So getting back to plants, what did they feel. Check out this video. Of course its a video of the Venus flytrap. But notice, that's it's not that the trap closes immediately, it's actually feeling where its prey is in relation to its leaves. So what's going on here? How does it know where it's being touched? So, here we have a close up of the Venus flytrap. We could see it's general structure. We have two symmetrical lobes which are separated by midrib. At the edge of these lobes are cilia, hair-like structures which actually close-like teeth making a type of jail cage for the enclosed prey. As the prey come along over the leaves, over the lobes of the Venus flytrap, they sometimes touch these large black hairs that are on both sides. And it's these hairs which are called the trigger hairs. It's the sensation of touching of these hairs which causes the flytrap to close. May not surprise you that one of the first people to study and report on the Venus Flytrap, again was none other than Charles Darwin. And he did this in a book that came out before the book we talked about earlier, before The Power of Movement in Plants. This was a book that was published in the 1870's called Insectivorous Plants by Charles Darwin. Darwin originally suggested that an impulse, somehow or another, must travel rapidly to enable the very quick closing of the fly trap. But with all of Darwin's talents, he could not figure out what the signal was. He tested many, many, many options. For example, he even put meat on the fly trap. He thought somehow on there it was some chemical coming out of the meat, out of the animal that was triggering it to close. But that didn't work. So he was left pretty much empty handed. But it was one of his colleagues who came up with the answer. It was a doctor, a physician named Burdon-Sanderson who published in 1878 the following paper in the proceedings of the Royal Academy of Sciences. And I think it's sometimes interesting to read these early scientific papers. This paper was entitled A Note on the Electrical Phenomena Which Accompany Irritation of the Leaf of Dionaea. Dionaea is the a Latin name for the Venus flytrap. And here's what he writes. Okay, let's go through this old English as he writes it here. If the leaf being so placed on the electrodes that a normal leaf-current is indicated by a deflection leftwards, the fly is allowed to creep onto it. It is observed that the moment the fly reaches the interior, so as to touch the sensitive hairs on the upper surface, the needle swings to the right, and leaf at the same time closes on the fly. What I neglected to tell you is that Burdon-Sanderson was one of the first people to actually study electrophysiology. He was one of the first scientists to notice action potentials, these change in electricity in muscles of animals. And he applied the same technology to studying plants using the Venus flytrap. So what he goes on to say is the same series of phenomena, which is the actual potential, the electrical current, present themselves if the sensitive hairs of a still expanded leaf are touched with a camel-hair pencil. In other words, in simpler English, what Burdon-Sanderson discovered in 1873 was that the impulse that causes the closing of the flytrap is electrical. And it actually resembles the signal of an animal nerve and muscle. What he and Darwin also showed was that it was not enough to touch only one hair. When one air was touched there wasn't an electric signal that went through the entire plant. It takes the touching of one hair and then several seconds afterwards a second hair that causes the depolarization. To get the depolarization, two hairs have to be within 20 seconds of each other. If one hair is touched and a long time passes before the second one is touched, let's say 30 seconds or a minute, there's no depolarization and the trap doesn't close. What this ensures is that the fly or the prey that is coming in the trap is of a certain size, it's rather large. That makes it worthwhile for the plant to close. If it's a very small bug that's doesn't have a lot of protein, not very nutritious, it'll touch one hair. But the small bug will take a long time til get to the second hair. By the time it touches the second hair, it won't cause it to be closed. So the trigger hairs have to be touched, close to each other in order to cause the depolarization, in order to cause the closing. You may notice here that there is a phenomenon that I don't want to go in to now, maybe in one of the later lectures. What we're seeing here is a type of memory. The plant, the Venus flytrap, remembers that the first hair was touched, stores this memory and if a second one is touched within 20 seconds, then it will close. If it's more than 20 seconds, it forgets that the first one was touched. We'll come back to that in later lectures. So it's not only that the touching causes the closing. It's not only that the touching causes an electric signal. This electric signal is very similar, or the mechanisms are very similar to what's happening in human nerves and muscles. Recently, scientists have shown that you can even just add an electric impulse by putting electrodes in the Venus flytrap, and without touching any of the hairs, this electrode signal will close it to close. If you put on certain medicines, certain drugs which stop human nerve signaling. For example, there are chemicals which will stop ion channels from opening in humans. These will also stop the ion potential, the amplitude of the action potential in the Venus flytrap or inhibitors of what are called aquaporins. Aquaporins are the type of channels which like water in and out of cells. So, inhibitors of human aquaporins also inhibit the closing of the Venus flytrap. I just want to emphasize this, the same chemicals, the same drugs that inhibit action potentials or that inhibit the movement of water in humans, influence plants the exact same way. Which leads us to realize that the basic cellular mechanisms are conserved between plants and humans.