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Tropism (Types, positive & negative)

Let's explore (+ & -) photo, geo, chemo, thigmo, & hydrotropism. Created by Mahesh Shenoy.

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Video transcript

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 gonna learn about a particular kind of plant moment which is called tropism or tropic moments 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 aren't 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 moment 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 it's also a turning moment which is moving away from the sunlight I'll be 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 chute you will see that it won't grow horizontally instead it will turn up again this is a tropic moment or tropism because it's a turning moment 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 moment 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 moment too words gravity so there's also tropism all right now let's take a different example what about the touch-me-not my Moza leaves we know you know as a child you may have done this right touch-me-not planned so you go and touch it and you'll find that the leaves closed 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 does that make sense like in these cases so this is not a tropic moment let me you another example this is a Venus flytrap the moment of light comes it traps it is this a tropic moment 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 stimulated which direction you stimulated it's just very close it has nothing to do I mean it doesn't depend upon the direction of the stimulus so again this is not a tropic moment 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 moments are 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 moments are growth related movements which we see growth in plants as you can see over here and more examples we will see that but in nastic moments we don't usually see growth in plants anyways we don't have to worry too much about nastic moments 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 moments the names of these depends on the stimuli okay for example over here the stimulus for that bending movement or turning moment is light or photo so these tropic moments are called phototropism so this is called photo tropism 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 gravitropism some people also call this geotropism just to keep it simple so we can also call it Geo tropism 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 moment because if you could see this in action we would have seen that things start turning and bending around the same 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 ride 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 root 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 moment 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 Sigma and so we will call it Sigma tropism figma tropism another example which I won't draw which you can explore yourself is something called a hydro tropism can you guess what that is that is movement towards or away from water hydro stands for water so you usually see hydro tropism in roots again if there is some water somewhere we will see the roots start growing too let's look at one more example which can be seen in the reproductive part of the plant will 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 a 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 X cells 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 this chemicals you know what's gonna happen as this pollen grain matures let me zoom in over here yeah so as it secretes the chemicals the pollen grain a tube starts growing from this pollen alright and the tube starts following it 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 part over here the sperm cells can easily follow into that pollen tube and then go and fuse with the egg all right so let's zoom back so can you see this moment of Poland 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 or here another such example can be seen again in the rules 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 the turning moment away from the stimulus a 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 chemo tropism chemo tropism 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'll 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 chute over here it is bending two words the sunlight and so we will say this is positive tropism so the chutes show positive phototropism does that make sense similarly if you look at the roots they are bending away from the sunlight and therefore the chutes 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 the stimulus right since the roots are growing two words gravity or towards earth we will say the roots show positive geotropism does that make sense similarly what about the chutes the chutes are showing negative geotropism because they're going away from the earth can you look at these examples and try and figure out quickly whether each 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 thing more tropism over here once it touches the rock the roots are going away from it so this is negative figma tropism over here notice the pollen grains sorry not the point the pollen tube is growing two words the chemicals released by the ovary and so this is positive chemo tropism and over here again the roots are moving away from this acidic not acidic this is some harmful chemicals so since they're moving away this is an example for negative chemo tropism and that's pretty much it for this video