Plant Life Cycle: The Alternation of Generations

All Embryophytes and some algal Viridaeplantae undergo a specialized type of life cycle known as Alternation of Generations. This term reflects the fact that the generations change ploidy, with a diploid generation (the sporophyte) giving "birth" to a haploid generation (gametophte), and that haploid generation giving "birth" to the next diploid (sporophyte) generation, and so on.

Alternation of Generations: An Analogy

The typical human is diploid. Here are Diploid Brad and Diploid Paris.

But these are not typical humans. Having evolved from vegetables to mimic humans, they still undergo alternation of generations as they reproduce. Here's how it works.

In the springtime, Diploid Brad and Diploid Paris begin to feel frisky. Lesions known as sporangia begin to develop on specialized areas of their "skin", and inside the sporangia, diploid cells undergo meiosis to produce haploid cells called spores.

After a certain period of maturation, the spores are ready for distribution into the environment. The sporangia rupture, releasing the tiny, blue spores into the air, where they are carried to new and exciting environments.

The tiny spores swirl out into the night, diaspora of the human vegetable mimics. Wind carries them everywhere. Some land on the tops of buildings, and there remain until the elements dry and kill them. Some end up in Fido's food bowl, and are ingested along with the Alpo. But a few lucky individuals are carried far away, and land on a suitable patch of moist soil. And here the spores germinate and grow via mitosis to become the haploid gametophyte of the species.

Lacking the charisma and good press of its parent sporophyte, the gametophyte looks nothing like its sporophyte parent.

Brad's male microspores will develop into males...

and Paris's female megaspores will develop into females.

The male Smurf gametophytes have male sex organs known as antheridia that produce sperm.
The female Smurfette gametophytes have female sex organs known as archegonia that produce ova.

In their damp little glade, the small forest of Smurfs and Smurfettes wait for that fateful event they need to reproduce: a good rainstorm.

And then it rained! (Use your imagination.)

Fortunately, the picture is blurry. Because with the rain, the males release their sperm into the wild storm, where they swim wildly, searching for the females. The sperm travel up the legs of the Smurfettes, and find the archegonia, wherein lie the waiting ova.

Fertilization takes place inside the Smurfette's archegonium, and a new zygote is born.

Sadly, Smurfette cannot give birth to her developing zygote, and as it grows, it gradually crushes her out of existence, erupts from her quivering shell as a new, diploid sporophyte.

  • Once (the diploid sporophyte) has grown to its adult size and is sexually mature, sporangia will erupt along the new sporophtyes body again, and the cycle will continue.

    Here's a generalized plant life cycle:

    And to keep track of this complex cycle, you must learn the following vocabulary...


    A Tour of Alternating Generations

    First stop, The Bryophytes (as represented by the Liverworts)

    Next stop, The Seedless Tracheophytes (as represented by a Fern)

    Next, The Gymnosperms (as represented by a Pine)

    Last stop, The Angiosperms (as represented by the Lily)