Plants can be classified into these major groups:
ALL of these diverse plants undergo a life cycle that includes an Alternation of Generations.
The structures and life cycle stages in all these plants are homologous and have the same names, but they may look VERY different from one another, across taxa.
All Embryophytes and some algae undergo an Alternation of Generations.
Each generation of offspring has a different ploidy than the parent 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...
Sometimes it's hard to envision how this cycle works, because plants seem to introduce a "middle man" where animals go straight from haploid gamete to diploid zygote.
To get around this sticking point, I offer you...
Looks can be deceiving.
Although Diploid Brad and Diploid Paris may LOOK like animals, they are not.
They are highly derived plants that have evolved animal mimicry in order to get more attention and fertilizer.
As plants, they still undergo alternation of generations.
An alert scientist can spot a mimic by observing its life cycle.
Small lesions known as sporangia begin to develop on specialized areas of their epidermis.
Inside each sporangium, diploid cells divide to produce haploid cells called spores.
Because Diploid Brad and Diploid Paris produce spores, their life cycle stage is named the SPOROPHYTE GENERATION.
New Vocabulary Alert!
In our Brad/Paris species, the sporophyte generation happens to be dioecious:
After a period of maturation, the spores are ready for distribution into the environment.
The sporangia rupture, releasing spores into the air where they can be carried by the wind to new and exciting environments.
Wind carries them everywhere.
Some land on the tops of buildings, and there remain until the elements kill them.
Some end up in Snuffy's dog food bowl, and are ingested along with the kibbles.
But a few lucky spores are carried away to land on a patch of moist soil.
And here the spores germinate and grow (via mitosis) into haploid organisms that look completely different from their diploid parents.
These new, haploid organisms are the GAMETOPHYTE GENERATION.
In our Brad/Paris species, the gametophyte generation also happens to be dioecious:
In their damp little glade, the small forest of Smurfs and Smurfettes wait for that fateful event they need for reproduction:
A GOOD RAIN STORM.
At last it rained! (Use your imagination.) (But not too much.)
Fortunately, the picture is blurry. Because with the rain, the males release their sperm into the storm.
The sperm swim valiantly, searching for the females.
Following a chemical trail sent out by the Smurfette gametophytes, the sperm wiggle up the legs of the Smurfettes and...BEYOND.
Deep in the BEYOND, the sperm encounter the female sex organ, the archegonium.
Inside the archegonium, one sperm and one egg fuse to form a diploid zygote.
The stage is set for tragedy. (At least if you're Smurfette.)
Without a muscular, innervated uterus, Smurfette cannot give birth to the diploid offspring that will develop from the zygote.
As her child grows, it gradually crushes her out of existence, erupting from her quivering shell.
Thus begins the life of the new SPOROPHYTE.
The diploid sporophyte will grow to adult size.
When it is sexually mature, sporangia will erupt from his epithelium.
And the Life Cycle spins on.
To keep track of this complex cycle, we need to use a complex terminology.
II. TERMS RELATED TO THE (DIPLOID) SPOROPHYTE GENERATION:
III. GENERAL TERMS:
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.
They can be very comfy.
The Liverwort will show us the generalized life cycle of a bryophyte.
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 liverwort gametophytes, don't they?
For good reason.
Recall the link between similar ontogenies and common ancestry.
Seedless Tracheophyte gametophytes are often bisexual: each gametophyte has both antheridia and archegonia.
The fern will represent a generalized life cycle of a seedless tracheophyte:
2. Gametophytes of the Vascular Plants II: The Gymnosperms
In the gymnosperms (from the Greek gymnos meaning "naked" and sperm meaning "seed"), 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
The female gametophyte
3. Gametophytes of the Vascular Plants III: The Angiosperms
The angiosperm sporophyte produces a unique structure for sexual reproduction: the flower.
All the parts of the flower are modified leaves.
From outermost to innermost, these are
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 is reduced to 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 sac directly produces a single ovum.
What do SPOROPHYTES 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.
Notice the reversal in size and lifespan from bryophytes to tracheophytes.
In tracheophytes, the sporophyte is the dominant, long-lived generation.
Made it all the way to the end? Here's a little reward. A video of fern alternation of generations.
(Can you find the factual boo boo in this video? Two points extra credit if you can!)