Plants are able to sense and respond to physical stimuli, like touch or wind. Growth or movement in response is called thigmotropism. Climbing plants use this in order to climb. They grow out tendrils, like you can see here, and when they touch a surface or something to grab onto, there's a response that causes them to grow and wrap around, and grip that object or surface, whatever they're touching. Venus flytraps will also use this in order to shut on an unsuspecting insect or some form of prey. And this actually requires something pretty special because, as you can imagine, if you've ever tried to swat a fly, they can move pretty quickly. So, the plant has to respond to that sensation of touch and move very fast to shut closed on the fly. So what a plant is going to need to do is use an action potential, and this is an electric signal, like what goes through the nerves in our bodies, but plants don't have nerves. They actually will move their action potential through plasmodesmata. So it's not going to be quite as fast as moving through a nerve. However, it will still be quite fast signal, and if you're wondering how they generate that electrical signal, it's actually done by moving ions across the membrane. The specifics of this get pretty complicated. I wouldn't worry about it too much here. Just know that by moving ions around the membrane, they can create electric signals that they're able to transmit. And we will learn much more about action potentials when we talk about the nervous system.
Now it's also worth noting that plants that get a lot of physical stimulus, for example, generally speaking, they often will not grow as tall. This is because of their response to wind. If they're getting a lot of physical stimuli, it could be from lots of wind gusts. And in order to not get blown over or have the plant body get broken in the wind, plants will actually restrict their vertical growth so that they are less susceptible to being damaged by the wind. So this comes in many different flavors and varieties. All you really need to know is that plants are able to sense and respond to physical stimuli.
Now plant growth is hormone regulated, and it is going to be regulated by a hormone, or a class of hormones, called cytokinins, which actually regulate growth by regulating the cell cycle. If you don't remember the cell cycle very well, I highly recommend you go back and check out the videos on cell division, especially the video on regulation. If you remember that stuff, hopefully, you recall that there is a gate at G2. This is sort of like a fail-safe mechanism to make sure that, right before mitosis happens (that's what that 'M' is), that all systems are go; this is the point of no return, and cytokinins are actually going to cause cells to pass that G2 checkpoint and continue dividing. They are the gatekeepers essentially. This hormone is produced in the roots and is transported through xylem to target tissues. It is going from roots up. And it's thought that in terms of apical dominance, which you might recall is in part regulated by auxin, cytokinins actually play a role in there too. And the ratio of cytokinins to auxin is, in part, what determines that apical dominance. It's thought that, cytokinins play a role in growing outward, in growing bushiness, whereas auxin plays a role in vertical growth.
Alright. With that, let's flip the page.