeds, perisperm, true, false and parthenocarpic fruits, the advantages and viability of seeds, apomixis and polyembryony. It follows the NCERT Class 12 chapter Sexual Reproduction in Flowering Plants. NEET often asks post-fertilisation facts on coconut endosperm, perisperm, the scutellum, the coleoptile and apomixis.

1. Post-fertilisation Events

• Post-fertilisation events: the development of the endosperm and embryo, the maturation of ovule(s) into seed(s) and of the ovary into a fruit, which together follow double fertilisation.

2. Endosperm

• ★ Exam imp Endosperm development precedes embryo development, so that food is ready for the embryo.
• The primary endosperm cell divides repeatedly and forms the triploid endosperm tissue.
• Endosperm cells are filled with reserve food, used to nourish the developing embryo.

2.1 Free-nuclear and cellular endosperm

1. In the most common type, the PEN undergoes successive nuclear divisions and forms many free nuclei. This stage is the free-nuclear endosperm.
2. Cell wall formation follows, and the endosperm becomes cellular.
3. The number of free nuclei formed before cellularisation varies greatly.

2.2 Fate of the endosperm

• Splitting open seeds of castor, peas, beans and groundnut, and a coconut fruit, shows where the endosperm has been used up and where it persists.

3. Embryo

• The embryo develops at the micropylar end of the embryo sac, where the zygote lies.
• ★ Exam imp Most zygotes divide only after a certain amount of endosperm has formed. This adaptation assures nutrition for the developing embryo.
• Seeds differ greatly, but the early stages of embryo development (embryogeny) are similar in monocotyledons and dicotyledons.

3.1 Stages of embryogeny in a dicot

1. Zygote, at the micropylar end of the embryo sac.
2. Proembryo, a small group of cells formed from the zygote.
3. Globular embryo, a ball of cells carried on a suspensor.
4. Heart-shaped embryo, whose two lobes become the cotyledons.
5. Mature embryo, with radicle, plumule and cotyledons.

3.2 The dicot embryo

• A typical dicot embryo has an embryonal axis and two cotyledons.
• Epicotyl: the part of the embryonal axis above the level of the cotyledons. It ends in the plumule (stem tip).
• Hypocotyl: the cylindrical part below the level of the cotyledons. It ends in the radicle (root tip).
• The root tip is covered with a root cap.

3.3 The monocot (grass) embryo

• Monocot embryos have only one cotyledon.
• ★ Exam imp In the grass family, the cotyledon is called the scutellum. It lies towards one side (lateral) of the embryonal axis.
• At its lower end, the embryonal axis has the radicle and root cap, enclosed in an undifferentiated sheath, the coleorrhiza.
• The part of the axis above the attachment of the scutellum is the epicotyl.
• The epicotyl has a shoot apex and a few leaf primordia, enclosed in a hollow foliar structure, the coleoptile.
• Soaking seeds of wheat, maize, peas, chickpeas or groundnut overnight and splitting them shows the parts of the embryo and the seed.

4. Seed

• ★ Exam imp In angiosperms, the seed is the final product of sexual reproduction. It is often described as a fertilised ovule.
• Seeds are formed inside fruits.
• A seed typically consists of seed coat(s), cotyledon(s) and an embryo axis.
• Cotyledons are simple structures, generally thick and swollen because they store food, as in legumes.

4.1 Albuminous and non-albuminous seeds

4.2 Seed coat, drying and dormancy

• The integuments of the ovule harden into tough, protective seed coats.
• The micropyle remains as a small pore in the seed coat. It lets oxygen and water enter the seed during germination.
• As the seed matures, its water content falls; seeds become relatively dry, with 10-15 per cent moisture by mass.
• The general metabolic activity of the embryo slows down.
• The embryo may enter a state of inactivity, dormancy. If favourable conditions are available (adequate moisture, oxygen and suitable temperature), the seed germinates.

5. Fruit

• As ovules mature into seeds, the ovary develops into a fruit; the two changes proceed simultaneously.
• The wall of the ovary develops into the wall of the fruit, the pericarp.
• Fruits may be fleshy (guava, orange, mango) or dry (groundnut, mustard).
• Many fruits have evolved mechanisms for the dispersal of seeds.
• The number of seeds in a fruit is linked to the number of ovules in the ovary, since each fertilised ovule forms one seed.

5.1 True and false fruits

• In most plants, the other floral parts degenerate and fall off by the time the fruit develops.
• ★ Exam imp In a few species, such as apple, strawberry and cashew, the thalamus also contributes to the fruit. Such fruits are false fruits.
• Most fruits develop only from the ovary; they are true fruits.

5.2 Parthenocarpic fruits

• In most species fruits result from fertilisation, but a few species form fruits without fertilisation. These are parthenocarpic fruits.
• ★ Exam imp Banana is one such example.
• Parthenocarpy can be induced by applying growth hormones; such fruits are seedless.

6. Importance of Seeds

6.1 Advantages of seeds to angiosperms

1. Pollination and fertilisation are independent of water, so seed formation is more dependable.
2. Seeds have better adaptive strategies for dispersal to new habitats, helping the species colonise other areas.
3. Their food reserves nourish young seedlings until they can photosynthesise on their own.
4. The hard seed coat protects the young embryo.
5. Being products of sexual reproduction, seeds carry new genetic combinations, which lead to variation.

• Seed is the basis of our agriculture.
• Dehydration and dormancy of mature seeds allow them to be stored, to be used as food through the year and to raise the next season's crop.

6.2 Seed viability

• How long seeds remain alive after dispersal varies greatly.
• In a few species, seeds lose viability within a few months; in a large number they live for several years; some remain alive for hundreds of years.
• ★ Exam imp The oldest record is a lupine, Lupinus arcticus, dug out of the Arctic Tundra. It germinated and flowered after an estimated 10,000 years of dormancy.
• A recent record is a 2000-year-old viable seed of the date palm, Phoenix dactylifera, found during the archaeological excavation at King Herod's palace near the Dead Sea.

6.3 Huge reproductive capacity

• An embryo sac has one egg, an ovule generally one embryo sac, and an ovary one to many ovules, so a single tree can produce enormous numbers of seeds.
• Orchid fruits contain thousands of tiny seeds; so do the fruits of parasitic species such as Orobanche and Striga.
• A tiny seed of Ficus grows into a huge tree, which produces billions of seeds.

7. Apomixis and Polyembryony

• Fruit production without fertilisation is called parthenocarpy; seed production without fertilisation is apomixis.
• Apomictic seeds develop in several ways:

1. In some species, the diploid egg cell forms without reduction division and develops into an embryo without fertilisation.
2. More often, as in many Citrus and mango varieties, some nucellar cells around the embryo sac start dividing, protrude into the embryo sac and develop into embryos.

• Apomictic embryos form without fertilisation from the mother plant's own cells, so they are genetically identical to it; they can be called clones.

7.1 Apomixis and hybrid seeds

• Hybrid varieties of many food and vegetable crops are widely cultivated, and they have tremendously increased productivity.
• ★ Exam imp Problem: hybrid seeds must be produced every year. If seeds from hybrids are sown, the progeny segregate and lose the hybrid characters.
• Producing hybrid seed is costly, so hybrid seeds become too expensive for farmers.
• If hybrids are made into apomicts, the hybrid progeny show no segregation. Farmers can then reuse the hybrid seed year after year.
• So active research is going on worldwide to understand the genetics of apomixis and to transfer apomictic genes into hybrid varieties.
• Apomicts have several advantages in horticulture and agriculture; apomixis is found particularly in grasses.

8. Exam Essentials

Pairs to Match

Numbers to Remember

• Endosperm: 3n; zygote and embryo: 2n.
• Coconut water: thousands of free nuclei.
• Cotyledons: 2 in a dicot embryo, 1 in a monocot embryo.
• Moisture in mature seeds: 10-15 per cent by mass.
• Lupinus arcticus: about 10,000 years of dormancy; date palm seed: 2000 years old.
• Orchid fruit: thousands of tiny seeds; a Ficus tree: billions of seeds.

Examples to Remember

9. Quick Revision

• Post-fertilisation: endosperm and embryo develop; ovules become seeds, the ovary a fruit.
• Endosperm (3n) forms before the embryo, from the primary endosperm cell.
• Free-nuclear endosperm turns cellular; coconut water is free-nuclear, the kernel cellular.
• Endosperm is consumed in pea, groundnut and beans; it persists in castor and coconut.
• The embryo forms at the micropylar end; zygotes usually wait for some endosperm to form.
• Embryogeny: zygote, proembryo, globular, heart-shaped, mature.
• Dicot embryo: two cotyledons; epicotyl ends in the plumule, hypocotyl in the radicle with a root cap.
• Grass embryo: scutellum (one lateral cotyledon), coleoptile over the shoot apex, coleorrhiza over the radicle.
• The seed is the final product of sexual reproduction, a fertilised ovule inside the fruit.
• Non-albuminous: pea, groundnut. Albuminous: wheat, maize, barley, castor. Perisperm: black pepper, beet.
• Integuments form the seed coat; the micropyle stays as a pore for oxygen and water.
• Mature seeds dry to 10-15 per cent moisture and may become dormant.
• Ovary wall forms the pericarp; false fruits (apple, strawberry, cashew) include the thalamus; banana is parthenocarpic.
• Seeds are dependable, dispersible, nourished, protected and variable; Lupinus arcticus survived about 10,000 years.
• Apomixis (seeds without fertilisation) keeps hybrid characters from segregating; polyembryony occurs in Citrus and mango.

10. Solved Examples

Common Mistakes to Avoid