 Let's explore the idea of tropism. Tropism in plants is basically a growth moment in which the plants are either turning towards or away from some stimulus. Let's take an example. Consider a plant growing in sunlight. You would then find after some days that the shoot of the plant turns towards the sunlight and the roots of the plant will turn away from the sunlight. This is tropism. Why? Because first of all, you can see both the shoot and the roots are growing. So it's a growth moment. Then there is turning. This is turning towards the sunlight. This is turning away from the sunlight. And you can see the stimulus over here itself is sunlight. So this kind of tropism is called phototropism because the stimulus over here is sunlight. Photon, photo, okay? And whenever you are having tropism towards the stimulus, we call it as a positive tropism. So for example, the shoots over here are displaying positive phototropism because it's turning towards the stimulus. Whereas the look at the roots, it's turning away from the stimulus. Therefore it is showing negative phototropism. All right, let's take another example. If you take a plant and you keep it horizontal, after a few days, you will see that the shoot will grow up and the roots will grow down. This time the stimulus is not light. This time the stimulus is gravity. Since the plant is turning due to earth's gravity, it's called geotropism, geophorearth. Now, can you think about which is a positive and which is the negative geotropism? Look at the roots and the shoots and pause and try it yourself first, okay? Now look at the shoot. The shoot is going up, but gravity is down. So it's actually going away from the stimulus, away from the earth, away from gravity. So this is a negative geotropism. Whereas look at the roots, it's going towards the earth in the direction of gravity downwards. So the roots are displaying positive geotropism. All right, let's take another example. Consider a creeper that is turning along and growing. This is also tropism. This is called thigmotropism because the plant is turning based on the touch. So the moment it touches something, it turns and coils along it, making it creep and grow. And if you look at the roots, sometimes when the roots touch like say rocks, it'll go away from it. Again, this is turning due to touch, thigmotropism. And you can see this one displays positive thigmotropism because it's turning towards the stimulus. And look here, it's turning away so it is negative thigmotropism. Okay, one last example. When a pollen grain sits on the stigma of the flower, it starts germinating and then it eventually grows, a tube grows and it goes all the way to the ovary. But how does it know to grow and go towards the ovary? Because here there are some chemicals secreted and so it grows towards the chemical. So you can see there is tropism because it's turning happening over here. Another example over here is sometimes the roots will turn away from some harmful chemicals inside the soil. Again, tropism turning. Since here tropism is happening due to chemicals, this is called chemotropism. Again, you can see here it is going towards the chemical so it's called positive chemotropism. Over here it's turning away so this is negative chemotropism. All right, now I want you to look at this example, the example of mimosa leaves. When you touch the mimosa leaves, it closes. Can you pause and tell me which kind of tropism is this? And whether it is a positive or a negative tropism. Why don't you pause and think about it? All right, at first I thought this is thigmotropism because it's touching, I'm touching it, right? But it's not tropism at all. You know why? Because look, tropism needs to be a growth moment. This is not a growth moment, it's just closing. It's not growth. And notice, tropism needs to be either towards or away from the stimulus. It's neither towards the touch nor away from the touch. So because of these examples, this is not a tropism moment.