Earlier in today's lecture, I said that proprioception is not only knowing up and down, but it's also balance. It's knowing where the body parts are when you're moving. When a ballet dancer leaps across the stage, and lands in an Arabesque, he's not only perfectly balanced. He's acutely aware of the position of every part of his body. He knows how far his leg is stretched behind him. I'm not going to show you that here. How high his hand is, and what is the angle of his torso. It's no surprise, though, that we regard plants as stationary beings. They're sessile organisms, eternally rooted. And incapable of locomotion. But as we saw in some of the time lapse movies. When we look at them patiently over a long period of time, their stationary stature gives way to an intricately choreographed festival of movement. Much like Baryshnikov springing to life in the first scene of a ballet. Leaves curl and unfold, stems move and twist, and flowers open and close. Just take a look at this movie of sunflower seedlings. We see these seedlings turning in circles. This is called circumnutation. It's a term that was coined by, again, none other than Darwin. A large part of the power of movement in plants, Darwin talks about plants moving in circles. What he would do, would be stay up for hours and with a glass plate above a plant, mark on the plate the position of the tip of the chute. And he noticed that plants would move in spirals and in circles. Here's actually a picture of one of his traces from his book. He did this with numerous plants and what he saw that each plant had its own characteristic shape of circumnutation, its own period, and its own speed. But every plant tested circumnutated. This led to two different hypotheses about why plants circumnutate. Darwin said in the 1880s that circumnutation is hard wired into the behavior of all plants. Almost 100 years later though, there was an alternative hypothesis by two Swedish scientists, Israelson and Johnsson, and they claimed that circumnutation is just a result of gravitropism. In other words, the spiral movement of the plant, is a result of plant growth overshooting where it should be. You might remember the first movie that we showed, showing that early plant gravitropism, the plant went like this, and then like this, and then like this. What Johnsson said was that as a plant somehow bent is falling a bit to the left, for example. Gravitropism would then make it bend upwards. But it then overshoots and then goes a bit to the right. Now the aminoplasts would fall again to the bottom, it would be responding to negative gravitropism, try to go up, overshoot and go a bit to the left. And back and forth, where you would get a waving movement which would be going in spirals. So what they were saying is that circumnutation is not hard-wired, but is only a result of gravitropism. So how can these two hypotheses be tested? How can we differentiate between them? We could differentiate between them using two tools. The first would be genetics. Can we find mutants that are defective in circumnutation or test mutants that are defective in gravitropism for circumnutation? What it actually ends up with is that if you take in an arabidopsis mutant that has no aminoplasts, arabidopsis mutant that has defective gravitropism, also have defective circumnutation. We see that actually sometimes in ornamental plants. For example, there's a morning glory that falls, or the chutes falls, it's a hanging plant. The reason is that this morning glory falls is that its chutes are defective in gravitropism. And these chutes of morning glory are also defective in circumnutation. So the genetics would support Johnsson's hypothesis that all of the turning is due solely to gravitropism. The other way of testing this hypothesis would be in space. And it was tested in 2007, interestingly in an experiment that was designed by the same Johnsson. His hypotheses as a young scientist in 1968, was finally tested in 2007 when he was a very senior scientist. So aboard the space sta, the International Space Station, they took seeds that were germinated in space. In other words that these seedlings had never been exposed to gravity, and then tested using a camera, whether there was any circumnutation. And what they found was that there were very, very minute movements of these seedlings in space. Almost undetectable, but there were small circular movements in the absence of gravity. Then when the scientists aboard the space station took the same seedlings and put them in the centrifuge, then the movements got much larger. So on the one hand, Darwin was correct. Circumnutation appears to be an inherent feature of all plants. Where was Johnsson correct? He was correct in that gravitropism then mediates the period, or increases how fast and how far it's going. So a plant can be pulled in many directions, as we've seen. Sun light coming from the side can pull it to bend towards the sun. Whereas, gravitropism at the same time, would be causing it to grow up. We saw with the cuscuta plant that a smell might be causing it to go to the side. But at the same time, its aminoplasts would be falling to the bottom, causing it to grow up. These often conflicting signals enable to a plant to situate itself in a position that's optimal for its environment. The tendrils of a vine, the part of a vine that circles a fence, search for the fence may be attracted to the shade of the neighboring fence, while gravity will enable it to wrap a twirly, will, will enable it to circle rapidly and twirl around the fence. A plant on a windowsill will be pulled by light to grow to one side toward the sunny part, while the force of gravity will influence it to grow up at the same time. Just like in Newtonian physics, a position of any part of the plant can be described at one time as a sum of all of the vectors that act upon the plant, to tell it both where it is. And what direction to grow. And a plant without proprioception would be unable to find its optimal place in its environment for continued growth. Thanks for being me with this with this lecture. We'll meet again next week.