The Plants & The Bees: Plant Reproduction – CrashCourse Biology #38

A couple of weeks ago, I talked
about a strategy for reproduction that the very first plants came
up with, called alternation of generations, a strategy that
nonvascular plants still use today. Hopefully this is
coming back to you. A plant can take two
different forms that alternate back and
forth between generations. The first form, the sporophyte, has diploid cells,
two sets of chromosomes. And the second form,
the gametophyte, has haploid cells,
just one set of chromosomes. Well, a lot can happen
in 470 million years. Today, vascular plants still use
the basic alternation of generations model, but they’ve tricked it
out so that it works a whole lot different than it did
back in the Ordovician swamps where planthood got its start. Compared with their small,
damp nonvascular brethren, vascular plants, with all their
cones and flowers and other flashy accessories, look like a
bunch of drag queens at a Carmen Miranda conference
and samba dance-off. Which might seem like overkill,
but we rely on these crazy kooks and their upstart
reproductive strategies for… well, pretty much for our
everything: the food we eat, the air we breathe, the bouquets
that we send to our wives and girlfriends when
they’re mad at us. Basically, what I’m saying,
is that we need vascular plants to have sex. So, as you recall, the
alternation of generations in nonvascular plants
is pretty straightforward. A gametophyte produces either
sperm or eggs which find each other if it’s wet enough for the sperm
to swim to the nearest egg. Once the egg is fertilized, the
gametophyte creates the sporophyte, which is a little capsule on a stalk
that has a bunch of spores in it. The spores are released into the
air, they land in a moist place, germinate, and BAM! A new
gametophyte generation is born. But nonvascular plants are what
you call gametophyte dominant: What you’re looking at when you
look at a moss or a hornwort or a liverwort is the gametophyte. It’s the form that has
only one set of chromosomes. For them, the sporophytes are
tiny and tucked away inside the gametophytes, which they rely
on for food, water, and protection. But in vascular plants,
it’s the opposite. They’re sporophyte dominant. When you look at a fern or a pine
tree or a morning glory, you’re looking at the sporophyte
generation, and the gametophytes are the teeny, tiny sex-making materials is has stashed
away in special parts. So, yes, all vascular
plants are sporophyte dominant, but that doesn’t mean they all
reproduce in the same way. No sir! The simplest form of
vascular plant are the ferns, which reproduce a lot
like nonvascular plants, in that they have spores that
grow on the underside of the frond, or fern leaf, which are released
into the wild blue yonder to find a nice soggy patch
of ground to germinate on. The spore then makes a tiny
gametophyte, which is only a few centimeters wide and has both
male and female reproductive organs on the underside of its leaves. If it’s moist enough, the sperm
on the boy side of the gametophyte will find the egg on the girl side,
and it will create a sporophyte, which is what we recognize as a fern. There’s a lot of fossil evidence
to suggest that at one point, there probably were ferns
that produced seeds, and that all the fancypants
vascular plants that do have seeds and flowers
evolved from them. But those seed-bearing
ferns are all extinct now, so we can just gaze longingly
at their fossils and wonder what their alternation of
generations looked like. But there are other groups of
plants that are more complex than ferns, and what they all have
in common is that they reproduce by creating pollen, which
contains the male gametophyte, and the female gametophytes, or ovules, which are
fertilized by the pollen. The complete, fertilized
cell grows into a seed, which ripens and can
produce a complete adult plant. So, reiterating: In your
more advanced vascular plants, that’s how the alternation
of generations works. The sporophyte generation
grows from a seed and produces tiny gametophytes,
either pollen or ovules. They then combine
to form another seed, which produces another sporophyte. This evolutionary change from
spores to seeds was a big deal, and it began with the gymnosperms. Their single-serving plant-making
packages cut out the middleman by allowing an adult plant to
grow immediately from a seed rather than having to
wait for a spore to go through that intermediate
gametophyte stage. It also means, in most cases,
that there doesn’t have to be water present in order to reproduce. Today gymnosperms include
conifers, gingkos, and tropical, palm-like plants called cycads,
and none of them produce flowers because they evolved before
flowers were invented. Instead, their reproductive
structures are cones. And you’ve seen a few
of these in your day. In fact, their name,
gymnosperm, means “naked seed,” and that comes from the fact
that their ovules develop exposed on the surface
of their cone scales. What we think of as cones
are the spiky, woody things that Boy Scouts are throwing
at each other at camp, right? But those things are actually
female cones, which house the ovules. The male cones are smaller and
kind of spongy and their job is to crank out pollen. All this pollen is
carried on the wind, and some of it might find
its way to a female cone, where it fertilizes the ovule,
located at the base of each of the scales of the female cone. As the fertilized embryo
matures inside the cone, it makes a seed containing
enough nutrients to sustain it for a while after it germinates. This seed has a tough, shiny casing
to protect it from the elements, and once it’s mature, the scales
of the female cone just peel back, and the seed falls to the
ground and makes a new tree! But some gymnosperms have evolved
the need for special conditions in order to reproduce.
Take the Lodgepole Pine. It’s a super tough tree that
evolved in a pretty dry climate where there’s lots of lightning
storms that regularly start fires that burn through a
forest every few years. Not only do Lodgepoles have no
problem withstanding a good low-intensity forest fire,
their female cones are serotinous, so they will only open
and drop their seeds when exposed to extreme heat. Now this sounds kind of crazy,
but really it’s super smart: because the Lodgepoles have evolved
to take advantage of forest fires. They know that the forest fire will
probably get rid of a lot of pesky underbrush that would crowd
out their babies, and maybe even it would kill some adult lodgepole
pines, so they just wait for the competition to be removed
before they expose their seeds. So, now I’m fixin’ to pull out
the big guns: the angiosperms. Because angiosperms are the
winners of the All-Invitational Plant Division of Things
That Live On Earth, at least for the past
140 million years or so. They’re rookies, really, but they
know what they’re doing. For starters, they have
seeds like gymnosperms, but they also have flowers. And flowers are awesome because
they don’t have to rely on the wind to carry their
pollen to another flower like gymnosperms do
with their cones. For the most part, flowers
put animals to work, toting their pollen from
one flower to another. In fact, angiosperms and flying
insects probably evolved together, or co-evolved, the flowers
providing food for the insects in the form of nectar, and the
insects providing transportation for the pollen to another flower’s
female reproductive parts. This, my friends,
is what we call mutualism, the interaction of two organisms
which mutually benefits both. Angiosperms reproduce by making
flowers that contain the gametophytes. In this case, the sporophyte
is made up of the stem, the roots, leaves
and even the flowers, all of the other parts of the plant
except the pollen and the ovum, which are the actual gametophytes. Some flowers contain both
male and female gametophytes. These are called perfect flowers.
No pressure, other flowers! Other flowers have both
male and female sex organs on the same plant,
but in different flowers. And some have male and female
flowers on entirely different plants. There are no rules with angiosperms.
They’re just wingin’ it. To see how flowers work, let’s
take a look at a perfect flower as our example, because a lot
of the garden flowers you see have both male and
female reproductive parts. Starting from the bottom up,
a flower has sepals, which look like leaves or petals,
but they’re usually green tissue that covered the flower
when it was a little bud. The petals are usually colored to
attract a certain kind of pollinator, like a flag. The male parts of flowers
consist of an anther, which produces the pollen and sits
on the end of a long filament attached to the base of the flower. This whole male reproductive set
up like this is called the stamen. Now, when it comes to ladyparts,
in contrast to gymnosperms, angiosperms don’t leave their
eggs hanging out all exposed. They lock their ovules down
in an ovary at the bottom of a vase-like structure, which
also has a neck called a style, and an opening at the
top called the stigma. Now all that’s left is
to get the male gametes, packaged up in their gametophyte,
the pollen, and have them carried to the female gametophyte,
the ovule, to fertilize them. This is pollination, and flowers do
it by luring animals with smells, colors, and food, and in return,
the animals mix and match the pollen with
different individual flowers. Bees are the most
famously successful at this, but lots of other
insects do it too, as well as birds like hummingbirds,
and even some bats. So, no matter who does it,
after fertilization happens, the ovule starts to swell, and the
ovule wall starts to toughen up because it’s going to become a seed. The ovary, meanwhile, starts to
grow around it, and become the fruit. Now, there are a bunch
of different types of fruit. A fruit is defined as
anything that the ovary, the protection around
the seed, turns into. So anything that contains
a seed is a fruit. And that’s a lot of different
things, including many, many things that we think
of as…not a fruit. To test your fruit skills,
how about a round of: Fruit and Not a Fruit! So, which one of these is the
Fruit and which one of them is the Not a Fruit? 1. A sandspur you get while walking
around at the beach or a carrot? Answer: A sandspur! The little
annoying thing that attaches to your pants is actually the
swollen up ovary of the flower. A carrot is a root of a plant. A stalk of celery or a
piece of dandelion fluff? The fluff! That little piece of
fluff is attached to a dry little fruit that contains the seed! Celery is the actual
stalk of the celery plant. A strawberry or a zucchini? The zucchini! A strawberry is
actually the swollen end of the stem of the strawberry flower,
so it doesn’t contain the seed. It actually has the
seeds on the outside. Each one of the hard little things
on the outside of the strawberry? Those are the fruit.
Some people argue about this, because what seems more
fruity than a strawberry? But zucchinis? They’re definitely
fruits because they contain seeds. Fruits are important to angiosperms,
because they like to get their seeds as far away from
themselves as possible so that they’re not competing
with their own offspring. So some fruits can be
carried away by the wind, while others move around
by being totally delicious, so they can be eaten by
an elephant and pooped out in an elephant turd far, far away. So that’s the steamy sex
lives of vascular plants. Mmm. Ah, that is good. Thank you for watching this episode
of Crash Course Biology. This really is, like,
a perfect nectarine! Thanks to everyone who helped
put this episode together, including this nectarine. And if you want to go check
out any of the angiospermy mess that is the sex lives of plants: there’s a table of
contents over there. If you have questions for
us, we’re on Facebook, we’re on Twitter, and we’re
always in the comments below. And we’ll see you next time.

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