Biology · Plant Diversity & Evolution
Alternation of generations and plant reproductive cycles
A tiny green fern gametophyte and a large leafy fern sporophyte can belong to successive parts of one reproductive cycle. Each is multicellular, yet their cells carry different numbers of chromosome sets. The word 'generation' here identifies that reproductive organisation, not simply an older and a younger plant.
The chapters on life's history and phylogeny explain shared ancestry and branching trees. Meiosis and inheritance supplies the distinction between one and two chromosome sets. Here, n and 2n are introduced again inside plant cycles. No botanical identification experience is assumed; the examples concern biological relationships and reproductive structures rather than cultivation.
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Follow the sexual land-plant cycle through meiosis, spore growth, gamete formation and fertilization without confusing body size with chromosome sets.
Two multicellular generations linked by two changes
In the basic sexual cycle, the sporophyte is the multicellular diploid generation: its ordinary cells have two chromosome sets, written 2n. Cells in its sporangia undergo meiosis to form haploid spores, with one set, n. A sporangium is a spore-producing structure; not every cell of its wall enters meiosis.
A spore can develop by mitotic growth into a multicellular gametophyte. In this basic model, growth preserves its haploid state. The gametophyte forms haploid gametes through mitosis and differentiation. Fertilization joins an egg and a sperm, restoring two chromosome sets in the zygote. Mitotic growth of that zygote produces the embryo and the next sporophyte. Later, the flowering-plant example distinguishes the extra nuclear fusion within its central cell.
These are the two set-changing transitions in the diagram: meiosis reduces 2n to n, and fertilization combines n plus n into 2n. The other labelled routes involve development within the same set category. DNA replication within a cell cycle is not an extra generation, and becoming larger does not automatically make a body diploid.
Two set-changing transitions in one cycle
Read the diagram in words
Sporophyte 2n → spore mother cell 2n → meiosis → spore n → mitotic development → gametophyte n → gametes n. Egg n plus sperm n form a zygote 2n; mitotic development forms a retained embryo and sporophyte 2n.
| Description | Ploidy |
|---|---|
| Multicellular sporophyte | 2n |
| Spore mother cell | 2n |
| Spore | n |
| Multicellular gametophyte | n |
| Egg | n |
| Sperm | n |
| Zygote | 2n |
| Retained embryo | 2n |
| From | To | Process | Specified route through |
|---|---|---|---|
| Multicellular sporophyte | Spore mother cell | Spore-producing tissue | — |
| Spore mother cell | Spore | Meiosis: 2n → n | — |
| Spore | Multicellular gametophyte | Mitotic development | — |
| Multicellular gametophyte | Egg | Gamete formation | — |
| Multicellular gametophyte | Sperm | Gamete formation | — |
| Egg | Zygote | Fertilization: n + n → 2n | — |
| Sperm | Zygote | Fertilization: n + n → 2n | — |
| Zygote | Retained embryo | Mitotic development | — |
| Retained embryo | Multicellular sporophyte | Mitotic development | — |
One chromosome set
Two chromosome sets
Spore develops without fusion
Gametes fuse
Stage
Chromosome sets
Basic sexual diploid land-plant cycle. Arrows are stage transitions, not material transport between organs or elapsed-time measurements. The later flower model separately treats the central-cell nuclear fusion; asexual and other departures are outside this cycle.
A spore develops; a gamete fuses
Both a spore and a gamete may be haploid in this cycle, but that shared chromosome state does not give them the same role. A spore begins gametophyte development without fusing with another spore. A gamete contributes to a zygote by fusion with another gamete. Asking what the cell does next is more useful than identifying it only by size.
The toy chromosome ledger assigns n = 3. A completed haploid spore nucleus has three chromosomes; a haploid gametophyte nucleus has three; an egg and a sperm each have three. Their fusion produces a zygote nucleus with six chromosomes. Different gametophyte cells do not pool their chromosomes simply because they belong to one larger body.
The ledger selects nuclei before their next DNA replication and does not display the replicated chromosomes inside meiosis. Thus its one-line chromosome symbols are not a picture of every stage of division. The real chromosome number varies among species, and polyploid life cycles require their own baseline. The set labels explain this constructed diploid cycle rather than assigning all plants six chromosomes.
Count sets per nucleus before counting a whole body
Read the diagram in words
Each selected spore, gametophyte, egg or sperm nucleus has three chromosomes. The zygote has six, with one of each numbered chromosome from each gamete. Forty haploid one-nucleus cells contain 120 chromosomes across their nuclei, while each nucleus remains n.
| Description | Ploidy | Chromosome sets per nucleus | Chromosomes per nucleus | Chromosome identity |
|---|---|---|---|---|
| Selected spore nucleus | n | 1 | 3 | 1a, 2b, 3a |
| Selected gametophyte nucleus | n | 1 | 3 | 1a, 2b, 3a |
| Egg nucleus | n | 1 | 3 | 1a, 2b, 3a |
| Sperm nucleus | n | 1 | 3 | 1b, 2a, 3b |
| Zygote nucleus | 2n | 2 | 6 | 1a, 1b, 2a, 2b, 3a, 3b |
| From | To | Process | Specified route through |
|---|---|---|---|
| Selected spore nucleus | Selected gametophyte nucleus | Mitotic development | — |
| Selected gametophyte nucleus | Egg nucleus | Gamete formation | — |
| Egg nucleus | Zygote nucleus | Egg contribution | — |
| Sperm nucleus | Zygote nucleus | Sperm contribution | — |
| Description | Assigned model value |
|---|---|
| Cells | 40 |
| Nuclei per cell | 1 |
| Chromosomes per nucleus | 3 |
| Count across all 40 nuclei | 120 |
| Ploidy | n |
Selected nucleus
Chromosome identity
Chromosomes per nucleus
Chromosome sets per nucleus
Count across all 40 nuclei
120 chromosomes across nuclei
Invented n = 3 case; every displayed nucleus is selected before its next DNA replication. Symbols name chromosome identities, not measured sequences or a full meiosis reconstruction. The sperm comes from another gametophyte in this example. The 40-cell body count concerns one nucleus per cell.
The same cycle can have very different visible bodies
In a moss, the conspicuous leafy body is usually a gametophyte, and a spore-producing sporophyte remains attached to it. In a typical familiar fern, the large frond-bearing body is a sporophyte; its gametophyte is much smaller and can live separately. In seed plants, the visible tree or flowering herb is a sporophyte, while the gametophytes are reduced and develop in association with sporophyte reproductive structures.
'Dominant generation' often names the generation that is more conspicuous or develops the more extensive body. It is not a claim that every species spends an identical fraction of its life in that generation, or that a small gametophyte has ceased to exist. Describing a particular cycle is clearer than treating dominance as a numerical score.
The diagram follows a sexual route. Plants can also reproduce through other routes, including vegetative growth, and some have departures from the standard sexual seed cycle. Those alternatives do not justify moving meiosis to gamete production in the displayed model. First identify which reproductive route is under discussion, then track its transitions.
Worked example
Six chromosomes, four spores and many cells
Assign a sporophyte n = 3, so its ordinary diploid nucleus has 2n = 6 chromosomes. One spore mother cell completes the two divisions of meiosis in the standard model and yields four haploid spores. Each spore nucleus has three chromosomes, rather than one quarter of six.
Suppose one spore develops into a gametophyte with 40 cells, each with one nucleus in the selected state. Each nucleus still has three chromosomes. Across those 40 nuclei there are 120 chromosomes, but the gametophyte is not 120-ploid: ploidy is the number of sets per nucleus here. An egg with three and a sperm with three give a zygote with six. Count nuclei, chromosomes per nucleus and chromosome sets separately.
Check your understanding
Where does meiosis occur in the displayed sexual plant cycle?
Show reasoning
In spore-producing sporophyte tissue, specifically the spore mother cells within sporangia. It produces haploid spores; the haploid gametophyte subsequently forms gametes without another reduction division in this cycle.
A haploid gametophyte grows from one cell to 40. Has growth alone changed it to 2n?
Show reasoning
No. Mitotic development preserves the haploid chromosome-set category in this model. More cells change the body's size and the total inventory across nuclei, not the sets per nucleus.
With n = 3, two selected gamete nuclei fuse. How many chromosomes are in the resulting zygote nucleus, and why?
Show reasoning
Six: each gamete contributes one three-chromosome set. Fusion restores two sets. This is a constructed chromosome number, not a measured number for a fern or moss.
What to carry forward
Meiosis makes spores, spores grow into gametophytes, and gamete fusion restores the sporophyte's two-set state.
References
Resources supporting this lesson’s explanations and examples.
- Biology 2e — Plant diversity and reproduction (2018)Mary Ann Clark, Matthew Douglas and Jung Choi · OpenStaxOpen textbook · selected sectionsEarly Plant LifeBryophytesSeedless Vascular PlantsEvolution of Seed PlantsGymnospermsAngiospermsPollination and Fertilization
- Australian bryophytes — Bodies, groups and liverwort poresHeino Lepp · Australian National Botanic Gardens and Australian National HerbariumExpert botanical guide · selected pagesWhat is a bryophyte?Which bryophyte is it?Thallose liverworts
- A community-derived classification for extant lycophytes and ferns (2016)Pteridophyte Phylogeny Group IOriginal community classification proposal · PDF