Post-Fertilisation Structures and Events

Table of Contents
1. What are Post-Fertilisation Events ?
2. Development of Endosperm
3. Development of Embryo
4. Development of Seed
5. Monocotyledonous and Dicotyledonous Seeds
6. Germination of Seed
7. Dormancy of Seed
8. Development of Fruit
9. Additional Illustrative Tables and Figures
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What are Post-Fertilisation Events ?

Post-fertilisation events in flowering plants are the sequence of structural and developmental processes that occur after successful pollination and fertilisation. These events include formation and development of the endosperm, development of the embryo, transformation of ovule into seed and ovary into fruit, and other phenomena such as dormancy, germination, apomixis and polyembryony. These processes ensure formation of a viable seed and its dispersal and establishment as a new sporophyte.

Development of Endosperm

After double fertilisation, one sperm nucleus fuses with the egg nucleus to form the zygote and the other sperm nucleus fuses with the two polar nuclei (or with the secondary nucleus) in the embryo sac to form the primary endosperm nucleus (PEN). The PEN undergoes divisions to form the endosperm, which acts as nutritive tissue for the developing embryo.

The endosperm is a tissue present in the seeds of flowering plants during seed development. It stores reserve food materials and provides nutrition to the developing embryo (commonly as starch, proteins or oils).

Mode of endosperm development is classified into three main types:

Types of Endosperm on the basis of Development

• Nuclear endosperm formation: The primary endosperm nucleus divides repeatedly without cell wall formation, producing many free nuclei in a common cytoplasm (free-nuclear endosperm). Later, cell walls may form around groups of nuclei to produce cellular endosperm.
• Cellular endosperm formation: Each nuclear division is immediately followed by cytokinesis so that the endosperm is cellular from the beginning. Coconut meat is an example where cellular endosperm predominates.
• Helobial endosperm formation (Extra Not in NCERT): An intermediate type where the first division of the PEN produces a large chalazal cell and a small micropylar cell; one segment shows nuclear type development and the other shows cellular type development.

Depending on the species and seed maturation, the embryo may utilise the endosperm completely (so the mature seed is non-endospermic) or endosperm may persist in the mature seed and serve as storage tissue.

Examples: In peas and beans the embryo consumes the endosperm during development; in castor, wheat, maize and barley the endosperm remains as a storage tissue in the mature seed.

Development of Embryo

The embryo arises from the zygote formed by fusion of the egg and sperm. Embryogeny usually begins once sufficient endosperm is present to nourish the developing zygote. Embryo structure and development differ between monocots and dicots: monocots have one cotyledon and dicots have two cotyledons.

Stages of Embryo Development in Dicots

Difference between Embryo and Endosperm

In brief, the embryo is the young sporophyte produced from the zygote and will develop into the seedling, whereas the endosperm is a nutritive tissue formed by fertilisation of polar nuclei and primarily functions to nourish the embryo. The embryo is diploid (usually), while the endosperm in most angiosperms is triploid (2n maternal + 1n paternal), though exceptions occur.

Development of Seed

After fertilisation the ovule transforms into a seed. Morphologically, the ripened ovule is called a seed; it is a mature, integumented megasporangium that contains the embryo and storage tissues. Seeds are characteristic of spermatophytes (gymnosperms and angiosperms).

Structure of Seed

A typical mature angiosperm seed has three main parts:

1. Seed coat

• The seed coat (testa and tegmen) develops from the integuments of the ovule and provides protection.
• In bitegmic ovules the outer integument forms a thick, hard outer layer called the testa and the inner integument forms a thin papery inner layer called the tegmen. In unitegmic ovules there is a single seed coat layer.
• The seed attaches to the fruit wall by a stalk called the funicle or funiculus; the point of attachment on the seed is the hilum.
• A small opening near the hilum, the micropyle, allows entry of water during germination. When ovules are anatropous there is often a ridge called the raphe.

2. Embryo

• The embryo is the young plant (future sporophyte) that develops from the zygote and contains the embryonal axis (tigellum) with cotyledons attached.
• The portion of the embryonal axis below the cotyledons is the hypocotyl, which bears the radicle (future root) at its tip.
• The portion above the cotyledons is the epicotyl, which bears the plumule (future shoot).

3. Endosperm

• Endosperm is the nutritive tissue that may be present in the mature seed or may be consumed during embryo development.
• Angiosperm seeds are classified by the presence or absence of endosperm at maturity:
• Non-endospermic (ex-albuminous) seeds: Endosperm is absorbed during embryo development and stored in enlarged cotyledons (e.g., many dicots such as gram, pea, bean). Examples include Capsella and most dicotyledons; note the exception Castor which retains endosperm.
• Endospermic (albuminous) seeds: Endosperm persists at maturity as the storage tissue and is used during germination (e.g., most monocots such as wheat, rice, maize; endospermic dicots include castor, papaya, cotton).

Note: Sometimes a persistent tissue derived from nucellus remains as a thin layer around the endosperm and is called perisperm (examples: betelnut, black pepper, castor).

Monocotyledonous and Dicotyledonous Seeds

• In monocot embryos there is a single cotyledon called the scutellum (example: grasses such as wheat).
• Opposite the scutellum a tongue-shaped outgrowth, the epiblast, may represent remnants of a second cotyledon.
• Some dicots show specific outgrowths such as a caruncle or strophiole (e.g., castor) near the micropyle; this spongy structure aids water absorption during germination.
• Based on cotyledon number angiosperms are classified as monocotyledons (one cotyledon) or dicotyledons (two cotyledons).

Table: Difference between Monocotyledonous and Dicotyledonous Seed

(i) Structure of Bean Seed (Dicotyledonous Seed)

Bean Seed

• Bean seed is kidney-shaped with a concave and a convex side. The concave side bears a white elongated scar called the hilum, marking the point of attachment to the funiculus.
• A small pore near the hilum, the micropyle, allows water entry during germination.
• The bean seed has two coats: a hard outer testa (variously coloured) and a thin white inner tegmen.
• Inside are two large fleshy cotyledons that store carbohydrates and proteins to nourish the embryo during germination.
• The embryonal axis bears the radicle (micropylar end) and the plumule (between the cotyledons). The sections of the axis are the hypocotyl and epicotyl as described earlier.

(ii) Structure of Maize Grain (Monocotyledonous Seed)

• A maize grain (Zea mays) is a dry, one-seeded indehiscent fruit of the caryopsis type in which the pericarp (fruit wall) is fused with the seed coat.
• The grain encloses two major parts: the persistent endosperm and the embryo.
• The endosperm is surrounded by an outer aleurone layer (one cell thick) whose cells contain protein bodies important during germination.
• The embryo has a shield-shaped cotyledon called the scutellum which presses against the endosperm and functions in absorption and transfer of nutrients to the growing embryo.
• The scutellum epidermis in contact with the endosperm is secretory and absorptive; it secretes hormones that induce synthesis of enzymes in the endosperm to mobilise stored food.
• The embryonal axis has a radicle covered by a protective sheath called the coleorhiza and a plumule covered by the coleoptile, a conical protective sheath that allows the shoot to emerge through soil during germination.

endosperm and an embryo. As seen in the longitudinal section, the major portion of grain is occupied by a large starchy endosperm.

Germination of Seed

Germination is the process by which a dormant embryo resumes growth and develops into a seedling (sporophyte). During germination the radicle emerges to form the primary root and the plumule forms the shoot; cotyledons may persist or degenerate depending on seed type.

Dormancy of Seed

• Seed dormancy is a common adaptive feature that prevents germination immediately after dispersal and allows seeds to survive adverse conditions for extended periods while remaining viable.
• The interval between seed maturation and germination is the dormancy period. Dormancy results from internal physiological or structural constraints that inhibit germination until conditions become favourable.
• Dormancy increases the chance of seed survival and successful establishment by allowing seeds to germinate at an appropriate time or place.

• Seed dormancy mechanisms include impermeable seed coats, physiological inhibitors, embryonic immaturity and requirement for specific environmental cues such as chilling, light or scarification.

Development of Fruit

After fertilisation the ovary develops into a fruit and the ovules develop into seeds. The pericarp (fruit wall) differentiates into layers (exocarp, mesocarp, endocarp) and fruits are adapted for dispersal by various agents (wind, water, animals).

Types of Fruit

Some fruits develop without fertilisation in a process called parthenocarpy, producing seedless fruits. Fruit development may involve only the ovary or contribute tissues from other floral parts.

Types of Fruit

• True fruits: Developed solely from the ovary (e.g., grapes, figs).
• False (accessory) fruits: Other floral parts contribute to the fruit structure (e.g., apple, strawberry).
• Parthenocarpic fruits: Fruits formed without fertilisation; often seedless (e.g., some bananas).

Additional Illustrative Tables and Figures

Table: Difference between Embryo and Endosperm

Table: Difference between Egg Cell and Secondary Nucleus

Table: Number of Chromosomes in Different Parts of a Flowering Plant

Table: Gametophyte