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Intro to vascular tissues (xylem & phloem)

Let's explore xylem and phloem (transport in plants). Created by Mahesh Shenoy.

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Video transcript

- [Instructor] When plants appear on the land for the very first time, these plants were extremely tiny like moss and algae. And for a few million years, there were no plants on our planet, which could grow taller than few inches. But why? Well, turns out growing tall has some challenges. However, with time, plants slowly started evolving and eventually they were able to overcome those challenges. And as you know, today we can have tall plants, plants which can grow into trees, trees which can grow hundreds of meters tall. And so the question we wanna try and answer in this video is what are the challenges that the early plants faced because of which they couldn't grow tall? And how did they overcome them? So, the biggest challenge that these early plants faced was a lack of a transport system. What's a transport system and why do they need them? Well, let's take an example and understand this. For example, consider my house plant. In order for this plant to stay alive, all the parts of it require food and water and minerals so that they can grow. But where are they available? Now you might know that most of the water and the minerals are available in the soil. So the soil contains water, and it has minerals in it. Minerals. And the plants have roots which can absorb these water and minerals, so great. But what about the rest of the part of the plants? Like the leaves, how do they get that water? Well, and you see, the plants need to transport that water from the roots all the way to the leaves and all the other parts. So can you see that a transport system is needed to transport this water and its dissolved minerals up? Right? Similarly, think about food. How do plants get their food? You might know that they photosynthesize. They use the energy of the sun to create their own food. And most of these photosynthesis happens in the leaves. So let's say some of the food is synthesized in these leaves. Consider these are food. These are basically sugars. Let's just call them as food as of now. Now, what about the parts of the plant that cannot photosynthesize? For example, roots can't photosynthesize because they're under the ground, they're not exposed to sunlight. There maybe certain growing parts of the plant which might require this food because they cannot photosynthesize. Maybe there are some leaves somewhere which are in absolute shade and because of it, they're not able to photosynthesize. So again, what to do? We need a transport system. The food needs to get transported to the different parts. So again, the food need to get transported down maybe to the roots sometimes. It might have to get transported up to some other parts. So can you see, a transport system is necessary. Without which, these plants cannot survive. And the earlier plants didn't have a transport system. And therefore, they are very tiny. The thing is, if you are very tiny, then, this transportation happens just by diffusion. Diffusion is a process in which stuff moves from a region of high concentration to lower concentration. So, if it's very tiny, these food particles, these sugars can easily move into the different parts of the plant. They can easily diffuse into it. Similarly, the water can easily diffuse to all the different parts of the plant. But as the plants get taller, then, diffusion becomes super slow and it almost becomes impossible to transport it just by diffusion. This is where specialized transport system is needed. And so, eventually, plants evolved this transport system. Today, they are called as vascular tissues. So the transport system evolved are called vascular tissues. And the word vascular, vascular means vessels. I like to think of them as pipes, so you can think of them as pipes. So piping system through which they can move the food and water all around their body. And what do these vascular tissues look like? Well, just like the name suggests, they look like pipes. So pipes which start from the roots. So let me show you the roots. And so, pipes that start from the roots and go all the way to each and every leaves. I'm not showing it for all the leaves, but it goes to each and every leaves. But wait, you maybe thinking, how can we use one pipe to transport both food and water? Wouldn't they get mixed up? Well, to make sure they don't get mixed up, we have two separate piping systems. So one for water and one for food. And we give them names. So the piping system that we use for water is called xylem. So xylem transports water. And the dissolved minerals. When we say water, it already has dissolved minerals. And for food, we have another tissue called phloem. And this one transports food, mostly sugars, but it can also transport other things like hormones, amino acids and other stuff, but mostly food. And the way I like to remember this, I remember watching this in some YouTube video, I like to remember this as fo for food and pho for phloem, so food, phloem, fo pho. And another reason why they have to be separate is because if you look at water, the water transport is only one direction, up. Can you see why? Because all the water and minerals are present in the roots. So, we just have to transport up to all leaves. But the food transport has to be in both directions. Sometimes, you would want the food to go up, maybe to the growing areas of the plants, sometimes, you want the food to go down in the roots, that's where the storing organs are. So, you see food needs to be transported in both directions and therefore, xylem is only one way transport up and phloem, well that is a two-way transport. So it makes perfect sense to have two separate vascular tissues altogether. And so this one line I have drawn, don't think of this as single pipe but imagine it's a bundle of a lot of xylem pipes and a lot of phloem pipes together. In fact, let me show you a little better. If I were to zoom in onto this, and if we could see inside, these are the xylem and the phloem bundles. Again, each one has a lot of xylem and phloem tissues inside. And if you could see from the top, if you could have a microscopic view of this, then, it would look somewhat like this. This is what you would see in a microscope. And what you're seeing over here, these are the vascular tissues. Again, let me zoom in even further. So if we zoom in one of these parts, you can see a lot of round things over here. These are the xylem, let me just mark them. These round things that you're seeing over here, these are the xylem tissues. They are the pipes that are coming out of the screen. Think of it that way. They're the one that transport water. These over here. And then, you can see pipes over here as well. You can see small, small round things over here as well, they are much thinner than xylem, these are the phloem. In between, you have other cells. But these are the phloem. And so, you can see a lot of xylem and a lot of phloem are bundled together, along with some other supporting cells as well. And that bundle is what I've drawn over here. So the single thing that I've drawn over here is both lots of xylems and lots of phloem together. And these pipes, they start from the roots and they end all the way into the leaves. So, let me show you a zoomed in version of the leaf. You may have seen a leaf like this. The veins that you see in the leaves are actually the xylem and the phloem. These are the vascular tissues. And then you can see they branch off, into becoming smaller and smaller, let me zoom in even further. And you can see them branching off like a network of roads connecting different different parts of the city. This way, they make sure a maximum number of cells are able to get the food and water. That's how the transport system in plants work. Isn't that amazing? And now that they have the developed the vascular tissues, nothing can stop them from growing tall, growing into trees, to grow hundreds of meters tall. Because of which, we have forests and jungles and everything. But finally, you might have another question. How do these transport systems even work? I mean in tall trees, how are they able to get that water from the ground all the way to the top? Is there some kind of a pump? And I guess that's where things get even more interesting. No, they don't have a pump. Yet, they use some clever mechanism to get their job done. And we look at these mechanisms in great detail in the future videos, okay? But one of the things about not having a pump is that this whole system becomes very slow. So plants, because they don't have a pump, they have slow transport system. But you know what? That's fine for them. The plants are saying, it's okay, we don't have a rush. We don't have to go anywhere. They have pretty low energy demands so they can easily survive with a very slow transport system. They don't need a pump. Unlike animals who have very high energy demands and animals do need a pump, that's why we have a heart, heart is our pump. But plants and trees, they don't need that. And as I said, we will look at how these things work without a pump in the future videos. Anyways, that's it for this video. So let's quickly summarize. What did we learn in the video? We saw that plants have developed a piping system to transport food and water which we call the vascular tissues. We have two different ones. One to transport water, unidirectional transport called xylem. And one to transport food in both directions, bidirectional of called phloem. And the way I like to remember this is fo for food, pho for phloem. And since they don't have a heart, their transport system is pretty slow, but that's fine because their energy demands are pretty low. As they don't have to go anywhere, they're pretty much still. And I would like to end by asking you to wonder about one thing. Can you imagine about how different our world would be without vascular tissues?