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Embryology1 (27-Feb-03)

Flower of Lilium

Pollination is the transfer of pollen grains to the stigma of a pistil (in the same flower or another, and in the same individual or another, depending on whether inbreeding or outbreeding is occurring). Flowers should be seen as the means of ensuring that a plant's pollen reaches as many stigmas as possible, whether on wind currents, or by co-opting animals (insects, birds, bats) to serve as pollen carriers. The same mechanisms also ensure that a plant's own stigmas receive the pollen they need in order for reproduction to proceed. Mature pollen grains comprise 2- or 3-nucleate microgametophytes, enclosed by the wall of the microspore that developed within an anther following meiosis. Once on a stigma the pollen grains may "germinate." The haploid microgametophyte emerges from the pollen grain as a narrow tube of cytoplasm that burrows down through the style into the ovary, assuming the pollen and stigma are genetically compatible.

Lilium ovule with megasporocyte

(This column) Lilium flower, and megagametophyte development.
Flower image © Royal Ontario Museum.

One to many ovules develop within the ovary of the one to many pistils that make up the gynoecium of a flower. An ovule is in effect a megasporangium enclosed by 1-2 integuments. Within the developing ovule a single cell enlarges and differentiates as a megasporocyte, that is, a cell competent to undergo meiosis. Meiosis results in four megaspores, each with the reduced (haploid) number of chromosomes. One or more of these megaspores then develop into the megagametophyte that at maturity consists of 4 to 7 or (sometimes many) more cells. Three of these comprise the egg apparatus, two synergids plus the egg, or definitive female gamete. Regardless how many additional cells make up the megagametophyte, a constant feature of flowering plants is a large central cell with one or more polar nuclei.

Pisum flower - shortly before beginning of ovule initiation

Pisum flower shortly before the beginning of ovule initiation. Which of the stamen primordia (globose bumps surrounding the gynoecium) do you think will become the single free stamen? © T. A. Dickinson

Lilium ovule with megagametophyte

The pollen tube is guided to an ovule where it delivers two sperm nuclei into close proximity to an egg nucleus and one or two other "female" polar nuclei. Flowering plants are unique in the way in which two fertilizations now take place. One sperm nucleus fuses with the egg nucleus to form the zygote that will develop into new embryo plant. The other sperm nucleus fuses with the polar nuclei to give rise to endosperm, the tissue that will nourish the developing embryo either during its maturation, or when it germinates (endosperm is what one eats when one eats coconut "meat," or any kind of grain). This double fertilization appears to be unique to flowering plants.

The development and morphology of ovules and pollen, micro- and megagametophytes, embryo and endosperm, and seeds and fruits all provide a wealth of characters at every taxonomic level. Understanding the origin of flowering plants will involve discovering the homologies between double fertilization and endosperm development in angiosperms and what appear to be as yet undiscovered features in modern or fossil gymnosperms.

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Further reading

Judd et al. (2002), Chapter 4 - pp. 87-90.

Koning, R. E. 1994. Pollen and Embryo Sac. Plant Physiology Information Website.

Williams, J. H. & W. E. Friedman. 2002. Identification of diploid endosperm in an early angiosperm lineage. Nature 415: 522-526.

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