Notes

This type of reproductive cycle is called alternation of generations,
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and it evolved first in algae, and many of them still use it today.
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However, the difference between algae and plants here is that,
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in algae, both generations look pretty much the same,
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while in land plants, all land plants, the alternating generations
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are fundamentally different from each other.
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And by fundamental, I mean that the two don't even share
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the same basic reproductive strategy.
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One generation, called the gametophyte, reproduces sexually
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by producing gametes, eggs and sperm, which you know are haploid
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cells that only carry one set of chromosomes.
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And the bryophyte sperm is a lot like human sperm, except they have
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two flagella instead of one, and they're kind of coil shaped.
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When the sperm and egg fuse, they give rise to the second generation,
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called the sporophyte generation, which is asexual.
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The sporophyte itself is diploid,
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so it already has two sets of chromosomes in each cell.
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It has a little capsule called a sporangium,
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which produces haploid reproductive cells called spores.
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During its life, the sporophyte remains attached to its parent
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gametophyte, which it relies on for water and nutrients.
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Once its spores disperse and germinate,
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they in turn produce gametophytes,
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which turn around and produce another sporophyte generation.
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And so on. Weird, I know, but that's the fun of it.
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Life is peculiar and that's what makes it so great.
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This means that the nonvascular plants that we all recognize,
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the green, leafy, livery or horny parts of the moss,
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liverwort or hornwort, are actually gametophytes.
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Sporophytes are only found tucked inside the females,
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and they're super small and hard to see.
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So in this gametophyte generation,
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individuals are always either male or female.
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The male makes sperm through mitosis in a feature
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called the antheridia, the male reproductive structure.
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While the female gametophyte makes the egg, also through mitosis,
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inside the female reproductive structures,
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which are called the archegonia.
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These two gametophytes might be hanging out right next to each other,
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sperm and eggs totally ready to go, but they can't do anything until
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water is introduced to the situation.
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So let's just add a sprinkle of water and take a tour
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of the bryophytes' sex cycle, shall we?
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By way of the water, the sperm finds its way to the female
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and then into the egg, where the two gametes fuse to create a
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diploid zygote, which divides by mitosis and grows into a sporophyte.
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The sporophyte grows inside the mother, until one day it
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cracks open and the sporophyte sends up a long stalk
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with a little cap on top called the calyptra.
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This protective case is made out of the remaining piece
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of the mother gametophyte, and under it a capsule forms
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full of thousands of little diploid spores.
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When the capsule is mature, the lid falls off,
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and the spores are exposed to the air.
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If humidity levels are high enough,
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the capsule will let the spores go to meet their fate.
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Now, if one lands on a basketball court or something,
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it will just die if it doesn't get water.
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But if it lands on moist ground, it germinates, producing
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a little filament called the protonema, that gives rise to buds.
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These eventually grow into a patch of moss,
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which is just a colony of haploid gametophytes.
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That generation will mate, and make sporophytes,
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and the generations will continue their alternation indefinitely!
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Now because nonvascular plants are the least complex kind of plants,
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their alternation of generations process
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is about as simple as it gets.
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But with vascular plants, because they have all kinds
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of specialized tissues, things get a little more convoluted.
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For instance, plants that produce unprotected seeds,
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like conifers or gingko trees, are gymnosperms,
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and it's at this level that we start to see pollen,
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which is just a male gamete that can float through the air.
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The pollen thing is taken to the next level with angiosperms,
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or flowering plants, which are the most diverse group
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of land plants, and the most recently evolved.
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So the main difference between the alternation of generations
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in vascular and nonvascular plants is that in bryophytes
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you recognize the gametophyte as being the...
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you know, the plant part. The moss or the liverwort or whatever.
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While the sporophyte is less recognizable and smaller.
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But as plants get more complicated, like with vascular plants,
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the sporophytes become the dominant phase,
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more prominent or recognizable. Like the flower of an angiosperm,
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for instance, is, itself, actually the sporophyte.
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Now I maybe just stuck a spoon in all the stuff that you learned
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and stirred it up to confuse you more.
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But we'll get into this more when we talk about
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the reproduction of vascular plants.
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But whether they have a big showy sporophyte like a flower
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or a little, damp gametophyte like a moss, all land plants
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came from the same, tiny little ancient nonvascular plant
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who just put their sperm out there, hoping to find
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some lady gametophyte they could call their own.
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And I think that's kind of sweet.