Why a Coconut Holds Both Liquid and Solid Endosperm

Double fertilisation leaves a zygote and a primary endosperm nucleus inside the embryo sac. From these, the ovule builds an embryo with its own food store and a protective coat, while the ovary around it swells into a fruit.

This part covers endosperm development, embryo development, seeds and fruits, and apomixis, polyembryony and parthenocarpy.

Endosperm develops before the embryo: the primary endosperm nucleus usually divides repeatedly without forming walls, giving free-nuclear endosperm that later becomes cellular; seeds that keep some endosperm at maturity are albuminous, while those whose embryo uses it all up are non-albuminous.

Endosperm development:

- The primary endosperm cell divides repeatedly to form triploid endosperm tissue that stores food
- Free-nuclear stage — nuclear divisions without cell walls, giving a mass of free nuclei
- Cellular stage — cell walls form later, from the edge towards the centre

Coconut shows both stages: the tender coconut water is free-nuclear endosperm made of thousands of nuclei, and the surrounding white kernel is cellular endosperm.

Seed types:

- Non-albuminous — the endosperm is completely used up before the seed matures, as in pea, groundnut and beans
- Albuminous — part of the endosperm remains, as in wheat, maize, barley, castor and coconut
- Perisperm — in seeds such as black pepper and beet, the nucellus persists as an extra food-storing layer

An everyday example. Drinking tender coconut water and then scooping out the soft white flesh means eating endosperm at two stages of development.

The substance. Endosperm forms before the embryo because the growing embryo needs a food supply ready and waiting.

The zygote divides to form a proembryo, which passes through globular and heart-shaped stages to become the mature embryo; a dicot embryo has an axis between two cotyledons, while a monocot embryo has a single shield-shaped cotyledon, the scutellum, with sheaths around its young root and shoot.

Embryogeny runs from zygote to proembryo, then the globular and heart-shaped stages, and finally the mature embryo.

Dicot embryo:

- An embryonal axis with two cotyledons
- Epicotyl — the part above the cotyledons, ending in the plumule, the shoot tip
- Hypocotyl — the part below, ending in the radicle, the root tip, protected by a root cap

Monocot embryo, as in grasses:

- One cotyledon, the scutellum, lying to one side of the embryonal axis
- The radicle and root cap are enclosed in a sheath called the coleorhiza
- The plumule, with a few leaf primordia, is enclosed in a hollow sheath called the coleoptile

An everyday example. Soaked chana seeds split easily into two halves — the two cotyledons of a dicot embryo — while a maize grain holds a single scutellum.

The substance. The coleoptile and coleorhiza are protective sheaths — they shield the tender shoot and root as they push through soil, and are not organs of the adult plant.

After fertilisation the ovule becomes a seed, its integuments hardening into the seed coat, and the ovary becomes the fruit, its wall forming the pericarp; a true fruit develops only from the ovary, a false fruit also involves other floral parts, and a parthenocarpic fruit develops without fertilisation.

Seed formation:

- The integuments harden into the tough seed coat
- The micropyle remains as a small pore that admits water and oxygen when the seed germinates
- The seed dries to about $10$ to $15$ percent moisture by mass, the embryo becomes dormant, and metabolism slows sharply

Fruit formation. The ovary matures into the fruit, and its wall becomes the pericarp, which may be fleshy, as in guava, mango and orange, or dry, as in groundnut and mustard.

Types of fruit:

- True fruits — develop only from the ovary, as in mango
- False fruits — the thalamus also contributes, as in apple, strawberry and cashew
- Parthenocarpic fruits — develop without fertilisation and are therefore seedless, as in banana; growth hormones can induce them

An everyday example. The juicy cashew apple is the swollen thalamus, while the true fruit is the kidney-shaped nut hanging beneath it.

The substance. Seedless bananas show that fruit growth can happen without fertilisation — hormones, not seeds, trigger the ovary to enlarge.

Apomixis is the formation of seeds without fertilisation, polyembryony is the presence of more than one embryo in a seed, and parthenocarpy is the formation of fruit without fertilisation; apomixis matters commercially because it would let farmers reuse hybrid seed without losing hybrid vigour.

Apomixis:

- A form of asexual reproduction that mimics sexual reproduction, found in some grasses and members of the sunflower family
- A diploid egg cell may develop into an embryo without fertilisation
- Alternatively, cells of the nucellus may grow into embryos

Polyembryony:

- Several embryos form in one seed, often from nucellar cells
- Common in citrus and mango

Parthenocarpy gives fruit without fertilisation, as in banana.

Why apomixis matters for hybrid seed:

- Hybrid crops give high yields, but their offspring segregate and lose hybrid characters
- Farmers must therefore buy fresh hybrid seed every season, which is costly
- If hybrids were apomictic, the embryos would be genetically identical to the parent, so hybrid vigour would pass unchanged to the next generation

An everyday example. An orange seed that sprouts two or three seedlings shows polyembryony; the extra seedlings are genetically identical to the parent tree.

The substance. Apomictic seeds are clones — they carry no new combination of genes, unlike seeds formed by fertilisation.