Embryology

Embryology is the study of the development of a fertilised egg (zygote) into a mature embryo. In angiosperms this follows sexual reproduction, which requires pollination and fertilization.

Pollination

Pollination is the transfer of pollen grains from the anther of a flower to the stigma of the same flower or of another flower of the same species.

Self-pollination

Transfer of pollen to a stigma of the same genetic individual:

• Autogamy: pollen falls onto the stigma of the very same flower, which requires the anther and stigma to mature together (homogamy) and be favourably positioned.
• Geitonogamy: pollen is transferred to a different flower on the same plant. Genetically this behaves like self pollination, since the parent is the same individual, even though a pollinating agent is needed to move pollen between flowers.

Cross-pollination (xenogamy)

Transfer of pollen between flowers of two genetically different plants of the same species, carried by an external agent:

• Anemophily (wind): flowers are usually small, inconspicuous, without scent or nectar, and produce huge quantities of light, dry pollen, as in grasses and maize.
• Hydrophily (water): rare, restricted to submerged or floating aquatic plants; hypohydrophily occurs entirely underwater (Zostera, Ceratophyllum) and epihydrophily occurs on the water surface (Vallisneria).
• Zoophily (animals): the commonest agent overall. Entomophily (insects) alone accounts for roughly 80 percent of animal pollination and favours brightly coloured, scented, nectar producing flowers; ornithophily (birds, for example sunbirds) favours large, bright red or orange, odourless, nectar rich flowers; chiropterophily (bats) favours large, dull coloured, strongly scented flowers that open at night.

Cross pollination promotes genetic variation and is favoured by devices such as dichogamy (anther and stigma maturing at different times), herkogamy (physical separation of anther and stigma) and self incompatibility.

Fertilization

After pollination, a pollen grain germinates on a receptive stigma, absorbing water and nutrients, and grows a pollen tube down through the style, carrying the two male gametes (sperm cells) toward the ovule. The pollen tube enters the ovule by one of three routes:

• Porogamy: through the micropyle, the commonest route in angiosperms.
• Chalazogamy: through the chalaza, at the base of the ovule opposite the micropyle.
• Mesogamy: through the side of the ovule, via the integument or funicle.