Previous: Communists, Nationalists, and China's Revolutions: Crash Course World History #37
Next: World War II: Crash Course World History #38



View count:1,438,515
Last sync:2022-12-06 21:30
Hank introduces us to one of the most diverse and important families in the tree of life - the vascular plants. These plants have found tremendous success and their secret is also their defining trait: conductive tissues that can take food and water from one part of a plant to another part. Though it sounds simple, the ability to move nutrients and water from one part of an organism to another was an evolutionary breakthrough for vascular plants, allowing them to grow exponentially larger, store food for lean times, and develop features that allowed them to spread farther and faster. Plants dominated the earth long before animals even showed up, and even today hold the world records for the largest, most massive, and oldest organisms on the planet.

Table of Contents
1) 3 Tissue Types 02:37
2) Primary Growth 03:04
3) Secondary Growth 03:28
4) Dermal Tissue 04:47
a) Epidermis 04:54
5) Parenchyma Cells 05:39
6) Vascular Tissue 05:58
7) Xylem 05:58
8) Collenchyma 07:10
9) Sclerenchyma 07:35
10) Ground Tissue 08:25
a) Mesophyll 08:17
b) Photosynthesis 08:47
11) Phloem 09:54


Crash Course is on Patreon! You can support us directly by signing up at

Want to find Crash Course elsewhere on the internet?
Facebook -
Twitter -
Instagram -

CC Kids:
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's said to have used it on the wounds of his soldiers.

And this is snake grass, otherwise known as horsetail. Or to the kids, it's pop grass 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, most 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 like (you know), good for them.

But these things alone can't explain vascular plants' evolutionary success. I mean, algae was photosynthesizing long before plants made it fashionable and as we learned last week, non-vascular plants have reproductive strategies that are tricked out six ways from Sunday.

So, like, what gives? The secret to vascular plants' success is their defining trait: conductive tissues that can take food and water from one part of a plant to another part of the 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 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 three blue whales laid end-to-end. 

The most massive organism is the 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 sea grass living in the Mediterranean dating back two hundred thousand years. 

We spend 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 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 die down to the root or die completely after one growing season.

Pretty much everything you see growing in a backyard garden - herbs and flowers and 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 your shrubs and 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 and store nutrients and also is home to specialized cells called meristems that are responsible for creating new growth. But the 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 first 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 support 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 hair-like 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 for using it for lunch. 

Finally, in the root 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 a 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's 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 three thousand gallons of water every day. A large oak tree, just one tree, can transpire 40 thousand 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. 


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 in young structures like new shoots. Celery is mostly make 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 hardy cell walls as a support structure and new cells from a fresh layer during the next growing season push the old, dead layer outward. In the 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, known as 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 found in the ground, and it's actually just defined as any tissue that's not dermal or vascular. Regardless of this low billing, though, it's where the money is, and by money I mean food.

The mesophyll is chock-full of parenchyma cells of various shapes and sizes, many of them 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 on the epidermis called stomata. Around each stoma are two guard cells at each end which regulate its size and shape. When conditions are dry, 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've 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 if it's the growing season or down the 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 equals winning. And I'm not just talking about them sweeping the contest for biggest, heaviest, oldest living things, although 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 incidents 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 the 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. We'll see you next time.