How pollen reaches the stigma, how a pollen tube delivers gametes, and what a flower becomes afterwards.
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Pollination is the transfer of pollen from an anther to a stigma. Self-pollination is within the same flower (or same plant); cross-pollination is between flowers of different plants of the same species. Agents: wind (anemophily), insects (entomophily), water (hydrophily), birds and animals. Cross-pollination brings new combinations of characters, so offspring are usually more varied and vigorous.
Give four features of a wind-pollinated flower.
Small, dull flowers; no scent or nectar; light, dry, powdery pollen in large amounts; long feathery stigma; exposed anthers.
Give four features of an insect-pollinated flower.
Large coloured petals; scent; nectar; sticky or spiny pollen; sticky stigma.
Why is cross-pollination considered better than self-pollination?
It produces varied offspring that may adapt better to changes.
A pollen grain landing on a suitable stigma germinates: a pollen tube grows down the style and enters an ovule through the micropyle, carrying two male gametes. One fuses with the egg to form a zygote (2n); the other fuses with the two polar nuclei to form the endosperm nucleus (3n). Using two fusions is called double fertilisation.
A plant has 24 chromosomes in its body cells (2n = 24). How many are in (a) a pollen grain, (b) the zygote, (c) the endosperm nucleus?
(a) n = 12. (b) 2n = 24. (c) 3n = 36 (n + two polar nuclei of n each = 12 × 3).
Through which opening does the pollen tube enter the ovule?
The micropyle.
What is the difference between pollination and fertilisation?
Pollination is the transfer of pollen to the stigma. Fertilisation is the fusion of a male gamete with the egg.
The zygote grows into the embryo; the ovule becomes the seed; the ovary wall becomes the fruit wall (pericarp); the ovary becomes the fruit. Sepals, petals, stamens, style and stigma usually wither and fall. A fruit that develops without fertilisation, and so has no seeds, is called parthenocarpic (e.g. banana, seedless grapes).
Complete: ovule → ____; ovary → ____; zygote → ____.
Seed; fruit; embryo.
What is parthenocarpy? Give an example.
Formation of fruit without fertilisation, giving a seedless fruit, e.g. banana.
1. Define pollination.
2. Distinguish self- and cross-pollination.
3. Name three agents of pollination.
4. How is a hydrophilous flower adapted for pollination?
5. What is a pollen tube? What does it carry?
6. What fuses to form the zygote?
7. What is double fertilisation?
8. If 2n = 16, what is the chromosome number in (a) pollen, (b) endosperm?
9. Name the fruit wall.
10. Why do sepals and petals wither after fertilisation?