When you picture a plant, what comes to mind?
Perhaps an exquisite orchid blossom or a majestic willow tree?
Or maybe a trendy houseplant like a Monstera or a succulent?
Well, it is time to update, and broaden, that picture!
Because plants aren’t only flashy and flowering.
Today, we’re giving some attention to the unsung heroes of the botanical world
— those essential plants that were the first to join the party,
but that tend to lie low.
They help ecosystems thrive, offer us knowledge about plant evolution,
and make great tree accessories.
We’re talking bryophytes and seedless vascular plants,
the categories that include mosses and ferns, liverworts, and more.
So let’s get low— to the ground, that is.
This is gonna be mossome.
Hi! I’m Alexis, and this is Crash Course Botany.
[THEME MUSIC]
Remember that tiny alga whose tasty snack started the development
of all the plant life on Earth hundreds of millions of years ago?
Well, the bryophytes and seedless vascular plants
were some of the first plants to arise from that event
—and many of them still exist today!
In fact, some of the characteristics that are essential to plants as we know them
first developed in these early plant types.
So, bryophytes and seedless vasculars
can tell us a lot about how plants evolved over time,
like how an older relative can tell you what life was like
before cell phones or the Internet, or microwave popcorn.
We can get a feel for how these plant types came to be
by referring to the plant kingdom’s phylogenetic tree,
a diagram that shows us how organisms evolved over time.
The branches on the tree represent the major plant groups.
Let’s start with the bryophytes, which occupy the lowest branch.
Time moves vertically here,
which means that they evolved first, around 500 million years ago.
This might make it tempting to say that bryophytes are “less evolved”
than the flowering plants, the highest branch.
But we do not use that kind of language on Crash Course;
it is rude.
And also, not scientifically accurate.
The ancestors of today’s bryophytes were here before the flowers,
but the bryophytes and flowers alive right now
have been evolving for the same amount of time
since they went their separate evolutionary ways
at that bottom branch point.
Plants are not all on a slow and steady march toward evolving flowers —
bryophytes are on their own path,
and they are doing just fine, thank you very much. Hmmf.
And that path is represented by a single branch coming off of the tree,
meaning that bryophytes are monophyletic — they share a single common ancestor
and are all more closely related to each other than to any other plant group.
So what are bryophytes anyway?
Well, most notably, they don’t have
the typical organs that most plants have today,
like stems, leaves, and roots.
And they don’t contain vascular tissue,
the water and transport system found in all other plants,
which developed later with seedless vasculars.
You see, bryophytes were
the first plants to start growing on land instead of in water.
Before that, there was pretty much no multicellular life on land at all
—animals all lived in water because, well, that’s where the food was.
We have the development of land plants to thank for all the land animals that exist,
including humans.
But in order for bryophytes to survive on land,
they needed to evolve ways to stay hydrated in the dry air.
So they developed root-like structures called rhizoids,
and leaf-like structures called phyllids, which bryophytes still have today.
Unlike true roots, rhizoids’ primary purpose is to anchor bryophytes to surfaces —
they don’t absorb much water.
So moist surfaces are prime real estate for bryophytes —
you’ll find them clinging to the ground, rocks, and larger plants
to absorb water and nutrients directly through the surfaces of their phyllids.
The most common types of bryophytes are mosses,
with about twelve thousand species.
And one of the most important things that mosses contributed
to the development of land plants is the stomata—
those mouth-like pores that allow gases to move in and out of the plant.
These helped plants survive on land
now that they were surrounded by gaseous air and not liquid water.
And today, mosses are super environmentally important.
Like, peat, which is mostly made of sphagnum moss,
stores 500 billion tons of carbon,
which is essential for minimizing the amount of carbon dioxide
that would otherwise be in our atmosphere,
driving global climate change.
Let’s be real, I could spend all day on mosses —
but they are just the first stop on our bryophyte journey.
The next branch on the phylogenetic tree is liverworts.
Despite their unfortunate name, liverworts do tons of cool, useful stuff,
like helping rocks break down into soil,
decomposing logs to return nutrients to the ground,
and serving as materials for bird nests.
And then there are hornworts, the rarest bryophyte —
with only a couple hundred species that typically live in the tropics.
They’re named for — you guessed it—
little horn-like protrusions.
And some can form symbiotic, or mutually beneficial,
relationships with bacteria.
The hornworts provide housing, and the bacteria provide nitrogen,
which is an important part of chlorophyll, the pigment involved in photosynthesis,
among other things.
So while mosses, liverworts, and hornworts may be low to the ground,
they are mighty.
They can survive extreme Arctic winters, and they’re often the first plants to re-grow
in ecosystems that have been disturbed by fire or floods.
You know those people who are the first to show up and help after a disaster?
They’re like the bryophytes.
And while bryophytes are diverse, they do all have some things in common.
Like, they all create spores, single cells coated in a
super tough, armor-like molecule that are an integral part of the bryophyte life cycle.
In fact, when we learned about their life cycle,
it unlocked some super important information about how all plants grow.
And that knowledge is thanks to the curiosity of an amateur botanist
named Wilhelm Hofmeister.
He was born in 1824 in Germany, where he developed an affinity
for plants in his father’s garden.
And despite having no formal training in botany,
Hofmeister’s observations through his trusty microscope led him to discover
a whole bryophyte life cycle.
Let’s go to the Thought Bubble…
Check it out: This moss is about to reproduce.
So it builds two chambers on the surface of its phyllids
— or leaf-like thingies.
One contains an egg, and the other contains sperm,
each with half the DNA needed to make a plant.
Now, the sperm swims from its chamber to the other,
which it can do in a wet environment.
And to be clear, the sperm can belong to this moss,
or it can swim over from another moss.
Once the sperm arrives, it fuses with the egg to make a zygote
— a cell that contains all the DNA needed to make a baby,
thanks to the combination of the two DNA halves.
The zygote then grows into a long, thin stalk with a capsule on top—
not very moss-like at all.
And this homebody stays attached to its parent moss its whole life.
But when the baby grows up,
the cells inside its capsule create spores, which are released to the wind.
When a spore lands somewhere nice and moist,
it germinates and starts to grow, eventually building moss.
Hofmeister called this multigenerational life cycle
the alternation of generations.
The cycle alternates between two different types of generations:
one with only half its DNA — called the gametophyte generation
because it makes gametes, or eggs and sperm.
And one with a full set of DNA, called the sporophyte generation
because it makes spores.
So if a parent plant made spores, its offspring would make gametes,
then its offspring would make spores, and so on.
Thanks, Thought Bubble!
While Hofmeister initially studied this phenomenon in so-called
“lower plants” like mosses,
he later discovered that all plants on Earth do some version of this cycle.
Plants are way more complex than we knew
—and they have complicated family lives.
We know from fossilized spores that bryophytes have existed for
around 500 million years.
In other words, they were around way before dinosaurs.
But there were still many more plant attributes
that had yet to evolve, one of them being vascular tissue.
So, a few hundred million years later, a cousin of the bryophytes showed up,
forming another branch on the plant family tree:
seedless vascular plants.
They reproduce via spores, like the bryophytes,
rather than seeds, which developed even later.
But, they were the first plants to form true roots, stems, and leaves
that contain vascular tissue,
so they’re considered the first vascular plants.
And this evolutionary change was a big deal for the plant kingdom.
With the ability to transport water and nutrients more smoothly,
plants could grow way bigger and could better survive
the dry conditions on land.
On top of that, seedless vascular plants —and all plants that evolved after them—
have a dominant sporophyte,
meaning that what we tend to recognize
as the plant is actually its sporophyte generation.
Having a dominant sporophyte introduced lots of advantages
— like, while the spore-making generation of a bryophyte
couldn’t grow very far off the ground,
the sporophytes of seedless vascular plants could grow much taller and larger,
with more room for spore-producing organs.
So more spores could be spread across larger areas,
enhancing plants’ options for flourishing on land.
Although many of the early seedless vascular plants have gone extinct,
you can still find some of them growing today.
Like, lycophytes were huge three hundred million years ago—literally.
They were tree-sized.
Their fossils tell us about the first forests on the planet and the earliest root systems.
Today, some have names like “clubmoss” and “spikemoss,”
but don’t be fooled by the misnomers
—they’re not true mosses because they have roots, stems, and leaves.
Another type of seedless vascular plant is ferns, also known as monilophytes.
Many types of ferns have large leaves called fronds
that slowly unfurl over time, but there are other kinds, too.
Some fern leaves are tiny and float on top of water,
and others have hairs shaped like egg-beaters.
And some ferns hardly make leaves at all,
like whisk ferns and horsetails.
Plants like Boston ferns are great at purifying the air.
And ferns of all kinds help scientists study climate change
because they’re extra sensitive to temperature and precipitation changes.
But we’re only just beginning to understand ferns’ interactions with animals,
and their role in cycling nutrients through the environment.
As you can tell, bryophytes and seedless vascular plants
are not simply low-to-the-ground lurkers.
Many are helpers in their environments,
restoring and maintaining their habitats and providing homes for other organisms.
And their long evolutionary history reveals the secrets of our environment
hundreds of millions of years ago.
So, when you come across a sporing plant, stop to say “I appreciate you!”
And most importantly,
tell them you understand that it’s not about the flowers,
but the fronds you make along the way.
Next time, we’ll be focusing on the other branches of plants’ phylogenetic tree
—gymnosperms and angiosperms, AKA the plants that took over the world.
Hey, before we go, let’s branch out!
What bryophyte can be used to dress wounds?
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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you can join our community on Patreon.