all right so we have two here we don't understand the two control mechanisms plants have one is called phototropism the other one is called gravitropism we're going to understand the experiments that led to the development specifically of one of them called photo trops but not both of them but yeah stimulus is any environmental Factor and you respond to stimuluses every single day you get cold I'm going to put on a sweater right um maybe if you're hungry that's a stimulus your internal stimulus your body's giv to you I'm going to go get some food all that kind of stuff your body responds to thousands of stimuluses every single day and sometimes your brain even tun certain stimulus out so for example clocks tick after a while you're you stop hearing the clock tick while the clock didn't literally stop ticking your brain just stop processing that stimulus so stimulus are very important they help organism survive and organisms need to respond to that stimulus another very common one is fish use chemicals in the water in order to detect things like their food or even a predator that they detect with their old factory sensors uh oh there's a big fish who wants to eat me right behind me I'm going to swim a away it's responding to that stimuli or a very Comics have right you are summer you turn your lights on a bunch of MTH and mosquitoes and flies they all go towards the light because they are attracted to that particular stimus plants also respond to stimuli and a tropism is a growth responding to some sort of stimulus so basically a plant is going to change its growth to respond to a particular stimuli common example roots roots are going to try to grow to where they can sense water because they need water in order to live so I'm going to grow to them first one we're going to look at is something called a positive phototropism this is a growth of the plant towards light so positive doesn't mean like good or bad right positive or negative positive is just like in physics we all love to a particular direction am I going towards the stimulus or am I going away from the stimulus this case plants are growing to W the light toward that stimulus so I get defined as positive phototropism we see right here I don't have any plants in my classroom but again if you walk down the Plant wall on the other side over here we would have the sun right here and all of these things are simply growing towards it yes yes okay so there we go that's all it is uh what is phototropism positive phototropism just the fact that plant is going to grow towards the sunlight that makes sense because it wants to maximize the amount of sunlight hitting its leaves to do the most amount of photosynthesis possible how is this actually able to do that well there is a chemical in the plants called oxin and oxin helps this thing grow towards the left so how does it kind of work well basically we have a sun right here that Sun starts shooting its Rays along this side of the plant the opposite side of the plant is where the oxin molecules move so they kind of move away from the light and what that causes to do is it causes these cells to elongate making this side of the plant significantly long then this side of the plant is pretty analogous to like a Bendy straw you use one of those right you pull up a Bendy straw and one side's bigger than the other that curves it in this particular direction and that helps it grow towards the side um don't have a very good visual for you but we got a slanky here we have a slanky that's like perfectly upright and I bend it towards here it it but we bend it in like one way or another uh again like this side of the spring is just covering more distance okay so this side gets pushed together this side expands away that's what's happening here this size all of the cells are just the exact same size all the cells are the exact same size Sun hits it this way you get a bunch of these molecules moving into these cells that causes them to become way bigger and that like pinches the one side to be small the other side becomes big and it curves it to this okay so there we go uh we don't have to understand the mechanisms of it super eloquently you just have to know that oxen goes to the opposite side of the light that causes it to expand and then that causes it to grow towards the Sun so that's the answer the question is how do we know that this is truly how this works we're going to go through some experiments that kind of LED scientist down this road first experiment we're going to look at is the men the myth the legend is Charles Darwin and his son Francis so little father son activity here grinding out some science in the lab uh they were using Oak seedblings talking about controlled manipulated responding variables uh the oak seedling is a controlled variable because through all of the trials of the experiments they use the same type of seed then they make a bunch of different manipulations one time they don't do anything to the plant they just see what they how it normally grows so they can compare it one they simply cut off the top of it one they put an opaque cover over the top of the plant one they put a transparent cap over the plant and other one they put on the base so it's easier just to kind of talk about these things when looking at the diagram that I've given to you it kind of looks like this to see what happens so what's the manipulated variable well how they're changing the plant from cutting it off to putting the opaque cap transparent Cap all that kind of good stuff that's what we're manipulating what's the responding variable is how the plant grows so we're going to see that the control the plant that had no manipulations done to it it's growing towards the sun there we go we know that this process actually occurs then we have the one where the tip is removed uhoh keeps growing straight it lost the ability to grow towards the Sun by cutting off the head same with the opaque C since Sun cannot penetrate the opaque C it just grows straight up the transparent cap that does allow sun to get in we get this thing growing towards the Sun and lastly when we cover the base of the plant this bad boy can still grow towards the Sun so what's a conclusion he can draw from his experiment whatever is causing this phenomena to occur going towards the sun has to be located in the top of the plants not in the bottom or else this one wouldn't have been able to grow but it could the bottom clear doesn't have we not responsible for this and both times the top of the plant were manipulated in one way or another it wasn't able to grow towards the sun first thing that Francis and and his father Charles figure out is simply just the fact that whatever is causing this mechanism to occur has to be in the of that's I got next we get Peter boen Jansen Jenson he also kind of picks up where Darwin and sun go and he keeps using Oak seedlings and he's going to take it one step further he has two different membranes he has a Mya membrane myia is just rocks so it's a rock membrane that is not permeable meaning chemicals can't go through it gelatin um or agar something like that just like jello things can move through a gelatin barrier chemicals can go through it all that kind of stuff so we're again just going to look at it but he just has two Oak seedlings he's just manipulating what membrane is present on them and it kind of looked like this we see that the the oak seedling that has the gelatin block this thing is permeable or allows stuff to go through it then we see the you can do the experiment with glass or Rock uh this is nonpermeable [Music] this one grows straight so what he kind of discovered from this is whatever is causing this effect must be a molecule or a chemical that has to be able to move from one part of the plant to the other part of the plant if we're able to move we can grow towards the sun molecules are free to move throughout this plant all good in the hood we're able to do it if we have this separated where molecules can can't move they can't move from one side of the plant to the other we no longer see any of this phototropism occur so what he basically discovers is that we're not sure what molecule it is but it has to be some sort of a chemical causing this phenomena toer poem then we get into britz went Brit went is now using agar AAR is another like jelly like thing just like the last guy was using gelatin uh we use agar a lot in biology it's a very good medium for growing lifeon if you ever grow like bacteria or something like that you're probably doing it egar that means this thing is permeable and chemicals are free just to move through it if it wants so what this thing does what FR spon does is he takes an old seeding he puts agar on top of it again I'm just going to kind of look at at the experiment so he cuts off a seeding this is where all the chemical is we know the chemicals in there because that's what Charles and francis's experiment showed it has to be in the tip we know it has to be a chemical because that's what Peter boy and Dan just showed us there's a high concentration of the chemical inside the tip in the agar there would be a loow concentration wow there it is it's still coming in we're still talking about diffusion we're still talking about concentration gradients the chemical naturally diffuses from the tip of the oak seedling and goes into the egar yay we now have an agar full of this mystery chemical so then what he can do is he can put this egar full of the chemical on to different sides of the plant so one time he puts it on the left side of the plant and he knows notices if I put it on the left side of the plant it grows towards the right or it again curves towards the right if he does it's not on the diagram but he has again starting from here if he puts the AAR on the right hand side what he then sees is this thing grows it goes towards the left and if he puts it smack that right in the middle we have this thing he just puts the egar right in the middle of it it just keeps growing up straight because we're not elongating one side or the other so again he's the guy who kind of figures out that this chemical moves from one side to the other in order to figure out that it that's how it grows to the left and to the right he figures it the opposite side right if I put this thing on the left it go to the right meaning the Sun causes this chemical to move to the opposite side of the plant and he says all right we know it's a chemical we know where it is we know basic function he's the guy who coins the name p call this bad boy Ox so any questions about those three experiments uh any manipulated control responding variables anything about how they function people who did anything like that easy easy so again side method great slowly marging forward inching by getting one thing at a time good phototropism just how the mechanism actually works and the three scientists who help Discover it we also found out that not only do plants respond to light they also respond to gravity gravity has both a positive and a negative tropism here so we called gravit tropism because it's gravity gravity gravity of course of gravity I'm sure we all know is pushing downward so things that grow with the stimulus of gravity would be positive gravitropism good example Roots roots on the bottom of a tree right they grow downward in like all directions so those roots are growing with gravity there we go gravity tropism negative gravity tropism would be plants growing in the opposite direction so that would be like your stem your leaves anything that's going in an upward direction and what's actually quite amazing I don't know if anyone's again been on many hikes they go through like the mountain and stuff like that so like trees can get like three quarters knocked over they can literally be like this and eventually who they start growing upwards they do indeed continue growing upwards even if they start at like a 45° angle to the ground no matter what this tree will eventually start to go upwards and it looks very funny when you see this phenomena occurring no matter what even the tree gets knocked down a little bit it can still respond to gravity and grow in the opposite direction of it start growing straight upwards any actually seen this before like in a forest yeah okay cool uh it's very cool to kind of see an actual nature uh but how does it actually work same idea we have oxen oxen moves based off the force of gravity pushing it in One Direction or another so again uh we're just going to look at the diagram when we look at it so we see right here to grow upwards Against Gravity negative gravity tropism it's kind of hard to see but there's these blue dots which are The Oxen they migrate to the opposite side of the plant because the force of gravity pushes it downward that elongates this surface that bunches this one up and we get a net curving upwards we get the same phenomena kind of occurring here on the bottom side where we get gravity pushing it down we get this thing right here uh that purls up together this side gets elongated and that pushes us straight down there we go we have a plant right on the side it will grow up it will will go straight down it doesn't matter oxen is controlling it in both cases I'm not going to make you draw a diagram like this or anything help you visualize it you have to know for both phototropism and gravitropism the molecule oxin that is causing it to happen you just know the terms negative and positive mean in the context of these tropisms so if I just say hey what's a negative tropism or what's an example of negative tropism you are not choosing roots or something right you know it's having go the opposite direction um and you just know the three researchers that's basically all I have done like I said very short not a whole lot to talk about with to five any questions regarding top all right rest period is yours we still have I don't know what the time table for Friday's say but that only took 18 minutes