 Animals keep moving around, responding to the environmental stimuli. But what about plants? Do they move? The answer is yes. Even though it's not as obvious as the animals, plants do move. They also respond to various environmental stimuli. And so in this video, we're going to learn about a particular kind of plant movement which is called tropism or tropic movements. So we'll see what it is and we'll look at its different kinds. All right. So what is tropism? Tropism is a kind of movement. It's a kind of turning movement. So it's a turning movement towards, towards or away from. So towards or away from what? From some kind of stimulus, from stimulus. So let me give you an example. If you take a plant and keep it in sunlight, you will see as time passes by, the plant will grow and the shoot will start bending towards the sunlight. This is tropism because notice the plant is moving. It's a kind of turning movement. It is moving towards the sunlight. So sunlight becomes the stimulus. So this is tropism. Similarly, if you look at the roots now carefully, you will find that the roots will start moving away from the sunlight. This is also tropism because this is also a turning movement which is moving away from the sunlight. Let me give you another example. Say you try to take a plant and make it grow horizontally for some reason. You will now see as the plant grows, if you look at the shoot, you will see that it won't grow horizontally. Instead, it will turn up. Again, this is a tropic movement or a tropism because it's a turning movement. But what's the stimulus over here? The stimulus over here is gravity. You will see that this thing is always going to grow against gravity. So it's a turning movement away from gravity. And similarly, if you look at the roots, you will find that the roots will again start growing down. This is also a turning movement towards gravity. So there's also tropism. All right. Now let's take a different example. What about the Touch Me Not Mimosa leaves? We know as a child, you may have done this, right? Touch Me Not Plant, so you go and touch it and you will find that the leaves close. This is also movement. But do you think this is tropism? The answer is no. Even though the plant is responding to stimulus, you can see that the movement is neither towards or away from the stimulus. It doesn't matter where you touch that leaf. You will find that the leaf will just close. It has nothing to do. There's no directional movement. Like in these cases. So this is not a tropic movement. Let me give you another example. This is a Venus fly trap. The moment a fly comes, it traps it. Is this a tropic movement? The answer is no. Why not? Again, because notice there was a stimulus over here. But the movement is neither towards or away from the stimulus. Regardless of how you stimulate it, which direction you stimulate it, it's just going to close. It has nothing to do with it. I mean, it doesn't depend upon the direction of the stimulus. So again, this is not a tropic movement. So these movements, which do not depend on the direction of the stimulus, they are often referred to as nastic movements of plants. So the major difference between these two movements is that tropic movements or tropism depend on the direction of the stimulus, as we saw here. But nastic movements do not depend on the direction of the stimulus. Another difference is that usually, tropic movements are growth-related movements. We see growth in plants, as you can see over here. And in more examples, we'll see that. But in nastic movements, we don't usually see growth in plants. Anyways, we don't have to worry too much about nastic movements. Let's get rid of this. All right. Now, before we go ahead and look at more examples, let's see how to name these tropic movements. The names of these depends on the stimuli. Okay. For example, over here, the stimulus for that bending movement or turning movement is light or photo. So these tropic movements are called phototropism. So this is called phototropism. Phototropism. What about this one? What's the stimulus for these turning movements? Well, we just saw the stimulus is gravity, right? So we can call this gravity tropism. Some people also call this geotropism, just to keep it simple. So we can also call it geotropism, basically because gravity comes from Earth and Earth stands for geo. All right. Let's look at more examples. Check out this creeper plant. What do you see? You see that this creeper is coiling around this stem. This is also a tropic movement, because if you could see this in action, we would have seen that things start turning and bending around the stem. But what's the stimulus for this? What do you think? Can you pause the video and think about what is the stimulus? Well, notice until it touched that stem, there was no bending here. Once it touched, it started bending around it. It started coiling around it. So the stimulus over here is touch. Okay. Based on the touch, this tropic movement happened. Let me give you another example. If you look at the roots which are growing underground, suppose they find an obstacle. Suppose the root of that, one of the roots starts touching the obstacle. Then we will see that the roots will start growing away from that obstacle. Again, we see the touch is the stimulus and here the turning movement is away from the stimulus. Here it was towards. Here is away. So both of these are tropism. So what should we name this? We can call it touch tropism. But the Latin term for touch is thigmo. And so we will call it thigmo tropism. Thigmo tropism. Another example which I won't draw, which you can explore yourself, is something called a hydrotropism. Can you guess what that is? That is movement towards or away from water. Hydro stands for water. So we usually see hydrotropism in roots again. If there is some water somewhere, we will see the roots start growing towards it. Let's look at one more example which can be seen in the reproductive part of the plant. We'll talk about reproduction in great detail in separate videos. But in short, if a plant needs to reproduce, a pollen grain needs to land on the stigma of the flower. These pollen grains, of course, are actually much smaller than what I've drawn. I've just drawn them big so that we can see it properly. Over here is the ovary which contains the excels. And in the pollen grains contains the sperms. And so the sperms need to go and reach the egg. How do the sperms do that? How do they know where to go? This is where the ovary starts secreting some chemicals. So based on these chemicals, you know what's going to happen as this pollen grain matures? Let me zoom in over here. So as it secretes the chemicals, the pollen grain, a tube starts growing from this pollen. And the tube starts following. It grows according to that chemical released. It follows it, follows it, follows it, follows it all the way into the ovary. And now that there is a nice path over here, the sperm cells can easily follow into that pollen tube and then go and fuse with the egg. So let's zoom back. So can you see this moment of pollen tube towards the ovary is also a tropic moment. It is a turning moment, it is turning, and it is moving towards the stimulus in this case. What do you think is the stimulus? The stimulus was some chemicals released over here. Another such example can be seen again in the roots. If there are some harmful substances which release some harmful chemicals, the roots will sense that and this time they will start growing away from that. So again, this is a turning moment away from the stimulus, the stimulus being some chemicals. So what do you think we should call this tropism? Since this tropism is due to chemicals, we will call this chemotropism. Chemotropism. So these were some major examples of tropism and of course there are others as well which you can explore yourself. But before we wind up, one last thing to remember is when it comes to naming these, we like to differentiate between movement towards the stimulus and movement away from the stimulus. The way we like to do that is whenever the movement is towards the stimulus, we will call that as a positive tropism and whenever it's away from the stimulus, we call it negative tropism. So for example, if you look at the shoot over here, it is bending towards the sunlight and so we will say this is positive tropism. So the shoots show positive phototropism. Does that make sense? Similarly, if you look at the roots, they are bending away from the sunlight and therefore the shoots show, sorry, the roots show negative phototropism. Similarly, when it comes over here, if you look at the roots, this time you see the roots grow towards gravity. Gravity is a stimulus, right? Since the roots are growing towards gravity or towards Earth, we will say the roots show positive geotropism. Does that make sense? Similarly, what about the shoots? The shoots are showing negative geotropism because they're going away from the Earth. Can you look at these examples and try and figure out quickly which of them are positive and negative? All right. Over here, notice that the creeper is bending towards the stimulus. Once it touches, it wraps around that. So this is a positive thigmotropism. Over here, once it touches the rock, the roots are going away from it. So this is negative thigmotropism. Over here, notice the pollen greens, sorry, not the pollen, the pollen tube is growing towards the chemicals released by the ovary. So this is positive geotropism. Over here, again, the roots are moving away from this acidic, not acidic, this is some harmful chemical. So since they're moving away, this is an example for negative geotropism. That's pretty much it for this video.