6.3 Pollination and Fertilisation
Pollination is the essential preliminary process in flowering plants where pollen grains are transferred from the anther to the stigma. Once on the stigma, the pollen grain germinates to form a pollen tube that carries male gametes down through the style into the ovule. This process culminates in double fertilisation, a reproductive mechanism unique to angiosperms that produces both a diploid zygote and a nutritive triploid endosperm tissue.
Learning Objectives
- Define pollination and identify various pollinating agents.
- Describe the sequence of events in the formation of the pollen tube and male gametes.
- Explain the process of double fertilisation involving the fusion of two male gametes with the egg cell and polar nuclei.
- State the biological importance of double fertilisation for the survival of flowering plants.
The Sequence of Pollen Tube Growth and Double Fertilisation
Follow the pathway of the pollen tube through the style and see how two male gametes fuse with the egg cell and polar nuclei.
Pollination
Transfer of pollen grains from the anther to the stigma.
Pollen tube
Germinates from the pollen grain and grows down the style.
Double fertilisation
Two male gametes fuse with the egg cell and polar nuclei.
Short Explanation
What is pollination?
Pollination is the process in which pollen grains are transferred from the anther to the stigma. It is assisted by pollinating agents such as insects, mammals, birds, water, or wind. The presence of pollen grains on the stigma triggers fertilisation.
Pollen tube germination
Pollen grains germinate by absorbing secretions from the stigma, forming a pollen tube that secretes digestive enzymes to penetrate the style.
Male gamete formation
Inside the growing tube, the generative nucleus divides by mitosis into two haploid male gametes (\(n\)).
Entry into the ovule
When the tube reaches the ovule, it enters through the micropyle. The tube nucleus degenerates, allowing both male gametes to enter the embryo sac.
Double fertilisation
One male gamete fuses with the egg cell to form a diploid zygote (\(2n\)), and the other fuses with two polar nuclei to produce a triploid endosperm nucleus (\(3n\)).
Try to Answer First
Answer in your mind, then press “Check Answer”.
What structure does the pollen tube penetrate to enter the embryo sac?
What are the products of double fertilisation in angiosperms?
By what type of cell division does the generative nucleus produce male gametes?
Common Mistakes
- Confusing the roles of the tube nucleus and generative nucleus: the tube nucleus directs pollen tube growth and later degenerates, whereas the generative nucleus divides to form male gametes.
- Assuming that double fertilisation involves two separate pollen grains, rather than two male gametes derived from a single pollen grain.
- Stating that the endosperm is diploid (\(2n\)) instead of triploid (\(3n\)).
Concept Misunderstandings
Both male gametes fertilise two different egg cells to produce twins.
Only one male gamete fertilises the single egg cell to form a zygote; the other fuses with the two polar nuclei to produce endosperm tissue for nutrition.
All mature seeds contain large amounts of endosperm tissue.
In eudicots, the endosperm is fully absorbed by the embryo before seed maturation, whereas in monocots, endosperm persists as a stored food reserve for germination.
Summary
- Pollination: Transfer of pollen from anther to stigma via wind, water, insects, birds, or mammals.
- Pollen Tube Growth: Pollen germinates on stigma; pollen tube secretes enzymes to digest style tissues; generative nucleus divides by mitosis into two haploid male gametes (\(n\)).
- Micropyle Entry: Tube nucleus degenerates upon reaching the embryo sac; both male gametes enter.
- Double Fertilisation: Male gamete (\(n\)) + Egg cell (\(n\)) \(\rightarrow\) Diploid zygote (\(2n\)). Male gamete (\(n\)) + Two polar nuclei (\(2n\)) \(\rightarrow\) Triploid endosperm nucleus (\(3n\)).
- Significance: Restores diploid state, transfers genetic information, and provides endosperm nutrition for embryo survival and seed germination.
Short Activity
Objective: check your understanding of pollination, pollen tube growth and double fertilisation.
A. Objective Quiz
What is formed when the generative nucleus divides inside the growing pollen tube?
The second male gamete fuses with two polar nuclei during double fertilisation. What structure develops from this fusion?
B. Fill in the Blanks
When the pollen tube reaches the embryo sac, it penetrates the ovule through an opening known as the .
In double fertilisation, the fusion of one male gamete with the egg cell produces a diploid .
C. Matching / Drag and Drop
Match each structure or nucleus to its correct description or fate during pollination and fertilisation. On a phone, tap a choice first, then tap the matching box. Tap the Answer Choices area to return a selected item.
C. Match the Structure or Nucleus to Its Role or Fate
Write the correct structure or nucleus next to each description.
| Role / Fate | Structure / Nucleus |
|---|---|
| 1. Undergoes mitosis to produce two haploid male gametes | |
| 2. Directs pollen tube growth and degenerates at the micropyle | |
| 3. Formed from the fusion of one male gamete with the egg cell | |
| 4. Formed from the fusion of one male gamete with two polar nuclei |
Answer choices: Generative nucleus • Tube nucleus • Diploid zygote • Triploid endosperm nucleus
