Life cycles and alternation of generations

Two generations

Every plant that reproduces sexually passes through two kinds of stage.

• The gametophyte is haploid ($n$). It makes gametes by mitosis.
• The sporophyte is diploid ($2n$). It makes spores by meiosis.

Two events link them.

1. Meiosis turns the diploid sporophyte's cells into haploid spores. A spore grows into a gametophyte.
2. Syngamy, the fusion of two gametes, turns two haploid cells into a diploid zygote. The zygote grows into a sporophyte.

Both haploid and diploid cells can divide by mitosis. So either stage can grow into a many-celled body. A life cycle in which the two stages follow each other is called an alternation of generations. Plants differ in which stage grows big, and which stays tiny.

Haplontic

The free-living, photosynthetic plant is the gametophyte. The only diploid stage is the one-celled zygote. The zygote divides by meiosis at once, into haploid spores, and these grow into new gametophytes. There is no free-living sporophyte.

Examples: many algae, such as Volvox, Spirogyra and some species of Chlamydomonas.

Diplontic

The free-living, photosynthetic plant is the sporophyte. The gametophyte is reduced to one cell or a few cells. In flowering plants it is the pollen grain and the embryo sac.

Examples: all seed plants, the gymnosperms and the angiosperms, and a few algae such as Fucus.

Haplo-diplontic

Both stages are many-celled. Which one is the main plant depends on the group.

• In bryophytes, the main plant is the haploid gametophyte, green and thalloid or leafy. The sporophyte grows out of it, lives only a short time, and depends on it, wholly or partly.
• In pteridophytes, the main plant is the diploid sporophyte, green and with vascular tissue. The gametophyte, the prothallus, is small and short-lived, but it lives on its own.

A few algae, such as Ectocarpus, Polysiphonia and the kelps, are also haplo-diplontic.

Where each group fits

Which stage is the main plant?

Move the slider through the three patterns. The bars show how big each stage grows.

Try it in Python

The program sorts seven organisms by their life cycle, prints the main stage of each, and counts the patterns.

Practise with new numbers

Common mistakes

• "Meiosis makes gametes in every plant." In plants with a many-celled gametophyte, meiosis makes spores, and the gametophyte makes gametes by mitosis.
• "The zygote of Spirogyra grows into a plant." In a haplontic cycle, the zygote divides by meiosis, and the haploid cells grow into the new plant.
• "In ferns the gametophyte is the main plant." That is true of mosses. In ferns, the sporophyte is the main plant.
• "Haplo-diplontic means both stages are free-living." Both are many-celled, but the moss sporophyte depends on the gametophyte.
• Calling every alga haplontic. Fucus is diplontic, and Ectocarpus and the kelps are haplo-diplontic.

Remember these

• The gametophyte ($n$) makes gametes by mitosis. The sporophyte ($2n$) makes spores by meiosis.
• Syngamy makes a diploid zygote, and meiosis makes haploid spores.
• Haplontic: the gametophyte is the main plant, and the zygote is the only diploid cell. Example: Spirogyra.
• Diplontic: the sporophyte is the main plant, and the gametophyte has one or a few cells. Examples: seed plants, Fucus.
• Haplo-diplontic: both stages are many-celled. The gametophyte leads in bryophytes, and the sporophyte leads in pteridophytes.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. In a haplontic cycle, which is the only diploid stage, and what happens to it?

2. In a flowering plant, which structures are the male and the female gametophytes?

3. Both mosses and ferns are haplo-diplontic. How do their main plants differ?

4. Name one alga whose life cycle is diplontic.

5. Why can both the haploid and the diploid stages of a plant grow into many-celled bodies?