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Must-know facts
20 facts
- 1A typical anther is bilobed, dithecous and tetrasporangiate: four microsporangia, two per lobe.
- 2Microsporangium wall, outside to inside: epidermis, endothecium, middle layers, tapetum; the tapetum nourishes the pollen and its cells are often multinucleate.
- 3Exine is sporopollenin, absent only at germ pores; intine is cellulose and pectin.
- 4Over 60 per cent of angiosperms shed pollen at the 2-celled stage (vegetative + generative cell).
- 5Rice and wheat pollen lose viability within 30 minutes; pollen is stored in liquid nitrogen at −196 °C.
- 6Parthenium (carrot grass) came to India with imported wheat and causes pollen allergy.
- 7Hilum joins ovule to funicle; micropyle is at the tip; chalaza at the base; nucellus holds the embryo sac.
- 8One megaspore functions (monosporic); three free-nuclear mitoses give 8 nuclei; embryo sac is 7-celled, 8-nucleate.
- 9Egg apparatus = 2 synergids + 1 egg, at the micropylar end; filiform apparatus of synergids guides the pollen tube.
- 10Cleistogamous flowers (Viola, Oxalis, Commelina) are always autogamous.
- 11Geitonogamy is functionally cross-pollination but genetically autogamy; only xenogamy brings genetically different pollen.
- 12Water pollination: about 30 genera, mostly monocots; Vallisneria, Hydrilla, Zostera. Water hyacinth and water lily are not water-pollinated.
- 13Monoecy (castor, maize) prevents autogamy only; dioecy (papaya) prevents autogamy and geitonogamy.
- 14Syngamy gives a 2n zygote; triple fusion gives a 3n PEN; together they are double fertilisation.
- 15Coconut water is free-nuclear endosperm; the kernel is cellular endosperm.
- 16Albuminous: wheat, maize, barley, castor. Non-albuminous: pea, groundnut. Perisperm: black pepper, beet.
- 17Mature seeds hold 10-15 per cent moisture; the ovary wall becomes the pericarp.
- 18False fruits: apple, strawberry, cashew (thalamus contributes). Parthenocarpy: banana.
- 19Lupinus arcticus seed germinated after about 10,000 years; Phoenix dactylifera seed about 2000 years old.
- 20Nucellar polyembryony in Citrus and mango; apomixis in some Asteraceae and grasses.
Common traps
Where marks are lost
Saying the embryo sac is 8-celled because it has 8 nuclei.
It is 8-nucleate but 7-celled: the two polar nuclei share one central cell.
Calling geitonogamy cross-pollination in every sense.
It needs an agent, so it is cross-pollination functionally, but the pollen is from the same plant, so genetically it is autogamy.
Thinking the tapetum protects the anther and helps it dehisce.
Epidermis, endothecium and middle layers protect and help dehiscence; the tapetum nourishes the developing pollen.
Believing monoecious plants like maize prevent all self-pollination.
Monoecy stops autogamy but not geitonogamy; only dioecy (papaya) stops both.
Calling water lily and water hyacinth water-pollinated because they are aquatic.
Their flowers rise above water and are pollinated by insects or wind; Vallisneria, Hydrilla and Zostera are the water-pollinated examples.
Giving the endosperm the same ploidy as the embryo.
Endosperm comes from the PEN, formed by fusion of one male gamete with two polar nuclei, so it is 3n; the embryo is 2n.
Confusing perisperm with pericarp.
Perisperm is leftover nucellus inside the seed (black pepper, beet); pericarp is the fruit wall formed from the ovary wall.
Mixing up coleoptile and coleorrhiza.
Coleoptile sheathes the shoot apex (think 'ptile' like leaf above); coleorrhiza sheathes the radicle and root cap ('rhiza' = root).
Assuming apomictic seeds are genetically varied like normal seeds.
Apomictic embryos form without fertilisation, from a diploid egg or nucellar cells, so they are clones of the mother plant.
Key terms
18 terms
- Dithecous
- Of an anther lobe or anther with two thecae per lobe, as in a typical angiosperm anther.
- Tapetum
- Innermost wall layer of the microsporangium; it feeds the developing pollen and has dense, often multinucleate cells.
- Microsporogenesis
- Formation of four haploid microspores from a pollen mother cell by meiosis.
- Sporopollenin
- The highly resistant material of the exine, broken down by no known enzyme.
- Germ pore
- A gap in the exine without sporopollenin, through which the pollen tube emerges.
- Nucellus
- Food-rich tissue of the ovule, inside the integuments, that contains the embryo sac.
- Monosporic development
- Formation of the embryo sac from just one of the four megaspores.
- Filiform apparatus
- Thickenings at the micropylar tip of the synergids that guide the pollen tube in.
- Cleistogamy
- Bearing flowers that never open, so pollination is always within the flower.
- Geitonogamy
- Transfer of pollen to another flower on the same plant.
- Xenogamy
- Transfer of pollen to the stigma of a flower on a different plant.
- Self-incompatibility
- A genetic block on germination or tube growth of pollen from the same plant.
- Emasculation
- Removing anthers from a bisexual bud before they dehisce, in artificial crossing.
- Triple fusion
- Fusion of one male gamete with the two polar nuclei, giving the 3n primary endosperm nucleus.
- Scutellum
- The single, laterally placed cotyledon of a grass embryo.
- Perisperm
- Persistent remains of the nucellus in a seed, as in black pepper and beet.
- Parthenocarpy
- Development of a fruit without fertilisation, giving a seedless fruit such as banana.
- Polyembryony
- Presence of more than one embryo in a seed, as in Citrus.