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- [Hank] 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, Pop grass 'cause 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, it 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 convert 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' 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 plant success is 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 three 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 sea grass in the Mediterranean dating back 200 thousand years. We spend a lot of time congratulating ourselves on how awesomely magnificent and complex the human animal is, but you guys, I got to hand it to you. (upbeat music) 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 and 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 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 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 their most important task is to support the last organ: the leaf. This, of course, is where the plant exchanges gasses 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 worlds most important molecules: water. So, since plants are pretty much designed around water lets 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 feature 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 secret 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 root, the epidermis has similar features called root hairs that maximize the roots' surface area for absorption, just like we've seen in our own organ systems. This, of course, is where they 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 are 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 it's 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 dissolve 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 upwards. 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 great 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 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 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 it's 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 it's name, meso meaning middle, and phyll meaning leaf, this layer sits between the top and the 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 it's 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, it's where the money is, and by money, I mean food. And the mesophyll is chock full of parenchyma cells of various shapes and sizes and 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 opening in the epidermis called stomata. Around each stoma are two guard cells, connected at both ends that regulate it's 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 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 it's 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 equals winning. And I'm not just talking about them sweeping the contest 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. And that's why the world's most plant-rich habitats, like rainforests and grasslands, are so crucial to our survival. When those habitats change, everything changes. Weather, food supply, even 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.
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