Biology · Plant Reproduction, Signals & Responses
Flowers, gametes, pollination and fertilisation
A yellow grain rests on a stigma. This tells us that pollen arrived. It does not tell us that sperm reached an ovule or that an embryo began. To explain the outcome, follow the pollen grain, its tube, and the two sperm cells as different parts of one reproductive journey.
Plant Structure, Transport & Nutrition supplies meristems, tissue boundaries, water movement and source–sink transport. Development & Differentiation introduces changing cell identities; Meiosis & Patterns of Inheritance supplies haploid and diploid chromosome sets. The first lesson explains the plant life-cycle transition again before using n. Cell Regulation & Signalling is a useful return path for receptors and feedback, and Evolutionary Processes for the final lesson’s selection argument.
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Distinguish pollen transfer, sperm delivery and gamete fusion, then trace the different tissue origins within one selected flowering-plant seed.
Begin with the organs and the life cycle
An anther produces pollen; a stigma is a surface on which pollen can be received. The style connects the stigma region with the ovary, which contains ovules. Flowers differ in their arrangement and in which organs they possess. A flower drawing with both stamens and carpels is a selected example, not a requirement that every flower contain both.
The familiar flowering plant is generally the diploid sporophyte: its nuclei contain two chromosome sets, written 2n. Meiosis in reproductive tissues produces haploid spores. Mitosis and development produce the small gametophytes that make gametes. A pollen grain is a male gametophyte, rather than one sperm cell. The female gametophyte develops inside an ovule. These stages connect this chapter to the alternation of generations introduced in Plant Diversity & Evolution.
A label such as male gametophyte names a reproductive role and generation. It does not mean that every visible flower belongs to a separate male plant. The location of pollen-producing and ovule-bearing flowers differs among species. The relevant question here is which structure makes, carries or receives each reproductive cell.
Arrival, delivery and fusion are different events
Pollination is pollen transfer to a receptive destination; in the selected flowering-plant case it is transfer to a stigma. Following compatible interactions, a pollen tube can grow through maternal tissues toward an ovule. Its vegetative cell provides the growing tube; the sperm cells are the gametes carried within it. The tube is not a third gamete.
In the selected sequence, pollen first reaches the stigma, then a tube reaches the female gametophyte and releases two sperm cells. One sperm fuses with the egg to produce the zygote; the other fuses with the central cell to initiate endosperm development. These two fusions constitute double fertilisation. The diagram separates the two destinations, rather than showing both sperm entering the egg.
The sequence shows successful events. A grain can fail to germinate, a tube can fail to reach an ovule, or later development can fail. Self-incompatibility in some species can prevent self pollen from completing the process. Self-pollination and self-fertilisation therefore remain distinct descriptions. A count of visits or pollen grains cannot substitute for a count of fertilised ovules.
Pollen arrival and the two fusion destinations
Read the diagram in words
The pollen grain arrives at the stigma. A tube extends through the style toward an ovule and carries sperm p1 and p2. p1 fuses with the egg; p2 fuses with the central cell. The tube and maternal organ boundaries are separate from these gametes.
| Structure in this model | Schematic x coordinate | Schematic y coordinate | Schematic width | Horizontal radius | Vertical radius | Radius | Minimum vertical coordinate | Maximum vertical coordinate |
|---|---|---|---|---|---|---|---|---|
| Stigma | 0 | 7 | 4 | Not assigned here | Not assigned here | Not assigned here | Not assigned here | Not assigned here |
| Style | 0 | Not assigned here | 1 | Not assigned here | Not assigned here | Not assigned here | 2 | 7 |
| Ovary | 0 | 0 | Not assigned here | 3 | 2 | Not assigned here | Not assigned here | Not assigned here |
| Ovule | 0.8 | 0 | Not assigned here | 1.6 | 1.3 | Not assigned here | Not assigned here | Not assigned here |
| Female gametophyte | 0.8 | 0 | Not assigned here | 1.1 | 0.9 | Not assigned here | Not assigned here | Not assigned here |
| Egg cell | 0.35 | 0.25 | Not assigned here | Not assigned here | Not assigned here | 0.2 | Not assigned here | Not assigned here |
| Central cell | 1.15 | -0.25 | Not assigned here | Not assigned here | Not assigned here | 0.4 | Not assigned here | Not assigned here |
| Structure in this model | Schematic x coordinate | Schematic y coordinate | Schematic width | Horizontal radius | Vertical radius | Radius | Minimum vertical coordinate | Maximum vertical coordinate |
|---|---|---|---|---|---|---|---|---|
| Pollen grain: male gametophyte | -0.8 | 7.4 | Not assigned here | Not assigned here | Not assigned here | 0.35 | Not assigned here | Not assigned here |
- Pollen grain: male gametophyte
- Vegetative cell forms the tube
- Pollen tube
- p1 / p2
| Tube path point from the stigma end | Schematic x coordinate | Schematic y coordinate |
|---|---|---|
| 1 | -0.8 | 7.1 |
| 2 | -0.3 | 6.7 |
| 3 | -0.3 | 3 |
| 4 | 0.1 | 1.5 |
| 5 | 0.25 | 0.6 |
| Sperm contribution | Schematic x coordinate | Schematic y coordinate | Chromosome sets contributed | Fusion destination |
|---|---|---|---|---|
| Sperm p1: one set | 0.1 | 0.5 | 1 | Egg cell |
| Sperm p2: one set | 0.35 | 0.5 | 1 | Central cell |
| Event | Assigned event stage |
|---|---|
| Pollen arrives at stigma | 1 |
| Tube delivers two sperm | 2 |
| p1 fuses with egg | 3 |
| p2 fuses with central cell | 3 |
- Stigma
- Style
- Ovary
- Ovule
- Female gametophyte
- Egg cell
- Central cell
- Pollen grain: male gametophyte
- Pollen tube
- Zygote → embryo
- Initial endosperm
Selected successful common flowering-plant sequence; diagram units have no anatomical scale.
One seed contains tissues with different origins
For the selected common angiosperm pattern, the egg contributes one maternal chromosome set and the sperm one paternal set. The embryo begins with 1 + 1 = 2 sets. The selected central cell contributes two maternal sets and the second sperm one paternal set: the initial endosperm has 2 + 1 = 3 sets. These are tissue-specific set accounts, not a claim that the whole seed is triploid.
The seed coat develops from maternal ovule integuments. Its selected starting tissue is diploid maternal sporophyte tissue, with no paternal set added by either fertilisation. The three rows in the model therefore have different histories: embryo 1m + 1p, endosperm 2m + 1p, seed-coat tissue 2m + 0p. The model does not count later DNA replication, endoreduplication or all maternal tissues.
Endosperm ploidy and female-gametophyte development vary among flowering plants; 3n is a common selected pattern, not a universal value. Development also involves communication between embryo, endosperm and maternal tissues. Knowing a tissue’s chromosome-set origin is one part of its biology, not a full explanation of seed survival.
Different chromosome-set origins within a seed
Read the diagram in words
Embryo origin: one maternal plus one paternal set = two. Selected endosperm origin: two maternal plus one paternal = three. Maternal seed-coat tissue origin: two maternal sets plus zero paternal = two. p1 and p2 enter separate fertilisation products.
| Tissue origin | Maternal sets | Paternal sets | Total sets in the selected nucleus | Paternal contribution identity |
|---|---|---|---|---|
| Embryo origin | 1 | 1 | 2 | p1 |
| Endosperm origin | 2 | 1 | 3 | p2 |
| Maternal seed-coat tissue origin | 2 | 0 | 2 | No paternal contribution |
| Chromosome sets contributed | Displayed symbol |
|---|---|
| Maternal sets | Filled circle |
| Paternal sets | Hollow diamond |
- Maternal sets
- Paternal sets
- Total sets in the selected nucleus
- Different tissues; no whole-seed average
Selected common angiosperm pattern at initial tissue origins; no replication or later endoreduplication assigned.
Worked example
Account for the two sperm contributions
Label the sperm contributions p1 and p2. Assign p1 to the egg and p2 to the central cell. The embryo row receives p1 only; the endosperm row receives p2 only. Each sperm contributes one set to a different fertilisation product. Adding both paternal sets to the embryo would describe a different event from the model.
The seed coat has no arrow from either sperm. Its 2m entry belongs to maternal tissue already surrounding the ovule. Thus the embryo, endosperm and coat entries cannot be pooled into an average seed ploidy: they refer to different cells, and the diagram supplies no numbers or sizes of those cells.
Check your understanding
A pollen grain reaches a stigma. Which event is established, and which remains unestablished?
Show reasoning
Pollen transfer is established. Sperm delivery, gamete fusion and successful seed development are not established by arrival alone.
Why is the pollen tube drawn separately from the two sperm?
Show reasoning
The vegetative cell forms the tube that carries the gametes. Each sperm has a fusion destination; the tube does not become an additional gamete or fuse with the egg as a sperm.
The selected endosperm begins with three sets. Does that make its embryo triploid?
Show reasoning
No. The embryo follows the separate egg-plus-sperm account, with two sets. Three sets apply to the selected endosperm pattern; maternal seed-coat tissue has its own diploid origin.
What to carry forward
Trace structures and events separately before assigning the origins of embryo, endosperm and maternal seed tissues.
References
Resources supporting this lesson’s explanations and examples.