6.2 Development of Pollen Grains and Embryo Sac
In flowering plants, sexual reproduction requires the production of male and female gametes within specialised reproductive organs. The male reproductive units develop within the anther as pollen grains, while the female reproductive structure develops inside the ovule as an embryo sac. Both developmental pathways involve meiotic cell division to reduce the chromosome number from diploid to haploid, followed by mitotic divisions to form mature structures ready for pollination and fertilisation.
Learning Objectives
- Describe the stages of pollen grain formation from a microspore mother cell in an anther.
- Identify and describe the structural components of a mature ovule.
- Explain the development of an embryo sac from a megaspore mother cell inside an ovule.
- Compare the nuclear arrangements and cell types formed during the development of pollen grains and embryo sacs.
Comparison of Microsporogenesis and Megasporogenesis
Trace and contrast the sequential divisions — meiosis followed by mitosis — that produce pollen grains and the 8-nucleate embryo sac.
Pollen Grain Formation
One diploid microspore mother cell yields four viable haploid pollen grains. Each pollen grain contains two nuclei after one round of mitosis.
- 1Diploid microspore mother cell divides by meiosis.
- 2Four haploid microspores form a tetrad.
- 3Each microspore separates and develops into a pollen grain.
- 4Mitosis produces a generative nucleus and a tube nucleus.
Embryo Sac Formation
One diploid megaspore mother cell undergoes meiosis to yield four megaspores, but three degenerate. The surviving megaspore undergoes three rounds of mitosis to form eight haploid nuclei.
- 1Diploid megaspore mother cell divides by meiosis.
- 2Three of four megaspores degenerate; one survives.
- 3Surviving megaspore nucleus divides by mitosis three times.
- 4Eight nuclei arrange into seven cells.
Short Explanation
Formation of Pollen Grains in an Anther
Pollen grains are produced inside the lobes of the anther. Each lobe contains tissues that form four pollen sacs. Inside each pollen sac, there are hundreds of diploid (\(2n\)) pollen mother cells, also known as microspore mother cells.
Each diploid (\(2n\)) microspore mother cell divides by meiosis to produce four haploid (\(n\)) microspore cells. These four haploid cells stay grouped together as a tetrad. Each individual cell in the tetrad separates and develops into a pollen grain. The haploid nucleus inside each pollen grain divides by mitosis to produce two nuclei: the generative nucleus and the tube nucleus. When the pollen grain matures, the thick and waterproof wall of the pollen sac splits open to release the pollen grains.
Structure of an Ovule
Ovules develop inside the ovary from a layer of tissues. An ovary may contain one or more ovules. The funicle attaches the ovule to the ovary wall. The placenta is the area of attachment on the ovary wall where nutrients are supplied to the ovule via the funicle. The nucellus is a central mass of parenchyma tissue inside the ovule. Two protective outer tissue layers, the integuments, develop from the nucellus. The micropyle is a tiny opening at the tip of the integuments that allows the entry of air and water into the seed during germination.
Formation of Embryo Sac in an Ovule
The diploid (\(2n\)) megaspore mother cell divides by meiosis to produce four haploid (\(n\)) megaspore cells. Three of the four megaspore cells degenerate, leaving only one surviving, functional megaspore cell that continues to enlarge. The haploid nucleus of the surviving megaspore cell divides by mitosis three times consecutively, resulting in a single large cell containing eight haploid nuclei. The eight nuclei distribute and cellularise into specific structures: three antipodal cells, one egg cell, two synergid cells, and two polar nuclei.
Features of Pollen Grains and Pollinating Agents
Insect- and animal-pollinated flowers such as rambutan, durian, papaya, rose, sunflower and hibiscus have pollen grains that are rough and sticky, enabling them to adhere easily to pollinator bodies and the stigma. Wind-pollinated flowers such as corn, grass and paddy have pollen grains that are small, smooth and lightweight for easy transport by air currents. Gymnosperms like pines and firs do not have flowers; they produce pollens in male pollen cones and ovules in larger female seed cones.
Try to Answer First
Answer in your mind, then press “Check Answer”.
Name the diploid cell that divides to form pollen grains in the anther and the diploid cell that forms the embryo sac in the ovule.
Why does the nucleus of a haploid microspore divide by mitosis during the formation of a pollen grain?
After the completion of three mitotic divisions in the functional megaspore, how are the eight nuclei arranged within the embryo sac?
Common Mistakes
- Confusing the generative nucleus with the tube nucleus or thinking the pollen grain contains only one nucleus upon release.
- Assuming that all four megaspore cells survive after meiosis; in reality, three degenerate and only one develops into the embryo sac.
- Stating that the megaspore divides three times by meiosis instead of mitosis.
- Forgetting that the starting cells (microspore mother cell and megaspore mother cell) are diploid (\(2n\)), whereas the resulting spores and mature nuclei are haploid (\(n\)).
Concept Misunderstandings
A pollen grain is a single male gamete.
A pollen grain is a male gametophyte containing two haploid nuclei (the generative nucleus and tube nucleus). The generative nucleus will later divide to form two male gametes.
The eight nuclei in the embryo sac all develop into separate egg cells.
Only one nucleus forms the egg cell (female gamete). The others form two synergid cells, three antipodal cells, and two polar nuclei.
Mitosis only occurs in diploid cells.
In plant life cycles, haploid spores undergo mitosis to produce multicellular haploid gametophytes or multi-nucleated structures.
Summary
- Pollen grain formation: microspore mother cell (\(2n\)) \(\xrightarrow{\text{Meiosis}}\) tetrad of 4 microspores (\(n\)) \(\xrightarrow{\text{Mitosis}}\) pollen grains with a tube nucleus (\(n\)) and a generative nucleus (\(n\)).
- Ovule structure consists of funicle, placenta, nucellus, integuments, and a micropyle opening.
- Embryo sac formation: megaspore mother cell (\(2n\)) \(\xrightarrow{\text{Meiosis}}\) 4 megaspores (\(n\)) \(\rightarrow\) 3 degenerate, 1 survives \(\xrightarrow{\text{3 Mitotic divisions}}\) 8 haploid nuclei.
- Arrangement: 3 antipodal cells + 2 polar nuclei + 2 synergids + 1 egg cell.
Short Activity
Objective: test your understanding of pollen grain and embryo sac development.
A. Objective Quiz
How many mitotic divisions occur in the surviving haploid megaspore to produce the eight nuclei of the embryo sac?
Which structure in a mature ovule allows the entry of air and water into the seed during germination?
B. Fill in the Blanks
A diploid microspore mother cell divides by meiosis to produce a group of four haploid cells collectively known as a .
Inside the embryo sac, two nuclei remain in the centre to form the nuclei.
C. Matching / Drag and Drop
Match each cell or structure in the mature embryo sac to its correct location or description. 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 to Its Description
Write the correct structure next to each description.
| Location or Description | Structure / Cell |
|---|---|
| 1. The female gamete located between two synergid cells at the micropylar end | |
| 2. Three cells situated at the end opposite to the micropyle | |
| 3. Two cells that flank the egg cell at the micropylar end | |
| 4. Two nuclei located together in the centre of the embryo sac |
Answer choices: Egg cell • Antipodal cells • Synergid cells • Polar nuclei
