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,