>> Hi everyone, and welcome to the Penguin Prof channel.
Today I want to talk about a topic
that my general biology students have been asking me
for for quite some time.
We're going to talk about plant sex.
And actually we're going to be talking
about alternation of generations.
A game that plants are playing that I like to call
"hide the gametophytes."
So if you don't think
about plants very often they really do have kind
of a tough go.
Plants need to be able to find each other to have sex.
Plants also need their offspring to be dispersed far away
from the parent so they don't compete with each other.
You don't want to compete with your offspring for resources.
The problem is of course plants don't move.
And so the question is, "How are you going to get your gametes,
your sex cells -- How are you going to get those together?"
How are you going to find members of your own species?
How are you going to, you know, find another individual
to have sex with and, you know, what happens
when you finally do have offspring?
How do you get them far away from you?
These are difficult questions.
This story is really going to be about sex and life cycles
and like everything I think it's good to start
with something you're familiar with and then build information
that may not be so familiar so that your brain kind
of has something to attach it to.
So we're going to start with humans, actually.
These are human chromosomes organized
so you can see all 23 pairs.
And this is called a karyotype, by the way.
And when we talk about sex there's a little bit
of a problem with these chromosomes.
So what if we had this scenario?
We have an egg and we said that humans have 46 chromosomes.
Right? And then what if that egg gets fertilized by a sperm
and that sperm also had 46 chromosomes?
Okay. What you have there is a complete disaster
because then every time you have fertilization you would double
the number of chromosomes that an organism would have
and you just can't have that.
So instead what we have is an egg cell that has half
that number, only 23 chromosomes.
And a sperm that has half that number, only 23 chromosomes.
And then when you have fertilization it's,
oh, so much better.
Now you have the correct number of chromosomes again.
So the vocabulary that we use to talk
about chromosome sets is called ploidy and you're going
to see these terms a lot.
Haploid, the number of sets in sex cells.
So eggs or sperm, they have only one set of chromosomes.
We use the letter N for haploid and then diploid is two sets
and that we're going to find in all somatic cells,
other cells in the body.
The word soma, by the way, just means body.
There are other terms, by the way, especially in plants.
You'll see tetraploidy, polyploidy,
but for now we're just going to stick with haploid and diploid.
That's going to get us through the story that we need.
So the human life cycle
with which you are familiar looks something like this
where we start out with adults which are diploid.
The cells of your body have 46 chromosomes.
In cells of the ovary
or the testes you have a very special type
of cell division called meiosis.
All right?
So meiosis only occurs in gamete producing cells.
And meiosis is what takes the cell from diploid to haploid.
It also creates variety because there's a shuffling of genes,
right, because if you've got to reduce the number of chromosomes
by half then obviously your egg
or sperm only gets half of what you've got.
Right? So now we go
from a diploid state to a haploid state.
So now the egg and the sperm are each haploid cells.
And only at fertilization is the diploid state restored.
Right? So egg plus sperm, 23 plus 23,
now we've got a diploid.
We call it a zygote, a fertilized egg.
And for humans N is 23.
Right? So we have 46.
Just so you know, of course,
different species have different numbers of chromosomes.
So the N value will change, but the haploid diploid,
right, that stays the same.
So now we've got a fertilized egg.
It undergoes mitosis.
That's just replicative division, kind of like a cell,
you know, going through a Xerox machine basically.
And that happens over and over again and you end
up with a diploid adult.
Okay. So if we're going to simplify this --
So we're going to be able to compare animals
with which you're familiar to plants
which may be a little more unfamiliar.
We have this where you have a diploid adult, right,
everything above the line is diploid, 2N, we have meiosis
which is our magic cell division and the product of meiosis
for animals -- The products are gametes.
Eggs and sperm.
This is a big difference that we're going to see from plants.
So keep that little fact in mind.
For animals the egg and the sperm come together.
Fertilization, right?
We just saw this.
Now you restore the diploid condition.
You've got some mitosis and now you're back to an adult.
Okay. So hang on to your adenoids
because this is what the plants do.
Plants are kind of foreign to us
because they have a whole extra step.
We're going to start with the diploid adult
which would be equivalent to an animal.
We're going to call it a sporophyte.
Okay? And that is diploid.
This 2N should now be familiar.
That sporophyte is going to undergo that special type
of cell division called meiosis, but what you make
in a plant is not eggs and sperm.
The product of meiosis in plants is a spore.
The product of plant meiosis is spores.
These spores are haploid and they undergo mitosis
and they grow up in to a haploid body that we call a gametophyte.
Oh, my goodness.
So this would be -- An equivalent in humans would be
if you could imagine your eggs or sperm leaving your body
and without fertilization they grow up in to you.
But it would be a haploid you.
Okay. That is pretty weird,
and animals don't do that, but plants do.
And that's why plants make a lot of people a little bit crazy
as we try to understand them.
This gametophyte will undergo mitosis and produce eggs
and sperm which are called gametes just like animals
which will fertilize just like animals
and restore the diploid condition just like animals.
That diploid zygote, the fertilized egg,
undergoes mitosis and grows in to a sporophyte.
So this is what you've got to remember with plants.
Sporophytes produce spores.
Gametophytes produce gametes.
So if you can remember this, say it to yourself over
and over again, and keep in mind that the product of meiosis
in plants and animals -- The product is different.
In animals meiosis produces gametes, eggs and sperm.
But in plants the product of meiosis is spores.
If you can keep those things in mind as we go
through the various plant groups, you're going to be okay.
We're obviously not going to be able to go
through 480 million years
of plant evolution on a Youtube video.
So -- Though, you know, I would love to try.
We're going to talk about a few main groups of plants
that will illustrate the difference in the alternation
of generations story which is really what this video
of course is about.
We're going to start with the mosses.
Mosses are in a group called bryophytes
and with mosses we have other less familiar plants
like liverworts and hornworts.
They all have pretty sexy names.
When you look at a moss, mosses of course grow in very,
very moist places in the forest and things like that,
they need a lot of water because mosses are non vascular.
They don't have any vascular fluid carrying tissues.
That also keeps the size of mosses very small.
When you look at a moss what you're looking
at is actually the gametophyte stage.
So let's look at the moss life cycle and look at some
of the exciting parts of it and how it relates
to the diploid haploid story.
So this moss, as you see it growing on the log --
This is what you're actually looking at.
You're looking at a haploid gametophyte body.
Now gametophytes produce of course gametes, eggs and sperm.
So there are male and female moss plants.
And here are the male parts and here are the female parts.
These are all haploid.
At fertilization the egg and sperm of course come together
and form a diploid zygote.
So now we're looking at something that's diploid.
The thing is on a moss the diploid stage is really small
and we call it a sporophyte and it actually grows right on top
of the haploid gametophyte.
And most people have never seen it.
Now as always the sporophyte produces spores.
Right? And those spores are haploid
because it's the sporophyte that undergoes meiosis.
So here you go back in to the spore stage.
These are now haploid.
And they will grow up in to the haploid gametophyte which is
where our story began.
It's very unusual to actually see the sporophyte in nature.
Mostly you see the gametophyte.
That's all that fuzzy green stuff.
But this is what the sporophyte looks like.
And you'll only see it during certain times of the year.
So again the dominant phase in a moss is the gametophyte.
Now let's compare the story of mosses
to a more highly evolved plant, the fern.
And when you look at a fern this is the sporophyte stage
of the fern.
By the way, ferns, beautiful plants,
these are vascular plants.
So they can get much, much larger than mosses.
There's about 12,000 different varieties in this group
and they really started
to dominate during the carboniferous period.
That's about 380 million years ago.
The final life cycle we're going to start again
with the dominant phase,
the phase that you would see out in the forest.
And in the case of the fern that is not the gametophyte.
It's the sporophyte.
So what you think of as a fern is actually a sporophyte.
A sporophyte produces?
Spores. If you look on the underside
of the fern leaf you're going to see these clusters of sporangia.
They're in little circular disks that are called sori
and of course these undergo meiosis
to produce haploid spores.
And haploid spores grow in to a gametophyte.
This gametophyte is very, very small.
In fact, it's microscopic.
So unless you go out in the forest
with a microscope you're never going to see it.
This is called a fern thallus.
It's kind of a heart shaped thing.
Isn't that cute?
It's got archegonia which are female parts, and antheridia
which are the male parts.
They only find each other in water
which is why ferns are also very much linked
to moist environments.
Right? There are no desert ferns because of this requirement.
So the gametophyte you've got the male and the female parts.
You've got the sperm.
You've got the egg.
You've got fertilization.
Same old story.
Now we've got a diploid zygote.
And now we have the new sporophyte.
It actually grows on top of the gametophyte.
And as the fern gets larger and we start all
over again the diploid sporophyte is actually growing
right out of the gametophyte
and the gametophyte will whither away and die.
Isn't that just so dramatic?
We've got two more groups to go.
We're going to talk about the gymnosperms.
Now the gymnosperms are the cone bearing plants.
So here is a pine tree
and hopefully everybody is familiar with these.
When you look at cone bearing plants
or conifers these are all sporophytes.
Okay? The gametophyte has gotten smaller still.
The gametes are actually hidden inside the cones.
Oh, my goodness.
I know. These life cycles can get a little bit overwhelming
with all the terminology,
but they're actually just variations on the same stories.
So we're going to start with again the phase
that you see in nature.
This is the mature sporophyte.
Right? This is a tree.
And the tree in the cones we have of course meiosis
and the products of meiosis of course are spores.
And these are haploid.
This actually is going to take place within the cone.
And at fertilization of course there are male
and female cones, by the way.
Female cones are more conspicuous.
Male cones which produce pollen, they basically --
After they release their pollen they just whither away
and you don't really see them.
Of course conifers rely on wind for dispersal.
Right? So it's just a mass of pollen in the air
from these guys when they are breeding.
During fertilization the diploid state is restored, of course.
Now we have our zygote and it develops within a seed.
And that seed will sprout and grow an entirely new sporophyte.
The most advanced of all plants and certainly the most beautiful
by most people's estimation are the angiosperms.
And these are the flowering plants.
This is a cherry blossom tree and in nature what dominates
in angiosperms is the sporophyte.
And the gametes are actually hidden inside the flowers.
So the flower is the sex organ of the flowering plant.
And I've just got to say right now it's really no coincidence
that flowers are a symbol of love and romance.
If you give a woman a corsage,
that's not a subtle message, right?
You're basically saying, "Wear this sex organ and think of me."
Okay. So again the diploid state is what we're going
to start with.
The plant itself is a sporophyte.
Right? So here's a really nice flowering plant.
And within the flower, within the flower,
you have meiosis going on to form the egg
and in this case the pollen, the pollen grains.
And the plant has to pay
for that special touch in a lot of cases.
Now some angiosperms are also wind pollinated,
but a great majority of them make big bright showy flowers
to attract pollinators.
The problem is of course that that's expensive.
Flowers have to pay for service
in the cold hard currency of nectar.
Right? And of course if you're pollinated by birds it's going
to cost you even more.
But for the plants it appears to be worth it.
As the pollen is delivered from flowers far away, it will --
The pollen grain will stick on to the stigma.
It actually fits very precisely.
This is to prevent pollination across different species.
So it's kind of like a lock and a key right here.
And then the pollen grain will grow a tube
and grow all the way down to the egg.
You have fertilization.
Diploid condition is restored.
And now you have the development of the embryo within a seed.
And for even better dispersal the ripened ovary becomes a
fruit and the plant will pay even more
to get its offspring far, far away from the parent plant.
And you make it nice and juicy and tasty
so that seed dispersal can occur.
And birds and mammals will eat the fruit, digest the seeds,
activate the seeds and then deposit them in a nice pile
of fertilizer far, far away from the parent plant.
Oh, my goodness.
Even dispersal is expensive for plants.
So the summary of the story is that as we look
through evolutionary development of plants as plants become more
and more complex that increasing complexity means you have
to have more genes and the more genes you have the more likely
it is that you're going to be safer as a diploid sporophyte
than as a gametophyte.
In other words, the gametophyte phase
because you only have one copy if something bad happens
and you have a lethal error in a gametophyte stage you're out.
Right? But if you're a diploid sporophyte you have a
backup copy.
So you want to spend more and more time
as a diploid sporophyte.
The more genes you have.
And this certainly holds true for animals as well.
Right? So we spend more of our life cycle, much more,
as a diploid organism and a very,
very small amount of time as haploids.
So that is the story of the hide the gametophyte game
that the plants are playing with us.
I really hope that was helpful.
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