Seed and Fruit Development

From Fertilised Ovule to Finished Seed

What happens to a tiny ovule once double fertilisation is over? It becomes a seed: the structure that carries the next generation of the plant safely into the future. In angiosperms, the seed is the end product of sexual reproduction. Every seed you see, whether it is a large bean, a small mustard grain, or a thin rice kernel, started life as a fertilised ovule tucked inside an ovary.

Seeds always form inside fruits. A typical seed is built from three components:

Seed coat(s) (the tough outer covering) — formed when the integuments of the ovule harden during maturation. This coat protects the embryo from physical damage, desiccation, and pathogens.
Cotyledon(s) (the seed leaves of the embryo) — simple, often thick and fleshy structures that store food reserves. In legumes like peas and beans, the cotyledons are visibly plump because they have absorbed all the nutrients the embryo will need.
Embryo axis (the central structure that will develop into the root and shoot of the new plant) — the part that carries the plumule at one end and the radicle at the other.

Albuminous and Non-albuminous Seeds: Where Does the Food Go?

Not all mature seeds look the same on the inside. The key difference lies in what happened to the endosperm during development.

Non-albuminous seeds (also called ex-albuminous seeds) have no leftover endosperm in the ripe seed. The developing embryo consumed every bit of it before the seed finished maturing. All the food reserves now sit inside the swollen cotyledons. Pea and groundnut are familiar examples: split one open and you find two large, food-packed cotyledons with no separate endosperm layer in sight.
Albuminous seeds keep part of their endosperm intact because the embryo did not use it all up. This retained endosperm acts as a nutrient store that the seedling will tap into later, during germination. Wheat, maize, barley, and castor all fall into this category.

There is also a third tissue that shows up in certain seeds. In species like black pepper and beet, remnants of the nucellus (the tissue that originally surrounded the embryo sac inside the ovule) persist even in the mature seed. This residual nucellus tissue is called the perisperm (leftover nucellus that remains as a storage or structural tissue in the seed).

The Seed Coat and Micropyle: Built-in Protection and a Doorway for Germination

As the ovule ripens into a seed, the integuments (the protective layers that wrapped around the ovule) undergo a dramatic change. They harden and toughen up, forming the rigid seed coat that acts like armour for the embryo within.

One small but important feature survives from the ovule stage. The micropyle (the tiny gap between the integuments that originally let the pollen tube enter) remains as a small pore in the seed coat. During germination, this pore serves as the entry point for oxygen and water, both of which the embryo needs to wake up and start growing.

What Happens as a Seed Matures

During the final stages of development, the seed dries out. Its moisture content drops to roughly 10 to 15 per cent of its mass. This dehydration is not damage; it is a survival feature. With less water, metabolic activity inside the embryo slows to a near standstill.

At this point the embryo faces two possible paths:

Dormancy (a state of suspended activity) — If conditions around the seed are not right for growth, the embryo enters dormancy and waits. It can stay in this resting state for weeks, months, or even centuries, depending on the species.
Germination — If the seed lands in a spot with adequate moisture, oxygen, and a suitable temperature, the embryo resumes its activity and begins to grow into a new plant.

From Ovary to Fruit: A Parallel Transformation

While ovules are maturing into seeds, the ovary that holds them is undergoing its own makeover. The wall of the ovary thickens and develops into the pericarp (the wall of the fruit). These two processes, ovule to seed and ovary to fruit, run in parallel and are tightly coordinated.

The pericarp can take on very different textures depending on the species:

Fleshy fruits — guava, orange, mango: the pericarp is soft, juicy, and often attractive to animals, which helps with seed dispersal.
Dry fruits — groundnut, mustard: the pericarp is thin and papery or hard and woody.

Many fruits have evolved clever mechanisms for scattering their seeds far from the parent plant, whether through wind, water, or animal activity.

One interesting connection to think about: the number of seeds inside a fruit is directly linked to how many ovules were present in the ovary. Each fertilised ovule becomes one seed, so a fruit with many seeds came from an ovary that contained many ovules.

True Fruits, False Fruits, and Parthenocarpy

In most flowering plants, the petals, sepals, stamens, and other floral parts wither and fall off once the fruit starts forming. The fruit develops entirely from the ovary, and such fruits are called true fruits.

However, in a handful of species, the thalamus (the swollen receptacle or base of the flower) also contributes to the fruit body. The result is a false fruit (a fruit in which tissues other than the ovary, particularly the thalamus, form a significant part of the structure). Apple, strawberry, and cashew are well-known false fruits.

Fig 1.15: (a) Structure of seeds: bean, castor, and maize grain. (b) False fruits: apple (thalamus forms the fleshy edible part) and strawberry (thalamus forms the fleshy receptacle; the tiny achenes on the surface are the true fruits)

In an apple, for instance, the fleshy part you eat is largely thalamus tissue, while the actual ovary-derived portion is the thin core surrounding the seeds. In a strawberry, the red fleshy part is the thalamus, and the small, hard dots on its surface (called achenes) are each an individual true fruit containing a seed.

There is one more category worth knowing. In a few species, fruits develop from the ovary even though fertilisation never happened. These are called parthenocarpic fruits (fruits that form without fertilisation). Banana is a classic natural example. Because no fertilisation occurs, parthenocarpic fruits are seedless. This process can also be triggered artificially by applying growth hormones to unfertilised flowers, which is how commercially seedless varieties of certain fruits are produced.

Why Seeds Matter: Survival Advantages for Angiosperms

Seeds are not just containers for embryos. They provide a suite of advantages that have helped angiosperms dominate terrestrial ecosystems:

Independence from water for reproduction — Pollination and fertilisation in angiosperms do not require external water (unlike mosses and ferns, where sperm must swim). This makes seed formation far more reliable across a range of environments.
Effective dispersal strategies — Seeds come equipped with features that help them travel to new habitats: wings, hooks, fleshy coverings that attract animals, lightweight structures that catch the wind. This ability to colonise distant areas gives the species a better chance of survival.
Built-in food supply — The food reserves stored in the cotyledons or endosperm nourish the young seedling during the critical period after germination, before it develops leaves and can photosynthesise on its own.
Physical protection — The hard seed coat shields the embryo from drying out, mechanical injury, and microbial attack during storage and dispersal.
Genetic variation — Because seeds are the product of sexual reproduction, each seed carries a new combination of genes. This genetic diversity is the raw material for natural selection and helps populations adapt to changing environments.

Seeds as the Foundation of Agriculture

Think about how much of human civilisation depends on seeds. The ability of mature seeds to lose water and enter dormancy is what makes agriculture possible. A farmer can harvest seeds, store them for months (as food or as planting stock for the next season), and sow them when the time is right. Without dehydration and dormancy, seeds would sprout immediately after forming and could never be stored.

How Long Can a Seed Stay Alive?

The lifespan of a seed after dispersal varies enormously. Some species lose their ability to germinate within a few months. Others remain viable for several years, and a few can survive for astonishing stretches of time.

The most remarkable records include:

Lupinus arcticus (an Arctic lupine) — A seed excavated from Arctic Tundra permafrost germinated and flowered after an estimated 10,000 years of dormancy. This is the oldest confirmed case of a viable seed.
Phoenix dactylifera (the date palm) — A 2,000-year-old seed was discovered during archaeological excavations at King Herod’s palace near the Dead Sea. It too proved viable and was successfully germinated.

These extraordinary cases show that under the right conditions (cold, dry, and undisturbed), some seeds can remain alive far longer than any individual plant ever could. The seed, in a sense, gives the species a form of time travel.

The Enormous Reproductive Capacity of Flowering Plants

Consider the sheer scale of reproduction in angiosperms. Each embryo sac holds one egg. Each ovule holds one embryo sac. But an ovary can contain one, a few, or hundreds of ovules. A single flower typically has one ovary (sometimes more), and a tree can carry thousands of flowers.

Some plants take this to extremes. Orchid fruits, for example, each contain thousands of tiny seeds. Parasitic species like Orobanche and Striga produce similarly vast numbers. The fig tree (Ficus) develops from a seed so small you can barely see it, yet grows into an enormous tree that produces billions of seeds over its lifetime. This massive reproductive output is one of the strategies that has made flowering plants the most successful group of land plants on Earth.