The Sex Lives of Nonvascular Plants: Alternation of Generations – Crash Course Biology #36


Plants! You’re familiar with
their work: They turn all that carbon dioxide that we don’t
want into the oxygen we do want, they’re all around us,
and they’ve been around for a lot longer than animals. The plants that we see today probably
evolved from a single species of algae that noodged itself onshore
about 1.2 billion years ago. And from that one little piece of
algae, all of the half million or so species of plants
that we have today evolved. But of course all this
didn’t happen overnight. It wasn’t until about
475 million years ago that the first plants started to evolve.
And they were very simple didn’t have a lot of
different tissue types, and the descendants of those plants
still live among us today. They’re the nonvascular plants:
the liverworts, the hornworts and everybody’s best
friends, the mosses. Mmm, fuzzy! Now, yeah, it’s clear that
these guys are less complicated than an orchid or an oak tree,
and if you said they were less beautiful, you probably wouldn’t
get that much argument from me. But by now I think
you’ve learned enough about biology to know that when it
comes to the simplest things: sometimes they’re
the craziest of all. Because they evolved
early in the scheme of things they were sort of able
to evolve their own set of rules. So, much like we saw with
archaea, protists and bacteria, nonvascular plants have some bizarre
features and some kooky habits that seem, to us, like,
kind of, just, like, what? Especially when it
comes to their sex lives. The main thing to know
about nonvascular plants is their reproductive cycle,
which they inherited from algae but perfected to the point where
now it is used by all plants in one way or another,
and there are even traces of it in our own reproductive systems. Usually when we’re talking about plants, we’re
really talking about vascular plants, which have stuff
like roots, stems, and leaves. Those roots, stems and leaves are
actually tissues that transport water and nutrients from one
part of the plant to another. As a result, vascular plants
are able to go all giant sequoia. The main defining trait of
nonvascular plants is that they don’t have specialized
conductive tissues. Since they don’t
have roots and stems, they can’t reach down into the
soil to get to water and nutrients. They have to take moisture in
directly through their cell walls and move it around from
cell to cell through osmosis, while they rely on diffusion
to transport minerals. Another thing nonvascular
plants have in common is limited growth potential. Largely because they don’t
have tissues to move the good stuff around, or woody
tissue to support more mass, the way for them to win is
to keep it simple and small. So small that when you
look at one of these dudes, you sometimes might not
know what you’re looking at. And finally, nonvascular plants
need water for reproduction This is kind of a bummer for
them because it means they can’t really survive in dry places
like a lot of vascular plants can. But I’ll get back
to that in a minute. Other than that,
nonvasculars are true plants: They’re multicellular, they have
cell walls made of cellulose, and they use photosynthesis
to make their food. All the nonvascular plants
are collectively referred to as bryophytes, and who
knows how many different sorts there used to be back
in the olden days, but we can currently meet three
phyla of bryophytes in person: the mosses, in phylum Bryophyta,
the liverworts, in phylum Hepatophyta, and the hornworts
in phylum Anthocerophyta. Taken together, there are over
24,000 species of bryophytes out there: about 15,000 are
mosses, 9,000 are liverworts and only only about
100 are hornworts. Hornworts and liverworts, funny
names, but are named after the shape of their
leaf-like structures horns for the hornworts and
livers for the liverworts with “wort,” stuck on the end
there, which just means “herb.” And you know what moss looks
like, though some things that are called moss like “Spanish moss”
in the southern United States, and “reindeer moss” up in the alpine
tundra of Alaska, are imposters, they’re actually lichens and
lichens aren’t even plants! The very oldest fossils of plant
fragments look really similar to liverworts, but nobody really
knows which of the bryophytes evolved first and
which descended from which. We just know that something very
bryophytic-looking was the first plant to rear its leafy head
back in the Ordovician swamps. So, now we’ve got these ultra-old
timey nonvascular plants to provide us with some clues as
to how plants evolved. And like I mentioned,
the most important contribution to the Kingdom Plantae, and
everything that came after them, is their wonderfully
complex reproductive cycle. See, plants,
vascular and nonvascular, have a way more complicated sexual
life cycle than animals do. With animals, it’s pretty
much a one-step process: two haploid gametes, one from
the mom and one from the dad, come together to make a
diploid cell that combines the genetic material
from both parents. That diploid cell divides
and divides and divides and divides until, voila! The world is one marmot
or grasshopper richer. Plants, on the other hand,
along with algae and a handful of invertebrate animal species,
have evolved a cycle in which they take on two different forms
over the course of their lives, one form giving rise
to the other form. This type of reproductive cycle is
called alternation of generations, and it evolved first in algae,
and many of them still use it today. However, the difference between
algae and plants here is that, in algae, both generations
look pretty much the same, while in land plants, all land
plants, the alternating generations are fundamentally
different from each other. And by fundamental, I mean that
the two don’t even share the same basic
reproductive strategy. One generation, called the
gametophyte, reproduces sexually by producing gametes, eggs and
sperm, which you know are haploid cells that only carry
one set of chromosomes. And the bryophyte sperm is a lot
like human sperm, except they have two flagella instead of one,
and they’re kind of coil shaped. When the sperm and egg fuse, they
give rise to the second generation, called the sporophyte
generation, which is asexual. The sporophyte itself is diploid, so it already has two sets
of chromosomes in each cell. It has a little capsule
called a sporangium, which produces haploid reproductive
cells called spores. During its life, the sporophyte
remains attached to its parent gametophyte, which it relies
on for water and nutrients. Once its spores
disperse and germinate, they in turn produce gametophytes, which turn around and produce
another sporophyte generation. And so on. Weird, I know,
but that’s the fun of it. Life is peculiar and that’s
what makes it so great. This means that the nonvascular
plants that we all recognize, the green, leafy, livery
or horny parts of the moss, liverwort or hornwort,
are actually gametophytes. Sporophytes are only found
tucked inside the females, and they’re super
small and hard to see. So in this gametophyte generation, individuals are always
either male or female. The male makes sperm through
mitosis in a feature called the antheridia,
the male reproductive structure. While the female gametophyte
makes the egg, also through mitosis, inside the female
reproductive structures, which are called the archegonia. These two gametophytes might be
hanging out right next to each other, sperm and eggs totally ready to go,
but they can’t do anything until water is introduced
to the situation. So let’s just add a sprinkle
of water and take a tour of the bryophytes’
sex cycle, shall we? By way of the water, the sperm
finds its way to the female and then into the egg, where the
two gametes fuse to create a diploid zygote, which divides by
mitosis and grows into a sporophyte. The sporophyte grows inside
the mother, until one day it cracks open and the sporophyte
sends up a long stalk with a little cap on
top called the calyptra. This protective case is made
out of the remaining piece of the mother gametophyte,
and under it a capsule forms full of thousands of
little diploid spores. When the capsule is
mature, the lid falls off, and the spores are
exposed to the air. If humidity levels are high enough, the capsule will let the
spores go to meet their fate. Now, if one lands on a
basketball court or something, it will just die if
it doesn’t get water. But if it lands on moist
ground, it germinates, producing a little filament called the
protonema, that gives rise to buds. These eventually grow
into a patch of moss, which is just a colony
of haploid gametophytes. That generation will mate,
and make sporophytes, and the generations will continue
their alternation indefinitely! Now because nonvascular plants are
the least complex kind of plants, their alternation of
generations process is about as simple as it gets. But with vascular plants,
because they have all kinds of specialized tissues, things
get a little more convoluted. For instance, plants that
produce unprotected seeds, like conifers or gingko
trees, are gymnosperms, and it’s at this level
that we start to see pollen, which is just a male gamete
that can float through the air. The pollen thing is taken to
the next level with angiosperms, or flowering plants, which
are the most diverse group of land plants, and the
most recently evolved. So the main difference between
the alternation of generations in vascular and nonvascular
plants is that in bryophytes you recognize the
gametophyte as being the… you know, the plant part. The moss
or the liverwort or whatever. While the sporophyte is less
recognizable and smaller. But as plants get more complicated,
like with vascular plants, the sporophytes become
the dominant phase, more prominent or recognizable.
Like the flower of an angiosperm, for instance, is, itself,
actually the sporophyte. Now I maybe just stuck a spoon in
all the stuff that you learned and stirred it up
to confuse you more. But we’ll get into this
more when we talk about the reproduction of vascular plants. But whether they have a big
showy sporophyte like a flower or a little, damp gametophyte
like a moss, all land plants came from the same, tiny little
ancient nonvascular plant who just put their sperm
out there, hoping to find some lady gametophyte
they could call their own. And I think that’s kind of sweet. Thank you for watching this
episode of Crash Course Biology. And thanks to all the people
who helped put it together. There’s a table of
contents over there if you want to go review anything. And if you have any
questions for us, we’re on Facebook and
Twitter and, of course, we’re down in the comments below. Thanks a lot.

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