Seeds, Fruits, Apomixis and Polyembryony

A seed encloses a young sporophyte and stored resources

After double fertilisation, the fertilised ovule changes into a seed. The maternal integuments develop into protective seed coats, often differentiated into an outer testa and an inner tegmen when two distinct layers are present. The micropyle usually persists as a tiny opening that can permit water and gas exchange during germination. Inside lies the embryo, comprising its embryonic axis and cotyledon or cotyledons. Food reserves may reside chiefly in cotyledons, in endosperm or in both depending on the plant. The protective coat and stored materials make a seed a transportable, potentially dormant developmental unit.

Seeds are not simply enlarged egg cells. The embryo arises from the zygote and therefore contains maternal and paternal genetic contributions after sexual reproduction. The seed coat usually derives from the maternal integuments and is genetically maternal tissue. Endosperm, in the common form, derives from triple fusion and has its own distinct nuclear contribution. In some seeds, surviving maternal nucellar tissue forms the perisperm. Recognising the tissue of origin helps explain why different seed layers need not have the same ploidy or genetic composition.

Albuminous, exalbuminous and perispermic seeds

Albuminous seeds retain a substantial quantity of endosperm at maturity, as in maize, wheat, castor and several other species. Exalbuminous (non-albuminous) seeds, as in pea and groundnut, consume the endosperm during development and typically store reserves in the cotyledons. Perisperm is residual nucellus rather than endosperm; it occurs in some seeds such as black pepper and beet. The distinction is developmental, not merely about whether a seed contains food: all viable embryos need nourishment, but that food can remain in different tissues.

Seed maturation, dormancy and survival

As many seeds mature, their water content falls and metabolic activity slows. Some embryos enter dormancy, a condition in which a living seed does not immediately germinate even though it retains the capacity to do so. Dormancy can prevent emergence during unfavourable seasons and permits dispersal before growth begins. Successful germination generally requires adequate water, oxygen and a suitable temperature, together with satisfaction of any species-specific light or dormancy requirements. Seed longevity varies widely: some seeds lose viability within weeks or months, while properly stored seeds of other species remain viable much longer.

Seed viability and seed dormancy are not interchangeable. A dormant viable seed can potentially germinate once its physiological inhibition is lifted, but a dead seed cannot germinate merely because conditions improve. Historical claims about extremely old germinated seeds require direct dating and careful exclusion of contamination; a widely repeated report of 10,000-year-old Arctic lupine seeds was later overturned by radiocarbon analysis. More securely dated ancient seed germination records illustrate that long survival is possible in particular species and conditions, not a universal feature of seeds.

Seeds also aid colonisation of new habitats. Protective coats reduce damage, food reserves support seedling growth before photosynthesis is fully established, and dispersal adaptations can distribute offspring away from the parent plant. A large plant may generate enormous numbers of tiny seeds, as in some orchids or parasitic flowering plants. The number of ovules in a flower can set an upper bound on ordinary seed formation, but pollen delivery, fertilisation, embryo viability and resource availability all affect the number of mature seeds.

From ovary to fruit

During seed formation, the ovary commonly enlarges into the fruit and its wall develops into the pericarp. Fruit walls can be soft and fleshy or dry and tough, and many forms promote protection or seed dispersal. A true fruit develops primarily from the ovary. In an accessory or false fruit, additional floral tissues contribute significantly. The fleshy edible part of an apple, for instance, largely arises from floral receptacle or hypanthium tissue rather than the ovary wall alone. Such a fruit still commonly contains seeds derived from fertilised ovules, so 'false' does not mean artificial or biologically defective.

Parthenocarpy is fruit development without fertilisation, often yielding seedless fruit, as in many bananas. It can sometimes be induced by particular plant growth regulators under suitable conditions. This must not be confused with apomixis. In parthenocarpy, a fruit develops without successful fertilisation and normally contains no fertilisation-derived seeds. In apomixis, an embryo and seed develop without the usual egg–sperm fusion. A fruit can be seedless without the plant necessarily producing apomictic embryos.

Apomixis: a seed without ordinary fertilisation

Apomixis is a form of asexual reproduction that produces seed-like products containing embryos without the ordinary fusion of sperm and egg. In one pathway, an unreduced diploid egg cell forms and develops into an embryo without fertilisation. In another pathway, somatic nucellar cells around the embryo sac initiate embryos; such adventive embryos may occur alongside a sexually produced embryo. These pathways differ in developmental origin and sometimes in the fertilisation requirements of surrounding seed tissues, so they should not be treated as one identical mechanism.

An apomictic embryo arising from maternal diploid cells usually closely preserves the maternal genotype, apart from mutation or other biological variation. This is of interest in hybrid crop improvement: sexual seeds derived from heterozygous hybrids often segregate in the next generation, whereas a stable apomictic pathway could retain useful parental combinations. Developing such systems reliably is biologically challenging because embryo formation, endosperm development and seed maturation all require coordination.

Polyembryony is a number-of-embryos observation

Polyembryony means that a seed contains more than one embryo. It is not synonymous with apomixis. Multiple embryos may arise through different developmental routes, including formation of adventive embryos from nucellar tissue. Citrus and some mango varieties can show nucellar embryony and polyembryony. When a seed contains embryos from maternal somatic cells together with an embryo from fertilisation, these embryos can have different genetic origins. To identify an embryo as a clone of the maternal plant, its development and genotype must be considered; counting two or more embryos is not sufficient by itself.

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