Post-fertilisation: Structures and Events

The Journey from Fertilised Ovule to Seed and Fruit

Double fertilisation gives the embryo sac two brand-new cells with very different futures: the diploid zygote ( $2n$ ) and the triploid primary endosperm cell ( $3n$ ). But fertilisation is only the beginning. A great deal still needs to happen before a tiny ovule becomes a mature seed sitting inside a fruit. The endosperm must build up a food reserve, the zygote must grow into a complete embryo, the ovule must harden into a seed, and the ovary must reshape itself into a fruit. Taken together, all of these changes are called post-fertilisation events.

Building the Food Reserve First — Endosperm Development

Here is something worth noticing: the embryo does not start dividing right away after fertilisation. The endosperm (the nutrient-storing tissue that will feed the embryo) develops first. By the time the embryo begins growing, a ready supply of food is already waiting for it. This timing is no accident. It is an adaptation that guarantees the embryo will never go hungry during its early, critical stages.

How the Endosperm Forms

The primary endosperm nucleus (PEN), produced during triple fusion, kicks off the process. It divides over and over, but in the most common pattern of endosperm development, something unusual happens: the nuclei multiply without building cell walls between them. The result is a mass of free nuclei floating in a shared cytoplasm. This stage is called the free-nuclear endosperm.

Cell wall formation comes later. Once walls appear, the tissue is converted into cellular endosperm, where each cell is packed with reserve food materials.

You have actually seen both forms if you have ever opened a tender coconut. The sweet water inside is the free-nuclear endosperm, a liquid containing thousands of nuclei but no cell walls separating them. The firm white flesh (kernel) that lines the inner surface of the shell is the cellular endosperm, where walls have formed and cells are loaded with oils and nutrients. The number of free nuclei produced before cell walls finally appear varies a great deal from species to species.

Two Fates of the Endosperm

Not every seed keeps its endosperm all the way to maturity. What happens to it depends on how much the developing embryo consumes:

Endosperm fully consumed before the seed ripens — In plants like pea, groundnut, and beans, the growing embryo absorbs every bit of endosperm while still inside the ovule. By the time the seed is mature, no endosperm remains. All the food reserves end up stored in the thick, fleshy cotyledons instead. Such seeds are called non-albuminous (or ex-albuminous) seeds.
Endosperm persists in the mature seed — In plants like castor, coconut, wheat, rice, and maize, the embryo uses only part of the endosperm during its development. A significant portion survives into the ripe seed and acts as the food supply during germination. These are called albuminous seeds.

There is one more variation. In a few species such as black pepper and beet, it is not just the endosperm that sticks around. Remnants of the nucellus (the tissue that originally surrounded the embryo sac within the ovule) also persist in the mature seed. This leftover nucellus is known as perisperm.

From a Single Cell to a Complete Body Plan — Embryo Development

Where and When the Embryo Begins

The zygote sits at the micropylar end (the end nearest the opening) of the embryo sac. It does not rush into dividing. Most zygotes wait until a certain amount of endosperm has already accumulated around them before they begin their first division. This delay is a practical adaptation: the embryo does not start growing until its food supply is secured.

The Stages of Embryogeny

The entire process of embryo development is called embryogeny. Although mature monocot and dicot seeds look very different from each other, the early stages of embryogeny are surprisingly similar in both groups. In a dicotyledonous plant, the zygote passes through these stages:

Proembryo — the first few cell divisions produce a small cluster of cells
Globular stage — the cluster rounds into a ball-shaped mass
Heart-shaped stage — two bumps emerge on one side (these will become the cotyledons), giving the embryo the outline of a heart
Mature embryo — the cotyledons elongate, the embryonal axis differentiates, and all the basic parts of the future plant are in place

Anatomy of a Dicot Embryo

A fully formed dicotyledonous embryo (see Fig 1.14a) is made of two main parts: an embryonal axis (the central stem of the future plant) and two cotyledons (seed leaves that store or absorb food).

Looking at the axis from top to bottom:

Epicotyl — the region above the point where the cotyledons are attached. It ends in the plumule (the embryonic shoot tip that will grow into the stem and leaves after germination).
Hypocotyl — the cylindrical region below the cotyledon attachment point. It ends in the radicle (the embryonic root tip), which is covered by a protective root cap.

How a Monocot Embryo Differs

Monocotyledonous embryos (see Fig 1.14b) have only one cotyledon instead of two. In grasses (family Poaceae), this single cotyledon has a special name: the scutellum. It sits to one side (laterally) of the embryonal axis, pressed flat against the endosperm so it can absorb nutrients from it.

The monocot embryo also carries two protective sheaths that dicots lack:

Coleorrhiza — an undifferentiated sheath that wraps around the radicle and root cap at the lower end of the axis
Coleoptile — a hollow, leaf-like covering that encloses the shoot apex and a few young leaf primordia (the earliest leaf structures) at the upper end. The coleoptile pushes through the soil ahead of the delicate shoot tip during germination, protecting it from damage

Fig 1.14: (a) A typical dicot embryo; (b) L.S. of a grass embryo

Packaging It All Up — The Seed

The seed is the final product of sexual reproduction in flowering plants. Think of it as a fertilised ovule that has matured: it carries a dormant embryo, a food supply (or the remains of one), and a tough protective coat, all bundled together in a compact, durable package.

What Makes Up a Seed

Every seed has three basic components:

Seed coat(s) — formed from the integuments (the layers that originally surrounded the ovule). As the ovule matures, the integuments harden into tough coverings that shield the embryo from physical damage, pathogens, and drying out. The micropyle, the tiny pore that was present in the ovule, persists as a small opening in the seed coat. During germination, this pore lets water and oxygen enter the seed, triggering the embryo back to life.
Cotyledon(s) — simple, often thick and fleshy structures loaded with food reserves. Dicots have two; monocots have one.
Embryo axis — the miniature future plant, complete with its plumule (shoot tip) and radicle (root tip).

Albuminous vs Non-albuminous Seeds — A Closer Look

Feature	Non-albuminous (ex-albuminous)	Albuminous
Endosperm at maturity	Absent (fully consumed by the embryo)	Present (partially retained)
Food stored mainly in	Cotyledons	Endosperm
Examples	Pea, groundnut, beans	Wheat, maize, barley, castor, coconut

How Seeds Mature and Enter Dormancy

Two important changes take place as a seed ripens:

Water content drops sharply — Mature seeds are relatively dry, retaining only about 10 to 15 per cent moisture by mass. This dehydration slows down all metabolic activity, making the seed stable enough to be stored for long periods.
Dormancy may set in — The embryo can enter a state of suspended activity called dormancy (a pause in growth and development). The seed stays alive but does not germinate. Dormancy prevents the embryo from sprouting at the wrong time, say in the middle of a dry spell or the cold of winter. When conditions become favourable, with adequate moisture, oxygen, and a suitable temperature, the seed breaks dormancy and germinates.

This ability to dehydrate and go dormant is what makes agriculture possible. Farmers harvest seeds, store them for months, and plant them in the next season. Without dormancy, every seed would germinate the moment it formed, and long-term storage of grain would be impossible.

How Long Can Seeds Stay Alive?

Seed viability (the length of time a seed remains capable of germinating) varies enormously:

Some seeds lose the ability to germinate within a few months of being dispersed.
Seeds of many species stay viable for several years or even decades.
The most extreme examples are astonishing. A seed of the Arctic lupine, Lupinus arcticus, excavated from frozen tundra soil, germinated and flowered after an estimated 10,000 years of dormancy, making it the oldest viable seed on record. A more recent find is a 2,000-year-old seed of the date palm, Phoenix dactylifera, discovered during archaeological excavation at King Herod’s palace near the Dead Sea, which also successfully sprouted.

When the Ovary Becomes a Fruit

While ovules are turning into seeds inside the ovary, the ovary itself undergoes a parallel transformation into a fruit. The wall of the ovary thickens and differentiates into the pericarp (the wall of the fruit). Depending on the species, the pericarp can be:

Fleshy — soft and often juicy, as in guava, orange, and mango
Dry — hard or papery, as in groundnut and mustard

There is a direct relationship between ovules and seeds: each fertilised ovule develops into one seed. So the maximum number of seeds in a fruit equals the number of ovules in the ovary, though unfertilised ovules do not form seeds.

In most plants, the other floral parts (sepals, petals, stamens) wither and fall off as the fruit matures. The fruit is essentially what remains of the flower, built around the developing seeds.

True Fruits, False Fruits, and Fruits Without Fertilisation

True fruits — The majority of fruits develop solely from the ovary. These are called true fruits. Mango, groundnut, and mustard are all examples.
False fruits — In a handful of species, the thalamus (the receptacle, the platform on which the flower parts sit) also swells and becomes part of the fruit. In an apple, the crunchy flesh you eat is mostly the enlarged thalamus; the actual fruit (from the ovary) is the tough core containing the seeds. In a strawberry, the tiny “seeds” dotting the surface are the real fruits (called achenes), while the red fleshy part is the swollen thalamus. Cashew follows a similar pattern.
Parthenocarpic fruits — Normally, a fruit forms only after fertilisation. But in some species, fruits develop without any fertilisation at all. These are called parthenocarpic fruits, and since no fertilisation means no embryo, they are seedless. Banana is a well-known natural example. Parthenocarpy can also be triggered artificially by applying growth hormones, which is how seedless varieties of certain commercial fruits are produced.

Fig 1.15: (a) Structure of some seeds; (b) False fruits of apple and strawberry

Why Seeds Changed the Game for Flowering Plants

Seeds are far more than just the end product of reproduction. They give angiosperms a powerful set of survival advantages:

Independence from water — Unlike mosses and ferns, where sperm must swim through water to reach the egg, pollination and fertilisation in seed plants work without any standing water. This makes successful reproduction far more reliable across a wider range of habitats.
Built-in dispersal strategies — Seeds come equipped with mechanisms for reaching new habitats. Wind, water, animals, and even explosive pod-splitting scatter seeds far from the parent plant, helping the species colonise fresh territory.
A ready food supply — The endosperm or food-rich cotyledons nourish the young seedling during its earliest, most vulnerable days, before it can carry out photosynthesis on its own.
Physical protection — The hard seed coat guards the delicate embryo against mechanical damage, disease organisms, and harsh environmental conditions.
Genetic variation — Because seeds are products of sexual reproduction (involving meiosis and the fusion of gametes from two parents), every seed carries a unique combination of genes. This variation is the raw material on which natural selection acts, driving adaptation and evolution.

The Sheer Reproductive Power of Seeds

Consider the enormous reproductive capacity that seeds make possible. Each embryo sac has one egg, each ovule has one embryo sac, but an ovary can hold many ovules, a flower can have multiple ovaries, and a single tree can carry thousands of flowers. Species like orchids produce fruits containing thousands of tiny seeds each. Parasitic plants such as Orobanche and Striga do the same. Even the humble fig tree, Ficus, grows from a seed so tiny it is barely visible, yet it develops into a massive tree that produces billions of seeds over its lifetime.