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Biology: Plant Phylogeny, Generation Alternation


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About this Lesson

  • Type: Video Tutorial
  • Length: 10:25
  • Media: Video/mp4
  • Use: Watch Online & Download
  • Access Period: Unrestricted
  • Download: MP4 (iPod compatible)
  • Size: 111 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: The Colonization of Land by Plants (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.

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I want you guys to do me a favor. Don't condemn plants before you've learned about them. This drives me crazy with students. Maybe the scientific establishment is partly to blame about this. When you think of experimentation--everybody wants to experiment on something. Very rarely have I ever bumped into a student that said, "I want to be a botanist and work on broccoli." I don't know why you guys don't want to work on broccoli. I don't know why you guys don't want to work on plants. I think I know why--they're not dramatic. They don't run around. They make crummy pets. And the point is that plants are such a great system to think about and work with because of all the things that they did. Now, you're saying, "Plants don't do anything," and that's where you're wrong. Plants do a lot. You've just got to look closely.
Let's talk about plants, and let's talk about the evolution of plants. Now, you know that plants had a lot of problems as they moved to land, and yet, once we figured out that those charophytes were, indeed, the out group of plants, and were going to be the ones that we could base our monophyletic tree on, now we can start talking plants, and we can take a look at the way plants have diverged temporally, and this time, because of the fossil record, because plants make some pretty nice fossils, we can actually fill in some of the blanks.
So I'm going to go down to about 475 million years ago or so to the origin of plants as we now know it, and this first particular line of plants, the charophytes, is right here. So there's my out group. Now, I want to talk to you about the phylogeny of plants, and I want to talk to you about some of the things they have in common, and then we're going to spend a lot of time together talking about something that's really very exciting about plants. Hang on for that one.
The first branch that we go to with plants--remember, animals have branches, too. We talked about symmetry and radial symmetry and bilateral symmetry and diploblastic and triploblastic embryos. You guys know all that. Well, the plants have an interesting division. The very first division that comes up is this division of making vascular tissue. That's right here. What we have is that at about this point 400 million years or so ago, we have the formation of tubes--veins, vascular tissue. Now, you and I call that xylem and phloem because we're botanists, but most people who eat celery say, "Oh, I've got a vein in my teeth," not realizing that they really just have a vascular bundle in their teeth. So we have this first division where we get xylem and phloem.
Up at the top here in my monophyletic tree, there's the bryophytes, the mosses. Mosses that have no true roots, no true stems, no true leaves in the sense that you have leaves like maple trees and grass plants and things like that. But they're plants. They just lack vascular tissue, which tells you something about them. Since they can't conduct water, do you expect them to be tall or short? Well, if you don't know the answer to this one, you've never seen a moss, because mosses grow very short to the ground. Why? Because they don't have vascular tissue.
Well, then the next thing that happened was the development of a structure called the "seed." Remember, this is all about adaptation to life on land. Go back to mosses. Where do they live? They live in shady areas where it's wet. Why? They don't have roots. They don't have a way to get water around; they don't have vascular tissue. Vascular tissue sounds important, doesn't it?
Well, seeds are pretty important, too. So we come to this next division here where we get our seed plants. So we have a group of vascular plants, xylem and phloem plants, but they're seedless. You don't plant seeds to get ferns; you plant spores. More on that later. And then the seed plants divided up into those plants that form naked seeds, gymnosperm, and those that have hidden seeds, or seeds in a fruit, angiosperms. That word "gymno," you didn't know that meant naked, did you? Guess where the word "gymnasium" comes from? Yes, they used to run around naked. So here were have gymno sperms and the gymnosperms have their seeds in cones, and therefore they're naked, and the angiosperms have their seeds in fruits, and therefore they're covered. I could keep going. There's a subclassification of the angiosperms called monocots and dicots, but we're not going there.
So there is the plant phylogeny, and I have to tell you that if we're going to really truly study the evolution of plants--we'll do a lot on the plant body together. We'll talk about the development of the xylem and the development of the phloem and what it does, and we'll talk about the function of roots, and we will really put together a picture of the anatomy of the plant body. But I have to tell you something. Just like those animals moving from water to land, and the adaptations they had to make for development, and the adaptations they had to make for fertilization, and the adaptations that they had to make to get things growing within their bodies, and the adaptations that they had to make to feed their young with milk or to put yolk inside of a big egg, and the adaptations they had to make to feed their babies if they didn't have milk--all that has to do with reproduction and development, and it's true with plants, too. The idea that plants have evolved new reproductive systems is the single most important aspect of understanding their phylogeny.
Let me start out with the way plants reproduce, and this is going to be a surprise to you. You think of plants probably as seed, ground, plant, seed, ground, plant, seed, ground, plant. No. Plants have a very unusual life cycle. Here's the way it works. We're going to start out our discussion of plant reproduction with the part of the plant called the sporophyte. This word "phyte" means plant, and the sporophyte makes a spore. That's why it's called a sporophyte. All plants make spores. Pine trees, maple trees, flowers, they all make spores. You're saying, "Wait. They make seeds." Hang on. What is a spore? A spore is a single cell that can give rise to an entire organism. How is that different than a seed? A seed is an embryo. A seed contains an embryo. A spore is a single cell. There's something else I want you to know about spores--they're haploid. They're N. Wow. Could that mean that a sporophyte, which is 2N makes spores by meiosis? I hope that's what you said. I hope you said, "Wow, could meiosis be occurring here?" I'll be really psyched if you did. The answer is yes. So aren't these gametes then? That's the way you form gametes--by meiosis. Plants don't form gametes by meiosis. They form spores. Well then, how do plants have sex? Let's watch.
So when the spore lands it grows into something called a gametophyte. What do I mean by grow? By grow I mean it does mitosis. So holy mackerel, what does that tell you about this thing? This is a plant. It's a thing that grows, just like the sporophyte. It looks different than the sporophyte, and in mitosis there's no fertilization happening here. This must be a plant with half the number of chromosomes as the same plant up there. How bizarre. It gets better. The sporophyte makes spores; what do you think a gametophyte makes? You've got it--gametes. But wait a minute. How could something t hat's N make gametes, because aren't gametes supposed to be N? And the answer is, "you bet," so you tell me, how does gametophyte make gametes? Mitosis. Oh boy. Am I rocking your world here? Nobody said plants were the same as us.
Now comes the best thing of all. What do you think those gametes do? Yes, plants have sexual reproduction. So those gametes do fertilization. What do you get when you get fertilization. A zygote. What do you think that zygote grows into? How cool is that? Look at this thing. A plant has two different parts of its life that don't look anything like each other. It makes sperm and eggs with mitosis and these things called spores by meiosis. I'll be you can't wait to figure out what's going on here.
The Evolution of Life on Earth
The Colonization of Land by Plants
Plant Phylogeny and Alternation of Generations Page [2 of 2]

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