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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