Welcome back to the second week of What a Plant Knows. In this lecture today we're going to talk about what a plant sees. And to start the lecture I want you to think about this. Plants see you. Now of course I'm not talking about they see you like I see you or you see me on the lecture, but plants actually monitor their visible environment all the time. They see if you come near them. They see when you stand over them. They know if you've painted your house. They see if you've moved their pot from one side of the house to the other. They even know if you're wearing a red or a blue shirt. Now of course plants don't see in pictures like you and I do. So what do I mean when I say see? So this is the view from my office at Tel Aviv University. You can see here the Mediterranean in the background and the green part of the middle of the campus. We all agree that this is seeing for us. And if you were blind, this is what you would see, nothing, no picture only black. So we all agree that there's a difference between seeing and being blind. But let's redefine this. If we would take someone who's blind, who's only seeing this black here what you see on your screen, and through some type of intervention, allow them to just differentiate between different shades of gray. For this person, even though he doesn't see pictures, would this be considered a form of rud, rudimentary sight? Would this be a type of seeing? I think we can agree that seeing, gray and white and black is better than seeing nothing. So it must be a type of simple sight. And if we could now allow this person, to differentiate between red and blue, you maybe see the blue sky above and some colors below it. This is even a slight improvement on just seeing gray. So if we can agree that this is a form of sight, that sight even for people doesn't have to be only pictures, then I think we can definitely see, or agree that plants do have a form of vision. While plants don't see in pictures like we do. They do see in many ways in colors that we can only imagine. For example, plants see the same UV light that gives us sunburns. And infrared light that, heats us up. These are two types of light that we are blind to. Plants can tell when there's very little light, like from a candle or when there's a strong light in the middle of the day, or when the sun's just about to set in the horizon. Plants know if the light is coming from the left or from the right, or from above them. They even know if another plant has grown over them, blocking their light. And they know how long the lights have been on. What is sight for humans? So how did I catch this baseball? Light signals came from the ball, bounced off the ball into my eye, went from my eye through to the brain, the brain, computed what was happening, sent the signal out to my hand and luckily I caught the ball. So what we have, we have a detection of the light. A transfer of the light signal and some type of response. What we see as light is actually part of the electromagnetic spectrum and we actually only see a small part of that, what we call visible light. It goes from the blue light, which is about 400 nano-meters which are the shortest light waves, up to red light which is about 700 nano-meters which is the longest light waves. But this is only as I said a small part of the electromagnetic spectrum. Slow smaller waves or what we see as ultraviolet light, we can't see those, all the way down to x-rays, which go through our body and even from gamma rays coming from the universe. Longer light waves, or longer electromagnetic waves could be infrared heat waves, again which we're blind to. Microwaves which heat our food, radio waves which can be even a kilometer in length, which we don't see either. So we only see a small part of the electromagnetic spectrum. When light is coming off the baseball, the light bounces off the baseball, goes in through my eye and hits the retina, and from there it sends the signal to the optic nerve. Within the retina there are cells which are specifically attuned for what we call visible light and these are the cone cells and the rod cells. The rod cells let us see black and white and different shades of grey, while the cone cells let us see color. There are actually three types of cone cells, some that see red, some that see green and some that see blue. So, for example, red light coming off the stitches of the baseball are absorbed only by the red cones and then they send that signal to the brain which we translate as a red color. If we see purple light, for example, coming through, the purple light would be absorbed both by the red cells and by the blue cone cells and our brain translates that into purple. So human sight, again to repeat this. We have photoreceptors in the retina, which absorb the light. The light is signaled to, or tran, the light signals transfer to the brain. It's translated into pictures, and this signals some type of response, or in this case, I caught the ball. So we have perception, signal transduction, and the response.