Flowering, pollination and fertilisation
How flowers form, self- and cross-pollination, self-incompatibility, double fertilisation and endosperm, apomixis and parthenocarpy, with crop examples.
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Most of the food people eat, grains, pulses, fruits, nuts and oilseeds, exists only because a flower was pollinated and an ovule was fertilised. A crop can grow perfectly for months and still yield nothing if pollen is sterile, pollinators are absent or fertilisation fails during a few critical days. Understanding how flowers work is therefore essential for growing crops, breeding them, and producing seed.
Flowering, pollination and fertilisation are core plant physiology and botany topics, and the basis for much of plant breeding.
The flower
A complete flower has four whorls:
- Sepals (calyx), protecting the bud.
- Petals (corolla), often attracting pollinators.
- Stamens (androecium), each an anther on a filament; anthers make pollen.
- Carpels (gynoecium, pistil): stigma, style and ovary containing ovules.
Flowers with both stamens and carpels are bisexual (perfect), as in rice, wheat, tomato and most legumes. Others are unisexual: monoecious species carry separate male and female flowers on the same plant (maize, cucurbits, castor, coconut), and dioecious species have male and female plants (papaya in many varieties, date palm, spinach, asparagus, kiwifruit, hemp).
Flower formation is triggered by internal and environmental signals, including day length (photoperiod), cold (vernalisation), age and hormones. These are covered in the article on photoperiodism and vernalisation.
Pollination
Pollination is the transfer of pollen from an anther to a stigma.
- Self-pollination (autogamy): pollen reaches the stigma of the same flower or another flower on the same plant. It is favoured by cleistogamy (pollination inside a closed flower, as often in groundnut and some wheat and barley flowers), by bisexual flowers that open after pollination, and by anthers that surround the stigma, as in tomato.
- Cross-pollination (allogamy): pollen comes from another plant, carried by wind (maize, rye, many grasses and trees) or animals (bees and other insects, birds, bats).
Mechanisms that promote cross-pollination include unisexual flowers, dichogamy (anthers and stigmas maturing at different times: protandry, male first, as in maize, carrot, sunflower; protogyny, female first, as in pearl millet and avocado), herkogamy (physical separation of anther and stigma), self-incompatibility and male sterility.
About three-quarters of the leading food crops benefit at least partly from animal pollination, although the crops that supply most calories, the cereals, are wind- or self-pollinated.
Pollen–pistil interaction and self-incompatibility
A pollen grain that lands on a compatible stigma hydrates and germinates, and a pollen tube grows down the style, guided by signals from the ovule, to the micropyle.
Self-incompatibility is a genetic system that prevents fertilisation by pollen carrying the same allele at the S locus as the pistil:
- Gametophytic self-incompatibility: the pollen's own S allele decides compatibility; rejection usually happens in the style. Examples: many Solanaceae, Rosaceae (apple, pear, cherry), Poaceae such as rye.
- Sporophytic self-incompatibility: the genotype of the pollen parent decides; rejection usually happens on the stigma surface. Examples: Brassicaceae (cabbage, radish), Asteraceae (sunflower in wild forms).
Self-incompatibility is why orchards of many apple and cherry varieties need pollinisers, compatible varieties interplanted to supply pollen.
Double fertilisation
Flowering plants have a unique double fertilisation, discovered independently by S. Nawaschin and L. Guignard in 1898–99.
The typical embryo sac (female gametophyte) has seven cells and eight nuclei: three antipodal cells, a central cell with two polar nuclei, and the egg apparatus of an egg cell and two synergids. The pollen tube enters one synergid and releases two sperm cells:
- Sperm + egg → zygote (2n), which grows into the embryo.
- Sperm + two polar nuclei → primary endosperm nucleus (3n), which grows into the endosperm, the seed's food store.
In cereals the endosperm persists and forms most of the grain. In many legumes, the embryo absorbs the endosperm during seed development and stores food in its cotyledons instead.
Chromosome numbers in a maize grain
Maize has 2n = 20 chromosomes, so its gametes have n = 10.
- Embryo: egg (10) + sperm (10) = 20 chromosomes (2n).
- Endosperm: two polar nuclei (10 + 10) + sperm (10) = 30 chromosomes (3n): two sets from the mother and one from the father.
- Pericarp (the grain's outer layer): maternal tissue, 2n = 20, with no paternal contribution.
Because the endosperm carries paternal genes, pollen can directly change grain traits in the current season, a phenomenon called xenia. If a white-endosperm maize ear is pollinated by a yellow-endosperm plant, its grains turn yellow, because the yellow allele is dominant. This is why seed crops and specialty maize (such as sweet corn) must be isolated from other varieties.
After fertilisation: seed and fruit
The ovule becomes the seed, and the ovary wall becomes the fruit wall (pericarp). Hormones from developing seeds, chiefly auxins and gibberellins, drive fruit growth, which is why poorly pollinated fruits are often small or misshapen, with seeds only on one side.
Two important exceptions:
- Parthenocarpy: fruit develops without fertilisation and is seedless, as in banana, pineapple, many cucumbers grown in greenhouses, and some citrus and grapes. It can be natural or induced by hormone sprays.
- Apomixis: seeds form without fertilisation, so offspring are genetic copies of the mother. It occurs in some grasses, citrus (where extra embryos form from nucellus tissue, called polyembryony), mango and blackberry. Breeders are working to introduce apomixis into crops so that hybrid vigour can be fixed in seed.
Why it matters on the farm
- Stress at flowering, such as heat, drought or cold, causes pollen sterility and failed fertilisation; this is the most sensitive stage for grain yield.
- Pollinators must be present for insect-pollinated crops such as almond, apple, cucurbits, sunflower and many oilseeds; hives are often brought in.
- Isolation distances in seed production prevent unwanted cross-pollination.
- Hybrid seed production relies on emasculation, male sterility or self-incompatibility to force cross-pollination.
Key points
- Flowers may be bisexual or unisexual; monoecious plants carry both sexes, dioecious species have separate male and female plants.
- Self-pollination is favoured by cleistogamy; cross-pollination by dichogamy, herkogamy, self-incompatibility and male sterility.
- Self-incompatibility is gametophytic or sporophytic, controlled by the S locus.
- Double fertilisation: sperm + egg → zygote (2n); sperm + two polar nuclei → endosperm (3n).
- Xenia: pollen directly affects endosperm traits in the current season.
- Parthenocarpy gives seedless fruit; apomixis gives seed without fertilisation.
What the exams ask
- School-leaving and entrance papers: parts of a flower, types of pollination, agents of pollination, and double fertilisation.
- Degree and research-fellowship entrances: the embryo sac, self-incompatibility types, dichogamy, xenia and metaxenia, apomixis, polyembryony and parthenocarpy.
- Breeding and seed examinations: floral biology for hybrid seed production, isolation distances, and pollination management.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
What is the difference between pollination and fertilisation?
Pollination is pollen reaching the stigma. Fertilisation is the fusion of sperm with the egg (and with the polar nuclei) inside the ovule, which happens hours to days later, after the pollen tube has grown.
Why do some fruit trees need another variety nearby?
Many are self-incompatible or produce little viable pollen, so they set fruit only with pollen from a compatible variety that flowers at the same time.
Is a seedless fruit the same as apomixis?
No. Seedless fruits come from parthenocarpy, fruit growth without seeds. Apomixis produces seeds, just without fertilisation.
Sources
- Raven, P. H., Evert, R. F. and Eichhorn, S. E. Biology of Plants, 8th edition. W. H. Freeman, 2013.
- Raghavan, V. Double Fertilization: Embryo and Endosperm Development in Flowering Plants. Springer, 2006.
- de Nettancourt, D. Incompatibility and Incongruity in Wild and Cultivated Plants, 2nd edition. Springer, 2001.
- Klein, A.-M. et al. "Importance of pollinators in changing landscapes for world crops." Proceedings of the Royal Society B 274, 303–313 (2007).
- Poehlman, J. M. and Sleper, D. A. Breeding Field Crops, 5th edition. Iowa State University Press, 2006.
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