Alternation of Generations

All Embryophytes and some algae undergo an Alternation of Generations.
Each generation of offspring has a different ploidy than the generation before it.

Generations alternate from diploid to haploid to diploid to haploid, etc.

sporophyte (2n) --> gametophyte (n) --> sporophyte (2n) --> gametophyte (n) --> sporophyte (2n) etc...

In short:

The haploid (gametophyte) generation "gives birth" to the diploid (sporophyte) generation, and that diploid sporophyte generation "gives birth" to the next haploid gametophyte generation, and so on.

All plants go through this life cycle, though the (homologous) stages may look very different across taxa.

The generalized cycle of the Alternation of Generations:




To fully understand the cycle it might be helpful to use an animal-based analogy.

What doGAMETOPHYTES look like?

It depends on the plant taxon. Let's have a look.

I. Bryophyte (non-vascular plant) Gametophytes

In non-vascular plants (mosses, liverworts, hornworts), the gametophyte is the dominant generation. When you see a typical moss or liverwort, you are seeing a haploid plant that can live for many years.

II. Tracheophyte (vascular plant) Gametophytes

In vascular plants (tracheophytes), the gametophyte is reduced, and does not live more than a season. The more derived the plant group, the more reduced the gametophyte.

1. Gametophytes of the Vascular Plants I: The Seedless Vascular Plants
(from right to left: fern, horsetail, club moss)

They look amazingly like a liverwort gametophyte, don't they? For good reason. Recall the link between similar ontogenies and common ancestry. These are often bisexual: each gametophyte has both antheridia and archegonia.

2. Gametophytes of the Vascular Plants II: The Gymnosperms
In the gymnosperms ("naked seed") plants, the gametophytes are drastically reduced, and are no longer recognizable as plants (though, technically, they are).

Gymnosperms include such plants as cycads, ginkgoes, and conifers. Our representative, typical gymnosperm will be the pine.

The male gametophyte

  • Each pollen in the pictures above is homologous to the liverwort thallus, and the fern and horsetail gametophytes.

  • Pines are wind-pollinated, and they release huge numbers of pollen gametophytes in the spring.

    Click on the picture to see a video of spectacular pollen release.

    The female gametophyte

    Below, from left to right:
    • female gametophyte (microscopic view)
    • female strobilus, longitudinal section, showing location of gametophytes (inside ovule)
    • female strobili: three different years

  • The female gametophyte is surrounded by sporophyte integuments. This entire structure is known as an ovule.

  • After the gametophyte's ovum is fertilized to become a zygote, the ovule becomes a seed.

  • The seed coat is made of sporophyte tissue. Nutritive, fatty nucellus surrounds the embryo to give it a head start upon germination. (This is why pine nuts are so delicious!)

  • Mature seeds are released by the female strobilus to be dispersed by wind or animals.

    3. Gametophytes of the Vascular Plants III: The Angiosperms
    The gametophytes of flowering plants are even more reduced than those of the gymnosperms. The male is still a pollen grain, and fairly similar (in function) to pine pollen. But the female gametophyte no longer even develops archegonia. She's just a mass of cytoplasm containing eight identical, haploid nuclei.

    The male gametophyte

    The female gametophyte

    The archegonial symplesiomorphy: It looks similar across species.

    The archegonium is the sex organ (analogous to an ovary) of the female gametophyte. It looks remarkably similar across taxa, though its location becomes less obvious as the gametophyte becomes smaller in the more derived plants.

    Female liverwort: archegonia are on the underside of the female gametophyte archegoniophore :

    Female moss: archegonia are at the tip of a female gametophyte's shoot

    Bisexual Fern: archegonia are clustered around the notch of the little heart-shaped gametophyte

    Female gymnosperm: archegonia are inside the female gametophyte inside the ovule

    Female angiosperm: archegonia have been lost. The 8-nucleate embryo goes straight to gamete!

    What doSPOROPHYTES look like?

    This is the diploid generation. In bryophytes, the sporophyte is ephemeral and small. In tracheophytes, the sporophyte is the dominant, long-lived generation.

    Liverwort sporophyte: These are little teardrop-shaped capsules attached to the bottom of the female archegoniophore. They release their spores, then dry up and die.

    Moss sporophytes: As in the liverwort, these sprout out of the archegonium. They're little more than a stalk with a sporangium at the tip.

    Fern sporophyte: The fern plant you know and love is a sporophyte.

    Gymnosperm sporophyte: The large, (often) cone-bearing plant is the sporophyte

    Angiosperm sporophyte: The flowering plants you see around you are all sporophyte generation individuals.