11.1 Alternation of Generations
Key Takeaways
A sporophyte is diploid and produces haploid spores by meiosis.
A gametophyte is haploid and produces gametes by mitosis, and fertilization restores the diploid sporophyte.
Ferns have a dominant sporophyte, a small free-living gametophyte, and flagellated sperm that must swim through a water film.
Conifers have a dominant sporophyte, tiny dependent gametophytes, pollen, and naked seeds in cones, and they have no flowers and no fruit.
Flowering plants have a dominant sporophyte, the most reduced gametophytes, and seeds enclosed in fruit.
11.1 Alternation of Generations
Land plants alternate between two multicellular bodies. The sporophyte is diploid. The gametophyte is haploid. This pattern is alternation of generations. Chromosome number changes at only two steps. Meiosis halves it, and fertilization restores it. All other growth is mitosis, so each body stays at one ploidy until one of those events.
Spores come from the sporophyte
Ordinary sporophyte cells have two chromosome sets. Inside a sporangium, selected cells undergo meiosis and produce haploid spores. A spore can grow into a new multicellular body without fusing with another cell. That body is the gametophyte, and every nucleus in it is haploid because meiosis already happened. A spore is not a gamete and not a seed. It is the handoff from the diploid generation to the haploid generation.
Gametes come from the gametophyte
The gametophyte makes gametes by mitosis. Fern eggs form in archegonia and fern sperm form in antheridia. In seed plants, sperm form in the male gametophyte and the egg forms in the female gametophyte. The gametes are already haploid, so this division must not halve the chromosome number again. Animals are the contrast students import by mistake: animal meiosis makes gametes directly. Plant meiosis makes spores. Plant mitosis, inside the gametophyte, makes the egg and the sperm.
Fertilization fuses one sperm nucleus with one egg nucleus. The zygote is diploid, and mitosis builds the next sporophyte from it. That sporophyte makes spores only when it is mature enough for meiosis. By ploidy, the loop runs diploid sporophyte, haploid spore, haploid gametophyte, haploid gametes, diploid zygote, diploid sporophyte.
Ferns, conifers, and flowering plants
In all three groups the familiar plant is the sporophyte. The gametophyte is smaller: free-living in ferns, and dependent in conifers and flowering plants. Mosses are the one contrast: the green moss cushion is the gametophyte, and the stalked capsule is a dependent sporophyte.
Ferns
Fronds, the rhizome, and roots are sporophyte organs. Meiosis in sporangia, often grouped as sori under a frond, releases spores. A spore can grow into a small free-living prothallus. That gametophyte is a thin green sheet. It photosynthesizes and is not fed by the sporophyte. Its antheridia release flagellated sperm, which must swim to an egg through a film of water. Drought blocks that swim even if the prothallus is healthy. The zygote grows a new sporophyte, and the prothallus then withers. Ferns make no pollen, seeds, flowers, or fruit. Most ferns are homosporous: one spore type grows into a gametophyte that can make both eggs and sperm. Conifers and flowering plants make two spore types, small male microspores and larger female megaspores, so each has a separate male and a separate female gametophyte.
Conifers
The pine, spruce, or fir tree is the sporophyte. Its gametophytes are microscopic and dependent on the tree. They cannot live on open soil as a prothallus does. Male cones release pollen grains, and each grain is a male gametophyte carried by wind to an ovule on a seed-cone scale. The female gametophyte develops inside that ovule and is nourished by the parent. Sperm travel in a pollen tube, so conifers do not need an external water film. The ovule becomes a seed sitting exposed on the scale. The seed is naked because no ovary wall encloses it. Conifers have neither flowers nor fruit. A cone is not a fruit.
Flowering plants
The herb, shrub, or tree is the sporophyte, and the gametophytes are the most reduced of the three. The male gametophyte is the pollen grain. The female gametophyte is the embryo sac in the ovule. Both are dependent on the sporophyte. Flowers are the shoots that bear the organs where these gametophytes develop. After fertilization the ovule becomes a seed and the ovary becomes a fruit, so the seed is enclosed.
How the three cycles compare
| Feature | Ferns | Conifers | Flowering plants |
|---|---|---|---|
| Dominant body | Diploid sporophyte | Diploid sporophyte | Diploid sporophyte |
| Gametophyte | Small free-living prothallus | Tiny and dependent on the sporophyte | Most reduced: pollen grain and embryo sac |
| Sperm delivery | Flagellated sperm swim in a water film | Pollen tube, with no external water film | Pollen tube grows to the embryo sac |
| What is dispersed | Spores, and no seeds | Naked seeds in cones | Seeds enclosed in fruit |
| Flowers and fruit | Absent | Absent; a cone is not a fruit | Flowers present; fruit forms from the ovary |
A water film and a free-living green gametophyte identify a fern. Cones, pollen, and exposed seeds identify a conifer. A fruit around the seed identifies a flowering plant. The nuclear sequence is shared: meiosis makes spores, mitosis makes gametes, and fertilization rebuilds the sporophyte.
Warning
Spores are made by meiosis in the sporophyte. Plant gametes are made by mitosis in the gametophyte. Fertilization, not a second meiosis, restores the diploid sporophyte.
Reading the cycle on an item
Pair every stage with both a ploidy and a division. A cell entering meiosis in a sporangium is still diploid. The spores it produces are haploid. The gametophyte stays haploid while it is multicellular. A prothallus and a pollen grain are both haploid bodies. Diploidy returns only when gametes fuse.
Keep travel separate from fusion. A pollen grain can leave an anther, and a fern sperm can swim, without fertilization having occurred. Fertilization is the fusion that makes the zygote. Keep the spore separate from the seed as well. A fern spore does not contain an embryo. A seed contains the embryo of the next sporophyte, a food supply, and a coat.
On a dominant-generation item, pick the largest, longest-lived body. For these three groups that body is the sporophyte. On a reduced-gametophyte item, the flowering plant is the extreme. The fern gametophyte is small, but it is still free-living and photosynthetic. Conifer and flowering-plant gametophytes are housed and fed by the sporophyte.
A diploid cell in a fern sporangium finishes meiosis. Which statement correctly describes the sporophyte and the cells just produced?
The sporophyte is diploid, and meiosis has produced haploid spores that can grow into a gametophyte.
The sporophyte is haploid, and mitosis in the sporangium has produced diploid spores.
The sporophyte is diploid, and meiosis has produced eggs and sperm ready for fertilization.
The sporophyte is haploid, and meiosis has produced diploid gametes that fuse at once.
Which set of traits matches a fern, a conifer, and a flowering plant, in that order?
Naked seeds in cones; seeds inside a fruit; a free-living prothallus with swimming sperm.
A dominant gametophyte in all three; swimming sperm in all three; fruit in all three.
Fruit around spores; flowers without seeds; a dominant gametophyte as the familiar plant.
A free-living gametophyte and flagellated sperm that need a water film; naked seeds in cones and no flowers; seeds enclosed in fruit.
A classmate writes that plant eggs and sperm are produced by meiosis and that spores are produced by mitosis. Which replacement is accurate?
Spores are produced by meiosis in the sporophyte, and gametes are produced by mitosis in the gametophyte.
Both spores and gametes are produced by meiosis inside the gametophyte.
Spores are produced by mitosis in the sporophyte, and gametes are produced by meiosis in the same sporangium.
Fertilization produces spores, and those spores then fuse to build the gametophyte.
Sections you finish are checked off in the contents.