Post Fertilisation: Structure, Events, Diagrams, Solved Examples

Post Fertilisation: In angiospermic plants, the pollen grains produce two male gametes. One male gamete fuses with the egg cell to form a diploid zygote (fertilisation). In addition, two polar nuclei fuse with the other male gamete to produce a triploid primary endosperm nucleus (PEN). Due to the fusion of three haploid nuclei, it is called a triple fusion. After the formation of the zygote, post fertilisation events take place.

Further, the zygote divides to form an embryo by the process of embryogeny. The primary endosperm nucleus further divides to form the endosperm, which has been utilised to form a seed. So, all these changes from seed development to fruit formation are considered post-fertilisation events. Read this article to learn about the post-fertilisation structure and events.

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Post Fertilisation

The post-fertilization events include:

1. Endosperm development
2. Embryo development
3. Ovule maturing into Seed
4. Ovary maturating into fruit

 Let us now read all the post fertilisation events:

I. Endosperm

During embryonic development, one male gamete fuses with the two polar nuclei of the embryo sac, forming the primary endosperm nucleus, which repeatedly divides to form the triploid endosperm tissue. Food is stored in the cell walls of this tissue to supply the embryo with nutrition.

Ploidy of Endosperm

1. n in Gymnosperm
2. 2n in Oenothera
3. 3n in most of the angiosperm
4. 5n in Panaea, Plumbago, Fritillaria
5. 9n in Peperomia
6. 24576n in Arum maculatum

Families such as Orchidaceae, Podostemaceae, and Trapaceae lack endosperm

Development of Endosperm

The PEN is formed during triple fusion. The PEN undergoes division to form the endosperm. The cells of the endosperm are filled with reserve food material and are used for the nourishment of the developing embryo. The development of endosperm can be of various types:

i. Nuclear Endosperm Development: It is the most common type of endosperm development seen in most dicot plants. The primary endosperm nucleus undergoes successive nuclear division to give rise to nuclei. Finally, the wall formation begins and makes the endosperm a multicellular tissue—for example, Capsella, Maize, Cotton, Coconut milk, etc.

ii. Cellular Development: This type of development is seen in both monocot and dicot. In this, the primary endosperm nucleus divides many times, and each division is followed by wall formation. Examples: Petunia, Utricularia, Coconut (copra), etc.

iii. Helobial Development: This type of development is seen mostly in monocot. In this, the primary endosperm nucleus divides and forms two unequal size cells, one large and one small. Cell towards the micropyle is large. The large cell nucleus undergoes nuclear division and becomes multinucleated, while the lower nucleus undergoes few divisions or may not divide at all. Eventually, cell wall formation takes place. Examples: Asphodelus, Muscari, Saxifraga, and Echium.

Fate of Endosperm

I. Endosperm can be completely consumed for seed maturation; such seeds are called exalbuminous or non-endospermic seeds—examples: Pea, Gram, Few monocots- Orchids.
II. In some seeds, endosperm persists in mature seeds; such seeds are called albuminous or endospermic seeds. Example: Monocots like rice, wheat, maise, barley and few dicots like castor, sunflower, etc.

II. Embryo Development

As an embryo develops from a zygote, it undergoes what is called embryogenesis.

a. Embryogeny in Dicot Plants

In most angiosperm families, endosperm formation is initiated before the division of the zygote.

1. The zygote undergoes division to form two unequal cells. The larger cell towards the micropyle end is a suspensor cell, and a smaller cell towards the antipodal region is an embryonal cell.
2. The suspensor cell undergoes few transverse divisions to form a filamentous suspensor of 6-10 cells.
3. The first cell towards the micropyle end of the suspensor cell becomes swollen and functions as haustorium.
4. Located at the end of the suspensor, near the embryo, is the hypophysis. A radicle root cap develops later from the hypophysis.
5. The embryo cell undergoes two vertical and one transverse division to form eight cells arranged in two tiers of epibasal and hypobasal.
6. Epibasal tier form two cotyledons and plumule.
7. Hypobasal tier produces hypocotyl except for its tip.
8. An outer layer of octant embryonic cells constitutes the protoderm or dermatogen, while the inner layer produces the procambium and ground meristem. The globular embryo begins to develop cotyledons and takes on a heart-like shape.

Structure of Dicot Embryo

A dicotyledonous embryo consists of an embryonal axis (tigellum) and two cotyledons.

Tigellum has two parts: The part of the embryonal axis above the cotyledons is called epicotyl, with its tip as plumule (future shoot). The other part is present below the level of cotyledon and is known as hypocotyl. The tip of hypocotyl is called radicle, which is referred to as the future root.

b. Embryogeny in Monocots

The embryogeny in monocot is studied in Luzula forsteri and is called the Sagittaria type.

1. Zygote elongates and divides transversely and produces a suspensor cell towards the micropylar end and an embryo cell towards the chalazal end.
2. The embryo cell divides transversally again into a top and a middle cell.
3. The terminal cell divides vertically and transversally into globular embryos.

Structure of Monocot Embryo

1. Monocots have a single cotyledon called the scutellum situated at the lateral side of the embryonal axis.
2. The embryonal lower axis has a radicle and root cap enclosed in a sheath called coleorhiza.
3. The part of the axis above the scutellum is called epicotyl. In epicotyl, the shoot apex is enclosed by a hollow foliar structure, the coleoptile, which protects the plumule region.

III. Seed

Definition: A fertilised ovule is called a seed. Based on the availability of endosperm in the mature seed, seeds can be divided into three types. Seeds may be endospermic or albuminous (with endosperm), non-endospermic or non-albuminous (without endosperm) or perispermic (seeds in which nucellus is seen, an example, black pepper, coffee, castor and cardamom).

Transformation of Floral Parts After Fertilisation

Seed Coat: The two integuments present in the ovary mature to form a seed coat. Outer integument mature to form testa, and inner integument matures to form tegmen. During the germination process, the micropyle remains as a small hole that allows the movement of oxygen and water into the seed.

Seed Viability: The period for which seeds retain their ability of germination is called seed viability. Some seeds can remain active for several years. For example,

i. The oldest seed is of a lupine, Lupinus arcticus, mined from Arctic Tundra. It germinated and flowered after approximately 10,000 years of dormancy.
ii. Near the Dead Sea, 2000-year-old viable seeds of the Phoenix sapling were found during an archaeological dig.

IV. Fruit

As the ovule matures into a seed, the ovary develops into a fruit, which means that the change of ovules into seeds and the ovary into fruit occurs simultaneously. The ovary walls develop into the fruit wall called the pericarp. Based on fertilisation, fruit is of various types:

a. True Fruit: True fruit or eucarp is the fruit formed from the ripened ovary. Example: Mango, Coconut.

b. False Fruit: False fruit or pseudocarp refers to the fruit formed from the ripened ovary, but the edible parts are other flower parts. Example: Peduncle is associated with cashew nut fruit, Thalmus is associated with apple fruit, and Perianth is associated with morus fruit.

c. Parthenocarpic Fruit: The fruit developed without fertilisation is called parthenocarpic fruit. These fruits do not have seeds.

Summary

Soon, after the double fertilisation, post-fertilization events take place, i.e. the ovule matures to form a seed, and the ovary matures to form fruit, and the ovary wall develops into the fruit wall (pericarp). By fusing one male gamete with an egg cell, the zygote is formed. The diploid zygote then divides repeatedly to form the embryo, which is nourished by the triploid primary endosperm.

Seeds whose endosperm has been completely consumed for seed maturation are called exalbuminous seeds. Other seeds preserve their endosperm, which is called albuminous seeds. Fruit can be true fruit when seeds are produced through fertilisation. The fruit can be false if other parts of the flower are involved in fertilisation. Fruits can also develop without fertilisation if they are induced with a plant growth hormone-like auxin.

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Frequently Asked Questions (FAQs)

Q1. What happens to the ovule after fertilisation?
Ans. After fertilisation, the ovule matures to form the seed.

Q2. Which fruits are called false fruit?
Ans. When other parts of flowers are involved in the fruit formation, it is called false fruit, for example, Apple.

Q3. State the examples of true fruit?
Ans. Mango, coconut, kiwi, blueberry, etc., are few examples of the true fruit.

 Q4. What is the ploidy of endosperm in gymnosperm?
Ans. In gymnosperms, the endosperm is haploid (n).

Q.5. Which seed has the longest viability period discovered till now?
Ans. The oldest viable seed is of a lupine, Lupinus arcticus, mined from Arctic Tundra, which germinated and flowered after approximately 10,000 years of dormancy.

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