There’s this plant called jewelweed, also known as a “touch-me-not.”
Why, you ask?
Because when you touch its seed pods, they have a habit of exploding.
The seed pods fling the jewelweed seeds into the air to spread them far and wide.
And then there’s the seeds of the Osyris plant,
which develop faster if they’ve been roughed up a little
inside a bird’s digestive system.
After that, they tend to try to grow wherever the bird, uh, drops them.
Foxtail seeds hitch a ride on animals’ fur.
Maple tree fruits surf on the wind.
Oak tree fruits — or acorns — sprout where squirrels bury them.
Are you sensing a pattern here?
The pattern is: plants move.
They migrate and spread over the globe, just not in the same ways that animals do.
Though sometimes, it is via those animals.
Throughout land plants’ 500-million-year-long tenure on Earth,
they’ve devised a lot of clever ways to get around.
And when seeds, flowers, and fruit came onto the scene?
Well, that was a major plant moment.
Hi! I'm Alexis. And this is Crash Course Botany.
[THEME MUSIC]
In the last episode, we explored bryophytes and seedless vascular plants,
some of the earliest plants to thrive on land.
And today, we’re looking at the evolution of the seed,
which came about 150 million years later and changed everything.
When gymnosperms, or seed-bearing plants, appeared,
they opened up entirely new habitats and ecological opportunities for plants.
And then, angiosperms, which bear flowers and fruits,
came up with even more innovations for protecting and spreading those seeds.
Because of their evolutionary advantages,
these two types of plants dominate Earth’s ecosystems,
shaping how virtually every other organism lives and interacts.
And they are by far the most common types of plants
you’re likely to interact with in your daily life.
Before gymnosperms, plants could live on land,
but they needed a pretty wet environment — because in their reproductive cycle,
a plant’s sperm could only reach an egg by swimming.
But the evolution of pollen
— which coincided with the evolution of seeds —
meant seed plants were no longer dependent on water for reproduction.
That’s because pollen,
which produces not only allergies but also plant sperm,
could travel by wind rather than water.
So plants were able to expand into habitats previously inaccessible to them.
And all of this happened very gradually,
as new types of plants built upon and tweaked what came before.
Kind of like how newer phones have more powerful cameras,
or let you turn your face into a unicorn,
but they still have the foundational features of older phones.
You still need the part that lets you make phone calls.
You see, previously existing plants like ferns had produced spores,
or single cells crucial for the plant’s reproduction.
These tiny cells grow in special structures on the leaves called sporangia.
And progymnosperms, which evolved about 390 million years ago,
preceding gymnosperms — still used spores to reproduce.
But we can see from fossils that progymnosperms
started making two different types of sporangia.
When the progymnosperms evolved into the gymnosperms
—about 70 million years later— they specialized those two sporangia.
One produced little spores like their plant elders’ had.
But the other produced one big spore,
protected by a layer of tissue called the seed coat.
This structure as a whole
—sporangium-plus-spore-plus-seed coat—
is called an ovule.
And when it grows up and matures, it becomes a seed.
But let’s unpack that.
There’s the sporangium that makes little spores — these become pollen, which produces sperm.
And then there’s the sporangium that makes a big spore.
This spore transforms into the next generation of the plant
and then produces an egg.
When pollen lands on an ovule,
it sends its sperm down a little tube to fertilize the egg.
Inside the ovule, the fertilized egg grows into an embryo
which will eventually become a baby plant.
And once it contains an embryo like this, the ovule graduates into a seed.
The evolution of seeds created a huge advantage because
now embryos were protected and nurtured in their cozy seed coats.
This let them travel safely and well-fed as they were dispersed
— whether by bird or wind or explosion.
And that let these plants spread farther with more reproductive success
than others had up to this point.
Now, there are a few different types of these seed-bearing gymnosperms,
many of which are still around today.
Cycads are one of the oldest gymnosperm groups—
they were around even before the dinosaurs.
But they grow very slowly and only in specific habitats, so
most species are considered endangered today.
Then there are the gymnosperms you’re likely to recognize.
Conifers include trees like pines, cedars, firs, and cypresses
that make scale- or needle-like leaves.
They’re often adapted to harsh environments like freezing mountaintops or barren deserts.
And there’s a gymnosperm category you might know by scent: the ginkgo.
These trees have been known to produce stinky seeds,
but they’re widely grown in cities around the world
because of their resistance to pollution.
The last of the gymnosperm groups to evolve were the gnetophytes,
which includes species such as Ephedra,
the plant that produces the decongestant ephedrine,
and Gnetum, an important part of Indonesian cuisine.
So, now let’s visit the next branch on the plant family tree: the angiosperms.
They appeared about 85 million years after the gymnosperms,
or around 275 million years ago.
Angiosperm means “vessel seed,”
because their seeds are enclosed in a special container, or vessel.
The container is a plant ovary, or reproductive organ,
and when it ripens, it becomes a fruit.
And the ovary is part of a broader reproductive structure
completely unique to angiosperms: the flower.
So, to repeat: flowers are reproductive structures,
and fruit are ripe plant ovaries.
Beautiful and tasty.
Flowers and fruits helped the angiosperms reach a level of evolutionary success
and ecological dominance that hasn’t been repeated in the history of plants.
There are around 300 thousand species of angiosperms today,
compared to just a thousand gymnosperm species.
That’s a ratio of three hundred to one.
And unless you’re in the Arctic Circle,
nearly every plant in your local environment is an angiosperm
— oaks, cacti, grasses, orchids.
Almost every plant you eat is an angiosperm, too,
like kale, rice, apples, carrots, and soybeans.
But the amazing array of flowering and fruiting plants we have today
wouldn’t have been possible without those early gymnosperm seeds.
Until recently, we didn’t really know when the angiosperms came about.
But that all changed thanks to a prehistoric forest fire,
a modern-day microscope, and some brilliant botanists.
Let’s head to the Thought Bubble…
Until the 1980s, scientists hadn’t found a ton of flower fossils
that could tell them about early angiosperms.
And the ones they had found weren’t great.
Since flowers are so soft, they tend to go “splat” when they become fossils,
so we lose a lot of valuable information about their structure.
Enter Dr. Else Marie Friis, a Danish paleobotanist.
Friis and a colleague were exploring a quarry in Sweden
when they discovered that little bits of charcoal in the rock
contained countless tiny, flower fossils.
They determined that a forest fire
swept through the site around 85 million years before,
turning the flowers to charcoal.
This was great news for the botanists
because the charcoal hardened the soft features of the flowers,
preserving them in exquisite, three-dimensional detail.
So, they developed a method for carefully extracting and cleaning
the two-millimeter flower fossils,
and used microscopes to visualize all the tiny details of their structures.
What they saw was astounding —
diverse and complex flowers that were 85 million years old,
yet recognizable as belonging to the modern-day group
that includes witch hazels and peonies.
Their findings revealed this group of flowering plants
was thriving way earlier than previously thought.
And Dr. Friis continues her work on charcoal flowers,
discovering older and older fossils that can help us understand
more about the meteoric rise of the angiosperms.
Thanks, Thought Bubble!
So flowers—whether from millions of years ago or today—
all have structures that enable them to flourish on land.
A typical flower consists of four types of organs
that are arranged in concentric circles:
In the outermost circle are the sepals,
which typically resemble leaves and protect the flower bud when it’s closed.
Next come the petals — beautiful, showy organs that attract animal pollinators.
After the petals come the stamens, the reproductive organs that produce pollen.
And the innermost circle contains the pistil, with the ovary and ovules at its base.
This combination of organs allowed the angiosperms
to evolve specialized flower shapes to attract and reward pollinators.
Having an army of insects, bats, and birds accurately shuttling pollen between your flowers
is a way more efficient reproduction strategy than hoping your pollen
lands in the right place when it blows by on the wind.
It’s like putting the right address on an envelope, not just the zip code.
So angiosperms evolved the staggering diversity of flower shapes and sizes
we see today to maximize this pollination potential.
And similarly, fruits evolved to have scents that attract
particular animals to eat them.
Those animals then move to another location where, hopefully,
the new plant won’t be in competition with the old one,
and they poop out the seeds inside that fruit.
It’s not pretty, but it’s effective.
Over about 50 million years, the rise of angiosperms
triggered what scientists consider
an explosive increase​​ in the number of different land-dwelling species,
both plant and animal.
This is largely because interactions between angiosperms, pollinators,
and herbivores drove a process called coevolution,
where each group influenced the other’s development and diversification.
But it wasn’t only their appealing flowers and fruits
that made angiosperms so successful.
They also developed ways to harness photosynthesis
more efficiently and outgrow other plants.
Their leaves gradually evolved to have more veins
that transport water throughout the plant.
So when their stomata, or pores on their leaves,
opened to absorb carbon dioxide in the air,
they didn’t lose as much water — and could keep growing steadily.
On top of that, angiosperms refined a process of reproduction
called double fertilization,
which essentially allows one sperm cell to fertilize an egg and make an embryo
— while a second sperm cell creates tissue called endosperm
that can provide nutrients to that embryo.
This made the seeds more likely to grow into successful plants.
And corn, wheat, and rice, by the way,
are so nutritious to us because they’re basically all endosperm.
At the end of the day,
while angiosperms may be the most abundant plant type on Earth,
plants like bryophytes, seedless vascular plants, and gymnosperms still exist, and thrive.
That’s because one plant type isn’t better or more evolved than another
— they’re all different and have their own ways of surviving in different environments.
Though yes, seeds changed everything and flowers and fruits make my life better,
there’s room for all of us on this big green planet.
And that completes our tour through the major groups of plants.
Next, we’ll explore the many ways humans and other organisms interact with their botanical brethren.
Hey, before we go, let’s branch out!
What’s the main trigger for Giant sequoia trees to release their seeds?
WOO it’s getting hot in here— find the answer in the comments!
Thanks for watching this episode of Crash Course Botany
which was filmed at the Damir Ferizović Studio
and made in partnership with PBS Digital Studios and Nature.
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