I want to move now to talk about another response of plants to light and that's when to flower. In the early 20th century the tobacco farmers in Maryland discovered a new strain which they called Maryland Mammoth which rather than flowering in late summer and giving seeds continued to produce more and more and more leaves. You could see in this pictures, this led to a huge, huge plant which is called Maryland Mammoth. And you would think that the farmers would be happy with such a plant because you'd get more leaves per yield. But unfortunately because it never flowered and only got killed in the winter, they couldn't get seeds to propagate this plant. This was actually a huge problem and a very interesting question what caused this plant to stop flowering? And this was a question that was taken up by scientists in the United States Department of Agriculture. And what they discovered was that if they would take the Maryland Mammoth's plants and put them in a shed early in the afternoon, they can induce them to start flowering. And this is how they discovered what we called photoperiodism. Photoperiodism is the plants response to a changing day length so that we know that there are plants, for example, which will only flower when the length of day gets shorter. We call these short-day plants, for example, chrysanthemums, soybeans, or the Maryland Mammoth. On the other hand, there are other plants which we call long day plants which only flower as the days get longer. This is such as irises or wheat. These are usually plants that flower in the spring or early summer. There's a actually a third class of plants which don't really show photoperiodism. They will flower when their plant gets to a certain body size, such as tomatoes or dandelions. We called these day neutral plants. So photoperiodism helps a plant know what is the length of day, and when is the right time of the season in order to flower. Now plants adapted photoperiodism according to where they grew wildly. For example, in the middle of the summer in Canada, the days are much longer than what they would be in Florida. Whereas in the winter the days are longer in Florida than they are in Canada and it's also warmer. So for example, if you would take Maryland Mammoth which dies in the winter in Maryland when there might be snow on the ground and put it in Florida. There the short days in do induce flowering. And there wasn't the cold for it to die. So each plant, for example here we have a plant called cocklebuer which grows up in northern Canada northern United States. It will only flower under a very long day. If cocklebuer was grown in the southern United States or southern Europe it would never flower. So if we look at photoperiodism the question becomes what's important? Is it the length of the day? Or is it the length of the night? And what type of experiment could we do in order to check this question? So the experiment that was done by the scientists in the United States Department of Agriculture, was to turn on the lights in the middle of the night. So if you take a short day plant, for example, such as chrysanthemum. And put it in conditions where it should flower with a short-day. But turn on the lights in the middle of the night, you inhibit the flowering. And it's enough to turn on the lights for just a few minutes. On the other hand, if you take a long-day plant and keep it in conditions of short-day, these are conditions which would inhibit flowering. But you turn on the lights in the middle of the night, again, just for a few minutes. You can induce a small day plant to flower under conditions of short-day. So we learned from this is that the plants are not really sensing how long the day is, but how long the night is. Now from an agricultural point of view this is very important. Because this meant that for example that flower growers can induce their plants to flower whenever they want irregardless of the conditions outside by changing the lights in their greenhouses. So we could get chrysanthemums flowering all year long just by manipulating the light conditions. Now we saw earlier that plants bend to blue light. They differentiate between the colors. So what color of light are the plants seen here to induce flowering. So the obvious experiment would be to turn on different colors of light in the middle of the night and see if we could inhibit flowering in the short-day plant or induce flowering in a long day plant. So here's the experiment that was done in the same lab in the United States Department of Agriculture and now we're talking for the history buffs among you were talking about in the middle 20th century. What they did is turned on different colors of light. And what they found was that only one color could either inhibit or induce flowering. A flash of red light in the middle of the night would inhibit flowering in a short day plant. And it would induce flowering in a long day plant. Blue light wouldn't work. Any other color wouldn't work. Only red light. So already here we're seeing just like the weak in differentiate between blue and red light, plants also differentiate between blue and red light, but now here it gets even more interesting, because after you give the flash of red light if it's followed by a flash of what's called far red light. Far red light is about 730 nanometers. This is the light that we barely see as the sun is going down. It's very long wavelengths, we're basically blind to it. That would inhibit the flash of the red light. The far red light inhibits the action of the red light. So if you gave a red light, you inhibit the flowering of the short day plant. If you give red light and then far red light, it's as if you had never turned on the lights to begin with. It will flower. Or if we're looking at the long day plant, you give the red light it would induce the flowering. But if you give red and then the far red, you inhibit it. We can continue this game, if after the red, you give far red, and then again give red, you have either an Inhibited flowering in the short-day or induced it in the long day plant. Give again far red, you inhibit or you've canceled out the effect of the red light. So in other words, we're actually seeing here a type of memory. The plant remembers what the last color it seen. If you're giving it red light, for a long day plant, you're activating flowering. If you're giving it far red light, you're inhibiting the flower. Give it red light again, activating.