Vascular Plants = Winning! – Crash Course Biology #37


This is yarrow, a flowering plant found all over
the Northern Hemisphere. Its feathery leaves have
natural astringent properties, and its scientific name, Achillea,
comes from Achilles, the Greek hero, who is said to have used
it on the wounds of his soldiers. And this is snakegrass, also known
as horsetail or, to the kids, popgrass, because you can
just pop it apart, and then put it back together again. Although
on top there, it’s dead now. And this is a ponderosa pine,
one of my favorite trees. They can grow hundreds of feet tall, and on a warm day if you sniff
it, it smells like butterscotch. They all have different
shapes, sizes, and properties, but each of these things is a
vascular plant, one of the most diverse and, dare I say, important
families in the tree of life. Since their predecessors first
arrived on the scene some 420 million years ago, vascular
plants have found tremendous success through their ability to
exploit resources all around them. They convert sunshine into food.
They absorb nutrients directly through the soil without the
costly process of digestion. And they even enlist the help
of some friends when it comes to reproduction, so often when
they’re doing their thing it involves a third party.
Which, y’know, good for them. But these things alone can’t explain vascular plants’
extraordinary evolutionary success. I mean, algae was photosynthesizing long before plants
made it fashionable. And as we learned last week,
nonvascular plants have reproductive strategies that are tricked
out six ways from Sunday. So, like, what gives? The secret to vascular plants’
success is in their defining trait: conductive tissues that
can take food and water from one part of a plant
to another part of a plant. This may sound simple enough,
but the ability to move stuff from one part of an
organism to another was a huge evolutionary breakthrough
for vascular plants. It allowed them to
grow exponentially larger, store food for lean times,
and develop some fancy features that allowed them to
spread farther and faster. It was one of the biggest revolutions
in the history of life on Earth. The result? Plants
dominated Earth long before animals even showed up. And even today, they hold
most of the world records: The largest organism in
the world is a redwood in Northern California,
115 meters tall. Bigger than 3 blue whales
laid end to end. The most massive organism is a
grove of quaking aspen in Utah, all connected by the roots, weighing
a total of 13 million pounds. And the oldest living thing? A patch of seagrass
in the Mediterranean dating back 200,000 years. We’ve spent a lot of
time congratulating ourselves on how awesomely magnificent and
complex the human animal is, but you guys, I gotta
hand it to you. So you know by now, the more
specialized tissues an organism has, the more complex they are
and the better they typically do. But you also know that these
changes don’t take place overnight. The tissues that define vascular
plants didn’t evolve all at once, but today we recognize three types that make these
plants what they are. Dermal tissues make up their outermost layers and help
prevent damage and water loss. Vascular tissues do all of that conducting of materials
I just mentioned. And the most abundant
tissue type, ground tissues, carry out some of the most
important functions of plant life, including photosynthesis
and the storage of leftover food. Now, some plants never
go beyond these basics. They sprout from a germinated seed, develop these tissues,
and then stop. This is called primary growth,
and plants that are limited to this stage are herbaceous. As the name says,
they are “like herbs” small, soft and flexible, and
typically they die down to the root, or die completely,
after one growing season. Pretty much everything you see
growing in a backyard garden: herbs, flowers, broccoli
and that kind of stuff, those are herbaceous. But a lot of vascular plants
go on to secondary growth, which allows them to grow
not just taller but wider. This is made possible by the
development of additional tissues, particularly woody tissues. These are your woody
plants, which include shrubs, bark-covered vines called lianas,
and of course, your trees. But no matter how big they
may or may not grow, all vascular plants are organized
into three main organs, all of which you are intimately
familiar with, not just because you knew what they were when
you were in second grade, but also because you probably
eat them every day. First, the root. It absorbs
water and nutrients, and serves as a pantry of
leftover food, and of course, keeps the plant anchored
in the ground. Next, the stem. It contains
structures that transport fluids, stores nutrients, and also is
home to specialized cells called meristems that are
responsible for creating new growth. But their most important task
is to support the last organ: The leaf. This, of course,
is where the plant exchanges gases with the atmosphere and collects
sunlight to manufacture food, with the help of water and
minerals collected through the root and sent up through the stem. Now, each of these organs
contains all three tissues, which together work to
absorb, conduct, and exploit one of the world’s most
important molecules: water. So, since plants are pretty
much designed around water, let’s follow some H2O to see
how plants make the most of it. First, as with most organisms,
nothing can get in or out of a plant without getting past the skin,
in this case the dermal tissue. In smaller, non-woody plants,
most of this is just a thin layer of cells called,
fittingly, the epidermis. Naturally, this is great for
keeping the outside out and the inside in, but the
epidermis can also sport some snazzy features in
different parts of the plant. In leaves and stems, for example,
it often has a waxy outer layer called a cuticle that
helps prevent water loss. On some leaves, or on pods
that hold those valuable seeds, the epidermis can sprout hairlike
structures called trichomes that help keep insects at bay and
secrete toxic or sticky fluids. The same secretions that make
the yarrow useful for first aid, for instance, are
also what discourage ants from using it for lunch. Finally, in the roots, the epidermis
has similar features called root hairs that maximize the root’s
surface area for absorption, just like we’ve seen in
our own organ systems. This, of course, is where the plants
generally absorb the water they need. By the way, the cells that
make up this dermal tissue are the most basic, essential
building blocks of vascular plants, called parenchyma, or
“visceral flesh,” cells. These are the most abundant plant
cells, found not just in roots but also in stems,
leaves, and flowers. They’re thin and flexible
and can perform all kinds of functions depending
on their location. Now, after passing through the
skin of the root and through its starchy cortex, or outer layer,
water arrives in the first of two kinds of vascular
tissue: the xylem. The xylem’s main function is
to carry water and dissolved minerals from the root
up to the leaves. But, like, how?
How, by Zeus’ beard, can plants make water defy gravity? Well, a lot of the reason is that,
up top, the plant is continuously evaporating water through a
process called evapotranspiration. As water evaporates from the
leaves, which I’ll explain in greater detail when we get
up there, it creates negative pressure inside the xylem,
which draws more water upward. Plants can transpire truly
staggering amounts of water, and it’s because of this that
our atmosphere is habitable. A single acre of corn gives
off about 3,000 gallons of water every day.
A large oak tree, just one tree, can transpire 40,000
gallons in a year. Only 1% of the water that plants
absorb is actually used by plants, mostly in photosynthesis. The rest is slowly, and invisibly
released, providing one of Earth’s most crucial functions,
transporting water from the soil into the atmosphere,
where it then returns to the surface as rain, making
all life possible. Yeah. Chew on that as we
continue up the xylem. And as we get higher in the
plant, we begin to encounter a greater diversity of cells,
designed not only for moving stuff around but also for
providing structural support. For instance, elongated cells
with thicker cell walls, called collenchyma, help hold
up the plant body, especially in herbaceous plants and young
structures like new shoots. Celery is mostly made
up of these cells, so you already know
what they taste like. In larger, woody plants,
you also find sclerenchyma cells, especially in the xylem. These have even thicker cell
walls made from lignin, a super-strong polymer
that makes wood woody. What’s weird about
sclerenchyma cells, though, is that most of them when they
reach maturity, they die. They just leave behind their
hearty cell walls as a support structure, and new cells form
a fresh layer during the next growing season, pushing the
old, dead layer outward. In warm, wet years these layers
grow thick, while in cold, dry years they’re light and thin. These woody remains form tree
rings, which scientists can use not only to track the age
of a tree but also the history of the climate
that it lived in. Now, at the top of the
xylem, water arrives at its final destination: the leaf. Here, water travels through an
increasingly minuscule network of vein-like structures
until it’s dumped into a new kind of tissue
called the mesophyll. As you can tell from its
name, meso meaning “middle” and phyll meaning “leaf,”
this layer sits between the top and bottom epidermis
of the leaf, forming the bacon in the BLT that
is the leaf structure. This, my friends, marks our
entry into the ground tissue. I’m sure you’re as excited
about that as I am. Despite its name, ground tissue
isn’t just in the ground, and it’s actually just defined
as any tissue that’s either not dermal or vascular. Regardless of this low billing,
though, this is where the money is. And by money I mean food. The mesophyll is chock full o’
parenchyma cells of various shapes and sizes, and many of them are
arranged loosely to let CO2 and other materials flow between them. These cells contain the
photosynthetic organelles, chloroplasts, which as you know
host the process of photosynthesis. But, where is this CO2 coming from? Well, some of the neatest
features on the leaf are these tiny openings in
the epidermis called stomata. Around each stoma are two
guard cells connected at both ends that regulate its size and shape. When conditions are dry
and the guard cells are limp, they stick together,
closing the stoma. But when the leaf is flush with
water, the guard cells plump up and bow out from each other,
opening the stoma to allow water to evaporate and let
carbon dioxide in. This is what allows
evapotranspiration to take place, as well as photosynthesis. And you remember photosynthesis:
Through a series of brain-wrackingly complicated reactions sparked
by the energy from the sun, the CO2 combines with hydrogen
from the water to create glucose. The leftover oxygen is
released through the stomata, and the glucose is
ready for shipping. Now, if you’ve been paying
attention, you noticed that earlier I said that there are two
kinds of vascular tissue, and here the circle is
made complete as the sugar exits the leaf through the phloem. The phloem is mostly made
of cells stacked in tubes with perforated plates
at either end. After the glucose is
loaded into these cells, called sieve cells or
sieve-tube elements, they then absorb water from
the nearby xylem to form a rich, sugary sap to transport the sugar. This sweet sap, by the
way, is what gives the ponderosa its delicious smell. By way of internal
pressure and diffusion, the sap travels wherever it’s
needed, to parts of the plant experiencing growth during the
growing season, or down to the root if it’s dormant, like during winter,
where it’s stored until spring. So now that you understand
everything that it takes for vascular plants to succeed, I hope you
see why plants=winning. And I’m not just talking about
them sweeping the contests for biggest, heaviest,
oldest living things. Though, again,
congrats on that, guys. Plants are not only responsible
for, like, making rain happen, they’re also the first and most
important link in our food chain. That’s why the world’s
most plant-rich habitats, like rain forests and grasslands,
are so crucial to our survival. When those habitats
change, everything changes: weather, food supply, even the
incidence of natural disasters. So I, for one, welcome our plant
overlords, because they’ve done a great job so far,
making life on Earth possible. But, I know you’re curious,
how do different kinds of plants make more plants? That’s all
about the birds and the bees, which is what we’ll be
talking about next week. Thank you for watching this
episode of Crash Course Biology. And of course, thank you
to everyone who helped put this episode together. If you want to review anything,
there’s a table of contents over there, just click, and you
can go see the part of the episode that you want to
reinforce inside of your brain head. And if you have any questions,
we’ll be on Facebook or Twitter, or of course, down
in the comments below. And we’ll see you next time.

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