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  • 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.

This is yarrow, a flowering plant

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