Double Fertilisation, Endosperm and Embryo Development

The two male gametes have different destinations

A compatible pollen tube grows from the stigma, through the style and into an ovule, where it usually enters one synergid of the female gametophyte. The tube releases two haploid male gametes. One male gamete approaches the egg cell and fuses with the egg nucleus. Because each partner contributes a single haploid set of chromosomes, the product is a diploid zygote: n + n = 2n. This fusion is called syngamy. The zygote is the first cell of the new sporophyte generation and will develop into the embryo.

The second male gamete fuses with the two polar nuclei in the large central cell. In the common eight-nucleate embryo sac, each polar nucleus is haploid, so three haploid contributions unite: n + n + n = 3n. The resulting triploid nucleus is the primary endosperm nucleus (PEN), and the central cell becomes the primary endosperm cell. This event is called triple fusion. The PEN and zygote have different fates: the first initiates endosperm development, while the second initiates embryo development. The two fusions together constitute double fertilisation, characteristic of flowering plants.

Why nutritive endosperm develops before the embryo enlarges

After triple fusion, the primary endosperm nucleus divides repeatedly. In the common nuclear type of endosperm development, these early nuclear divisions occur without immediate formation of separating cell walls. The result is free-nuclear endosperm. Cellularisation may follow, producing endosperm tissue that stores or transfers nutrients to the developing embryo. Although details differ among species, endosperm generally begins developing before the embryo undergoes extensive differentiation. This temporal sequence helps ensure an early nutrient supply and illustrates that embryo and endosperm are physiologically linked but developmentally distinct structures.

A familiar example is coconut: the liquid inside a tender coconut represents a free-nuclear endosperm phase, while the white edible kernel is cellular endosperm. In some seeds, such as pea and groundnut, the developing embryo consumes most or all of the endosperm before maturation. Other seeds, such as maize, wheat, castor and coconut, retain endosperm that later helps support germination. Thus presence of endosperm at the time of fertilisation does not guarantee that a mature seed will still contain much of it.

Embryogeny: from zygote to organised body plan

The zygote lies near the micropylar end of the embryo sac. In many flowering plants it remains temporarily inactive while the initial endosperm develops, supporting the early nutritional needs of embryogenesis. The zygote then divides to form a proembryo. In a typical dicot embryogenesis sequence, subsequent coordinated divisions and changes in shape produce a globular stage, then a heart-shaped stage as the two cotyledon primordia develop, and finally an elongated differentiated embryo. These stages indicate changing tissue organisation, not independent embryos.

The mature dicot embryo

A typical dicotyledonous embryo has two cotyledons attached to an embryonal axis. The axis above their point of attachment is the epicotyl; at its upper end is the plumule, the embryonic shoot apex. The region below the cotyledons is the hypocotyl, which continues toward the radicle, the embryonic root. The root apex is protected by a root cap. Cotyledons may store reserves, absorb reserves or later become photosynthetic organs depending on the plant. The terms epicotyl and hypocotyl are positional: they describe regions of the embryo axis, not synonyms for the entire stem and root.

The monocot grass embryo

Monocot embryos typically possess one cotyledon. In grasses, the cotyledon is specialised into the scutellum, placed laterally against the endosperm and involved in reserve uptake during germination. The embryonic shoot apex and small leaf primordia are covered by the coleoptile, a sheath that protects the emerging shoot. The embryonic root region, including the radicle and root cap, is enclosed by the coleorrhiza. These sheaths differ in which organ they protect: coleoptile protects the plumule; coleorrhiza protects the radicle. Their names must not be exchanged merely because both end with similar syllables.

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