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Biology: Alternation of Generations: Ferns


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About this Lesson

  • Type: Video Tutorial
  • Length: 9:26
  • Media: Video/mp4
  • Use: Watch Online & Download
  • Access Period: Unrestricted
  • Download: MP4 (iPod compatible)
  • Size: 100 MB
  • Posted: 07/01/2009

This lesson is part of the following series:

Biology Course (390 lessons, $198.00)
Biology: The Evolution of Life on Earth (34 lessons, $64.35)
Biology: Alternation of Generations (3 lessons, $5.94)

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.

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You know, if you look at the fossil record of plants, which is very, very well preserved, we think of the idea of the mosses being the first divergent off of our monophyletic tree of plant phylogeny, then we find a very interesting plant. It's called Cooksonia. I don't want to tell you too much about Cooksonia except for one thing. Remember the moss, and the moss had the sporophyte, and the sporophyte came off of the gametophyte. So we had the gametophyte, and the sporophyte came off of there and it had the single unbranched kind of knob at the end. Cooksonia had branched sporophytes. Now, you may not be getting all excited over this, but it was a major development in plant evolution. So I want you to remember that this extinct plant seemed to be a branch that was going in a branched sporophytic direction. Got that? We'll be back. That was a little hint of what's to come.
We're talking about alternation of generations, and we're talking about how plants do this sexual life cycle as an adaptation to life on land, and an adaptation that can continue to develop. We're about to go to our next group of plants. Now, once again, let's take a look at our plant phylogeny and remember what happened. We, of course, had our charophytes, and they were our first group to branch off. They're our out group. And then we had the bryophytes, and remember the bryophytes with their story, the mosses, and the fact that they grow real low to the ground, but what that allowed to happen was sperm and egg mixing, and it mixed when the water covered them. What happened next was this incredible development in plantdom, and this incredible development was the ability to literally grab water out of the ground and shoot it up inches and eventually feet, because as these vascular plants started to develop, they could get taller and taller, and in fact, natural selection would select for the taller ones because they were competing for light.
But that presented a dilemma and the dilemma was this. How are you going to fertilize an egg if you can't depend on water? Watch what happens. Think of a moss. You're saying, "Wait. You mean to ask `fern', didn't you?" No, no, no. Think of a moss. Now you're thinking of a moss. What are you picturing? Well, let's go through our little life cycle again. Remember, we have sporophyte, produced spores by meiosis. The sporophyte is 2N, the spores are N. we get a gametophyte, and the gametophyte is going to produce gametes, and then we get sperm and egg, and that's going to give me a zygote, and the zygote is going to develop into my sporophyte, the zygotes going to be 2N. You know all that, and I've asked you to picture moss before, and I said, "What is a moss?" and you pictured it and it turned out that that was the gametophyte.
Ready? Picture a fern. Do you know that you're not picturing the gametophyte anymore? You're picturing a sporophyte. Yes, you are. You are picturing a sporophyte. Now, wait a minute. In the mosses, the gametophyte was the dominant plant. That's the one you guys pictures. But I just said that when you pictured a fern, you're picturing the sporophyte. Well, if that's the sporophyte, which in the mosses was this little thing, what could possibly be the gametophyte of a fern? The answer is you just stumbled on an evolutionary tendency, because as we went to the bryophytes, to the tracheophytes, a very interesting thing happened, and this is big. You had a reduction of the gametophyte and the sporophyte becomes the dominant plant. And now, lte's go back to Cooksonia. Remember Cooksonia with its branching sporophyte? Aha!
So we have the reduction of the gametophyte and the sporophyte was the big boy. Let's see why. Water. Adaptation to life on land. Sperm, eggs. Let's take a look. We're at a sporophyte right now. What do you remember about sporophytes? You remember that sporophytes are 2N. There's your fern, your typical little pretty fern. They're going to produce spores. Some of the vocabulary is different. This is not a course in vocabulary. They're called sporangia--don't worry about it. But I do have to tell you this, if you ever have ferns growing around your neighborhood, go outside and look on the back of the leaves, and on the back of the leaves you'll see a bunch of brown structures. These are called sori, and within those is where the sporangia are on the back of a fern's leaf. Go out and look. So in order to form spores we've done the process of meiosis. So what does that make the spore? N.
What's the problem here? The problem is we're about to make a gametophyte. What's the gametophyte? The gametophyte is going to make a gamete. What's a gamete? Sperm and eggs. So you've got to get the sperm to the egg. How are you going to do that if you can't depend on water? The answer is, you can. Let's just make the gametophyte tiny. Let's make the gametophyte about a half an inch tall, and let's make it down at the bottom of the fern, down in the bottom of the ground, and let's let it depend on water, and let's give it both organs. So let's have this particular gametophyte be hermaphroditic and let's let it have both antheridia and archegonia. How cool. Now, when the thing gets wet, what do you have? You have a medium for your sperm to swim and a place for them to swim.
Let's not lose track of what we've got here. We've got a spore, we've got a 1N. That 1N spore has grown into my 1N what? What is that? Well, this is the sporophyte, so this is the gametophyte. It's tiny. It's down at the bottom on the ground. You've never seen one of these--they're little. I have, but I know what I'm looking for. So the sperm can now swim and fertilize the archegonia, and what are you going to get there? You're going to get a zygote, and once you get the zygote after fertilization, what's going to happen next? You have a 2N zygote which is going to grow into your 2N sporophyte, and the cycle is going to be completed and continue. So you see, adaptations to life on land--very tricky. Not only have they evolved a large branching structure so that they can increase their exposure to sunlight and conduct water up, but they've gotten around the problem of what am I going to do with my sperm. What they've solved is they're going to let their sperm swim. They're going to have that gametophyte low, they're going to have a situation where you have both organs archegonium and antheridium in one plant, and now a continued progress of tracheophytic evolution. The most important thing here--the reduction of the gametophyte.
What do you think is going to happen next? We're going to get into seed plants. How different is this going to be? We'll see.
The Evolution of Life on Earth
Alternation of Generations: Mosses, Ferns, and Gymnosperms
Alternation of Generations: Ferns Page [2 of 2]

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