In 1924, Dr. Katherine Esau was working on a sugar beet farm in California, where some of the beet plants were suffering from a strange virus called curly-top disease.

Spread by insects called leafhoppers, it causes plants to have unusually curly leaves and stunts their growth.

By paying careful attention to the anatomy, or the internal structure of these plants, Esau discovered how the disease spread throughout the plant.

She wrote in her autobiography: “I began to realize that the virus must enter and must move in the plant along a pathway.

I figured out that if the leaf-hopper passes the virus by feeding, then the virus must be moving through the same system as the food moves.” In other words, the very same group of cells  that move the sugar produced by photosynthesis from the leaves to the rest of the plant was being hijacked to transport the virus all throughout it.

This discovery was huge for the field of  botany because it helped illustrate just how essential the knowledge of healthy plant anatomy is for understanding unhealthy plants —and helping them get better.

So, stick out your tongue and say “ahhh,” plants.

Let’s explore what’s going on in there.

Hi!

I’m Alexis, and this is Crash Course Botany.

[THEME MUSIC] Just like human bodies, plant  bodies are made up of tissues, which are organized groups of cells that have similar structures and functions.

They do everything from transporting nutrients throughout a plant’s body to making fruit taste delicious.

And just like in our bodies, different tissues work together to make organs function.

But in plant bodies, those organs aren’t hearts and lungs; they’re stems, leaves, and roots.

So, let’s zoom in on the stars of today’s show.

Please welcome: dermal tissue, the plant’s skin; vascular tissue, the plant’s veins; and ground tissue — everything in between.

Let’s start with the skin.

The dermal tissue covers the entire outside of the plant, forming the epidermis, which is coated in a layer of wax called a cuticle.

This outer layer prevents water from escaping, and nasty things, like viruses, from getting inside.

In other words, the cuticle runs a constant skincare routine for the plant, keeping it moisturized and free of gross stuff.

No wonder plants don’t get zits.

But they do have pores.

Tiny openings in the epidermis of leaves, called stomata, let in carbon dioxide gas, a key part of photosynthesis.

And personally, I love that stomata  look like creepy little mouths.

I mean, they may look like aliens, but this image could easily be from a tree in your backyard.

Plants also get little hairs sprouting out of their skin, called trichomes.

And like hairdos, there are trichomes for every occasion.

Some protect against plant-eating animals by secreting itchy toxins from  the leaf or stem epidermis, which, if you’ve ever run into a stinging nettle, you are already painfully familiar with.

Others act like sunscreen and protect the plant from solar radiation, or form a blanket to keep the plant warm during the freezing winter.

Meanwhile, the root epidermis produces billions of trichomes to suck up water in the soil.

One study estimated that if you strung the root hairs of a single rye plant end-to-end, it’d be eleven thousand kilometers long.

That’s a ponytail stretching a quarter of the way around the earth.

Ariana, who?

While we can observe a plant’s skin on the outside, we’ll have to peer inside to explore its other tissues.

Next up: Vascular tissue, which forms the  circulatory system, or the veins, of the plant.

Its cells are specialized for transportation, like how a subway system moves people where they need to go.

And there are two main types of cells that make up that vascular tissue: xylem and phloem.

Xylem cells transport water and nutrients from the roots to the leaves.

And get this: the cells do this by dying on purpose.

They digest their own insides to become hollow, and their strong cell walls make perfect water-transporting pipes.

As the plant grows, stem cell clusters called meristems generate new xylem cells, which form bigger pipes to suit the plant’s needs.

After water flows from the roots  to the leaves through the xylem, it can exit the plant through the tiny stomata mouths.

This allows the plant to cool down.

In other words, it lets it sweat,  just like we do.

But unlike us, they never have to worry about pit stains.

Phloem cells, on the other hand, transport stuff mostly in the opposite direction as xylem cells.

They move the sugar produced by photosynthesis from the leaves to the rest of the plant.

And while they don’t die on purpose like xylem does, they do clear out a lot of their cell contents to pass that sugar around —so much, in fact, that they need a buddy, called a companion cell, to help them with the tasks their organelles —or functional cell units —would normally do.

[Sings] We all need somebody to lean on.

Xylem and phloem are bundled together to form the veins of a plant —the subway map that connects the leaves, stem, and roots.

You can actually see and feel these beautiful vein patterns when you pick up a leaf.

OK, so the third and final tissue type is ground tissue, which is everything that isn’t dermal or vascular — basically, the meaty bits of the plant.

Well, not meaty exactly.

But ground tissue does tend to be juicy and delicious.

The three types of cells that make up ground tissue all have similar —but pretty funky-sounding—names.

Parenchyma cells are the most common ground tissue cell type in a plant.

When you bite into an apple, the juicy part beneath the skin is  made up of these kinds of cells.

They’re the main photosynthesizers in leaves, and they store starch, or extra sugar reserves, in roots like sweet potatoes.

We sometimes eat collenchyma cells too.

They’re in the strings of plants like celery and rhubarb.

These cells provide structural support to organs that are still growing, so they need to remain stretchy and flexible.

And lastly there’s sclerenchyma.

Sclerenchyma’s job is to support the plant when it faces too much weight or bending.

They’re like the folks at the bottom of the cheer pyramid.

We do occasionally eat these cells — they give pears their gritty texture — but they’re usually too tough to eat.

Like, I’m not trying to chow  down on a peach pit.

Sclerenchyma supports organs that are fully mature, and similar to xylem, it has cells that are dead inside (physically, not emotionally).

All this isn’t just fun trivia to plant-splain to your next date, or fodder for your  next Scrabble game — though “sclerenchyma” would definitely get you a new high score.

It’s also knowledge that, when applied, can help prevent plant diseases, like those studied by Dr. Esau.

And plant diseases are often a big deal for more than just plants.

They could lead to losses in food supply, damaged ecosystems, and more.

So, all throughout this episode, we’ve been yanking tissues out of the tissue box, one by one.

By which I mean — talking about dermal, vascular, and ground cells separately when, of course, they don’t function separately.

Just like in our bodies, tissues work together in plants to form an organ.

And the real botanical magic happens when we can observe this collaboration in action.

So for that, we’ll need a microscope… and some chopped-up plant organs.

Ahem.

Thank you.

We’ll start with a microscope slide of a corn stem.

This view is called a cross-section — we’ve made a cut horizontally across the stem, like a flat cucumber slice, and we’re facing it head-on.

Check it out: the dermal tissue layer is the  perfectly neat row of cells all around the edges — it’s giving the stem a tight hug.

The rest of the stem is filled with ground tissue — those big, juicy parenchyma cells, which help keep the plant upright and store water and nutrients.

And scattered throughout the ground tissue are the vascular tissue bundles, or veins, which look weirdly like they’re screaming.

I mean, I guess I would be too if someone just cut a  chunk out of me and put me under a microscope.

The xylem cells, which are moving water and nutrients up through the stem, are clumped toward the inside of the bundle, and the phloem, which is transporting sugars down through the plant, is clustered toward the outside.

The companion cells are the little guys scattered throughout the larger phloem cells.

There are even some sclerenchyma fibers helping out along the edges.

OK, next plant organ.

Chef, make me a leaf cross-section.

Thank you!

If you’ve doodled a leaf before, chances are you’ve drawn a line going down its middle.

Well, here’s that line at the microscopic level.

That big honking circle in the center?

That’s the major vein that runs through the center of the leaf.

And if we zoom in, we’ll notice its xylem and phloem cells snuggled together, just like they were in the stem.

The blade of the leaf extends from either side of that big vein, and smaller veins branch off from it that support the leaf and supply it with water and food.

So the xylem cells are transporting nutrients from the roots to the leaf, and the phloem are taking the freshly made sugar from the leaf to the rest of the plant.

Packed around the veins is the ground tissue, which is working so hard at photosynthesizing.

Each leaf parenchyma cell can hold hundreds of chloroplasts — that’s where all the photosynthesis happens and what gives leaves their signature green color.

And we’ll find those funky little stomata on the underside of the leaf.

Since water escapes  the plant through the stomata, it’s best for the plant to have them on the side of the leaf  that does not get blasted by the sun all day.

Last but not least, let’s check out a root cross-section.

It’s got a similar vibe to the stem, except its vascular tissue  is clustered in the center of the root.

This tissue acts as a filter for the  incoming water that the root is soaking up.

The vascular tissue is surrounded by a thick layer of ground tissue, and the epidermis wraps around the whole thing.

So by examining these organs under a microscope, we can recognize that plants aren’t just a mass  of cells—they’re highly organized.

Plants are made of organs, which are made of tissues, which are made of complex cell structures that work together and have a hand in everything that goes on in the plant, from how it eats to how it sweats.

The work of plant anatomists like Dr. Esau shows us that plant form and function are inextricably linked.

Without the intricacies of plant tissue, plants wouldn’t be able to survive and thrive.

And viruses, unfortunately, know  this about as well as we do.

But the more secrets we learn about the amazing relationship between how plants are built and how they work, the more we’ll be able to conserve them and all the types of life that rely on them.

Next time, we’ll be seeing these plants in action as they accomplish some of their most important processes: cellular respiration and photosynthesis.

Hey, before we go, let’s branch out!

At Dr. Esau’s first university job, what did the students use to illuminate their microscope samples?