Notice:  As of December 14, 2016, the MindBites Website and Service will cease its operations.  Further information can be found here.  

Hi! We show you're using Internet Explorer 6. Unfortunately, IE6 is an older browser and everything at MindBites may not work for you. We recommend upgrading (for free) to the latest version of Internet Explorer from Microsoft or Firefox from Mozilla.
Click here to read more about IE6 and why it makes sense to upgrade.

Biology: Plant Photoperiodism: Flowering Control


Like what you see? false to watch it online or download.

You Might Also Like

About this Lesson

  • Type: Video Tutorial
  • Length: 11:47
  • Media: Video/mp4
  • Use: Watch Online & Download
  • Access Period: Unrestricted
  • Download: MP4 (iPod compatible)
  • Size: 128 MB
  • Posted: 07/01/2009

This lesson is part of the following series:

Biology Course (390 lessons, $198.00)
Biology: Plant Systems and Homeostasis (14 lessons, $24.75)
Biology: Photoperiodism (2 lessons, $2.97)

Taught by Professor George Wolfe, this lesson was selected from a broader, comprehensive course, Biology. This course and others are available from Thinkwell, Inc. The full course can be found at The full course covers evolution, ecology, inorganic and organic chemistry, cell biology, respiration, molecular genetics, photosynthesis, biotechnology, cell reproduction, Mendelian genetics and mutation, population genetics and mutation, animal systems and homeostasis, evolution of life on earth, and plant systems and homeostasis.

George Wolfe brings 30+ years of teaching and curriculum writing experience to Thinkwell Biology. His teaching career started in Zaire, Africa where he taught Biology, Chemistry, Political Economics, and Physical Education in the Peace Corps. Since then, he's taught in the Western NY region, spending the last 20 years in the Rochester City School District where he is the Director of the Loudoun Academy of Science. Besides his teaching career, Mr. Wolfe has also been an Emmy-winning television host, fielding live questions for the PBS/WXXI production of Homework Hotline as well as writing and performing in "Football Physics" segments for the Buffalo Bills and the Discover Channel. His contributions to education have been extensive, serving on multiple advisory boards including the Cornell Institute of Physics Teachers, the Cornell Institute of Biology Teachers and the Harvard-Smithsonian Center for Astrophysics SportSmarts curriculum project. He has authored several publications including "The Nasonia Project", a lab series built around the genetics and behaviors of a parasitic wasp. He has received numerous awards throughout his teaching career including the NSTA Presidential Excellence Award, The National Association of Biology Teachers Outstanding Biology Teacher Award for New York State, The Shell Award for Outstanding Science Educator, and was recently inducted in the National Teaching Hall of Fame.

About this Author

2174 lessons

Founded in 1997, Thinkwell has succeeded in creating "next-generation" textbooks that help students learn and teachers teach. Capitalizing on the power of new technology, Thinkwell products prepare students more effectively for their coursework than any printed textbook can. Thinkwell has assembled a group of talented industry professionals who have shaped the company into the leading provider of technology-based textbooks. For more information about Thinkwell, please visit or visit Thinkwell's Video Lesson Store at

Thinkwell lessons feature a star-studded cast of outstanding university professors: Edward Burger (Pre-Algebra through...


Recent Reviews

Flowering biological clock
~ sdeniz

The video was straight to the point and clear. Focus on the biological clock of flowers and methods of tricking the plant.

Flowering biological clock
~ sdeniz

The video was straight to the point and clear. Focus on the biological clock of flowers and methods of tricking the plant.

You know, if you think about the idea of plant regulation, probably one of the aspects of plant development that is economically and, in terms of agriculture, the most important, is this aspect of flowering. What controls flowering in a plant? Now, I've already told you that somehow hormones must do this because it's a plant regulatory effect. But, how? How do hormones do that? You know, if you look through your hormone list there that you downloaded, you're not going to find a hormone that says, "Controls directly flowering." Let me tell you an interesting story.
I take a plant, and I put it in a room with light, and I cover up all its leaves but one. And I give that plant the right amount of light it needs, and I feed it and I water it. The plant will flower with just one leaf. But if I cover up all its leaves, it will never flower, besides the fact that it can't do photosynthesis.
Number two: and this is the coolest thing of all, if I take two plants, and I put one of them in the light, and one of them in the dark; and I bind their stems together somehow, so there's some way for these two plants to inter-communicate cell to cell, the most amazing thing of all is that the plant in the light will flower and so will the plant in the dark. What happened here? These plants communicated. There's only one way they can communicate and that's chemically. And plants communicate chemically with hormones. So, there's a plant hormone and we'll name it florigen. But no one's really found it yet. So, you future plant physiologists, that's the place to go.
Let's take a look at the history of plants. Now, I want to tell you that this whole idea of flowering is going to be controlled by something - there's a field that's just taking off in biology, it's the study of something called "circadian rhythms." "Circadian rhythms."
Circadian rhythms are defined as 24-hour built-in clocks. There's no coincidence that this term '24 hours' relates to the planet we live on. These seem to have been built into eukaryotic cells. I'm not talking about, like people, and I'm not talking about just your eukaryroids, but I'm not just talking about people and I'm not jut talking about organisms that say, go into some kind of ester-cycle and that's circadian rhythms. I am talking about eukaryotic cells that seem to have a programmed-in genetic cycle. Whatever that cycle might be, it's a 24 hour or so clock.
That's major. That is the basis of how we run our life functions. So we still have an awful lot to learn about circadian rhythms. But as I talk to you about flowering, I want you to understand something. It's the circadian rhythm that's going to make the flowering happen. But, it's literally other chemicals that might set that clock. That might start the 24-hour period, you see. Because, if I have a clock with its hands, it's going to be 24 hours as it turns around twice. But if I start it pointing in that direction, this clock will have started its 24 pass at a different time than this clock. So you have to understand, we're talking about setting clocks now. Let's set some clocks now, let's talk about setting clocks and plants.
In 1920, the first plant clock was discovered, and it was discovered by Garner and Allard and it was discovered when they were working on tobacco. See a lot of, tobacco is a huge crop when it comes to economics. And so a lot of our advances in plant biology have been with crops like tobacco that have economic importance. And they had this mutant tobacco that grew huge, it was a giant plant, a mammoth tobacco, and they were growing it in Maryland and they couldn't get it to flower. So they had the mutant that wouldn't flower. And they tried, and they tried and they tried, and they tried, all different environmental things and they couldn't get it to flower, but they finally did get it to flower. Where? Indoors. When? In December. And the when is the key. In the greenhouse, in December, it did flower. Why? Well, they realized why.
Turns out that this tobacco is what they called a short-day plant. And they discovered an aspect of plants called `photo'-`period', or photoperiodicity. Photoperiodicity: what does that mean? Well, photo, `light,' period. And they called it a short-day plant because it turns out that it needed, in their minds, it looked like it needed short days to form flowers. And when do you get short days? In December. And when the days were short enough, back in, when they were growing this thing, it was winter and the plant didn't grow anyway because it was dead. So, they had to get in indoors to find out this effect.
I like to use the example of my garden as often as I can. Right now, it's late summer, it's mid-summer right now, and my lettuce is shot. Now, I can plant lots of different things and have them grow, but lettuce grows best in the spring. And there's a reason for that. See, there's other plants, too. Not only are there short-day plants, but there are also long-day plants, and day-neutral plants. And if you think about natural selection, probably the day-neutral plants are generally going to be found in areas where the length of the day doesn't vary that much like it does in extreme northern, and extreme southern, areas. But, my lettuce is flowering right now in mid-summer, because it's the longest days of the year. And it being the longest days of the year, my lettuce is like, "Yeah, it's time to reproduce." Which ruins my lettuce because when the thing goes to flower and eventually produces seeds, the leaves which I eat get all bitter and nasty-tasting cause of other chemicals. And so, lettuce has had it. But it was great in April and May, and June.
Well, that all being said, that would be nice, except the 1940's came after the 1920's with the 1930's in between. And in the 1940's, we discovered something else. That this whole thing about day length was a myth. Now don't go throwing out your notebooks yet, because I'll explain it to you. It turned out in the 1940's that it wasn't the day length, but the night length. Now, let's re-look at our premise here. Our premise is short days and long days. Forget day-neutral. I mean, that's easy.
Short days are the same thing as long night. If you have a short day and it is 24 hours long, than the rest of it's night, right. Including twilight and dawn. And long day are short nights. Well, it turned out in a series of experiments done in the 1940's, that a very interesting thing came to light. And let's see what that is.
Let's take a look at these flowers right here. This iris is a long day plant. Take a look at it. When it is long days, and short nights, it flowers. And back in the 1920's, I would have said "well, of course, because in the short days and long nights, it doesn't flower." But here's what we found out. What was found out was that if you take this thing and give it a short day when it shouldn't flower, but in the middle of the night, you flashed it with light, a flash of light, it flowered. Wow!
This suggests that it is the amount of uninterrupted darkness, not day. It's the opposite of what we thought. It's not day, but uninterrupted dark. That, in this particular long-day plant is going to cause flowering here. So in this long-day plant, it's not a long-day. Look, in this long-day plant that's short-night and you've convinced this plant, you've tricked its clock. And you've convinced it by that momentary flash that it now has not had this long period of uninterrupted darkness, but two short periods of uninterrupted darkness just like it would in a long day.
Let's look at another example. Here's a plant that flowers in what? In short days. Right? So, therefore long nights. And if you take a look at this thing, well, than when it has short nights, what happens? No flowering. So, let's give it a short day, it should flower, but in the middle of the night, let's trick it. Let's flash it. No flowers for you! And so, therefore this thing won't flower when we flash it. Well, think of the economic importance of this. The chrysanthemum growers went nuts. The chrysanthemum growers, a chrysanthemum, they were long-days. They said, "Oh, this is a long day plant, right." Well, if it's a long day plant, let's just give it long days. We'll keep the lights on for ten to twelve hours. They didn't have to. They saved themselves tons of money. Why? Because they wanted it to flower, right, it's a long day plant, let's flash it in the middle of the night. And if we're going to flash it in the middle of the night, we don't have to lengthen the day any more. Flash it at night, it thinks it had a long day. How easy.
And here's one for you.... sugar cane. You know, they used to illuminate sugar care fields because they didn't want it to flower. Because sugar cane, see the illuminating would make it a long day, because sugar cane is a short day plant, right? So here's what they did. What they did with sugar cane was the night flashes, so they flashed at night... and guess what? No flowering. And that's a good thing because we don't want flowering sugar cane. We want cane-y sugar cane. No flowers. It's a waste of sugar. So you think you got the whole story now.
Well, I have one question I want to leave you with. What's causing this? What's causing this? Why would light change the biological clock of a plant. It's got to have something to do with chemicals. Which chemicals and how? That's going to be another story!
Plant Systems and Homeostasis
Photoperiodism in Plants: Control of Flowering Page [2 of 2]

Embed this video on your site

Copy and paste the following snippet: