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Biology: Lateral Meristems, Vascular Tissue


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About this Lesson

  • Type: Video Tutorial
  • Length: 13:54
  • Media: Video/mp4
  • Use: Watch Online & Download
  • Access Period: Unrestricted
  • Download: MP4 (iPod compatible)
  • Size: 149 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: Plant Development (5 lessons, $11.88)

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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Recent Reviews

Lateral Growth
~ sdeniz

Good use of images and straight to the point.

Lateral Growth
~ sdeniz

Good use of images and straight to the point.

Now you guys, I know you're experts on the primary plant body, but how do all these other things grow? For example, how does a root develop rootlets? How does a stem develop branches? How does a branch develop twigs and twiglets? These are pressing problems that we need to address. Let's do a root very quickly. The thing about a root that is so interesting--remember that dicot root and remember the fact that inside of the stele there was a layer of cells called the "pericycle?" Well, this pericycle is undifferentiated tissue. And what it can start to do--remember, it's inside of the root itself--what that literally starts to do is it starts to divide and grow a new root coming from this, and so we end up, because this is undifferentiated tissue, it can then divide and actually end up forming an entire new root with a new stele, this central cylinder called the stele. With all of this storage out here and look what you have. You have a rootlet where water can go in, get to the stele, that is continuous with the main stele, which will go up to the plant. That's pretty cool stuff, but it's even cooler in the stems.
Let's go back to this idea of the primary dicot. And you remember that the primary dicot had these vascular bundles, and in its primary body had vascular bundles arranged so that they had cambium in between them. Well, imagine what you could do if these vascular bundles, if the cambium connected like so. Wow. That would give you a ring of cambium. And then that ring of cambium might be able to make a ring of xylem and a ring of phloem. And we would have rings on a tree. And I know that sounds familiar to you, so let's see how that develops.
So we begin with a discussion of rings. Perhaps we'll end with one too. Let's see what cambium does. Cambium is literally undifferentiated tissue that will give rise to new xylem and phloem. That's exactly what it is, so that's why that is called procambium. And cambium, you remember, is lateral meristem. Let's see what cambium does. What cambium does--now this is an interesting graph. Let's think about it. This is the diameter of the tree. So remember I said the great things about dicots is they can get wider and wider and wider? So if I put this sideways and the tree is growing up, you can get the idea that this is growing wider this way.
So here's what cambium does. Cambium will do a mitotic division; it divides into two cells. And one of those cells stays cambium, and the other one is kind of like an undifferentiated tissue. We're referring to it here as D. But then D will turn into X and it will make new xylem inward. Well, then the cambium will divide again, and this time--and I don't want to say "this time" because this is the way it always happens--it ends up making, as you'll see, much more xylem than phloem, but at least this time in my diagram the cambium divides again, forms another D, which will then turn into phloem. And then the cambium divides again, and again D and X, and then D and then P, and eventually what ends up happening is the plant is getting wider and wider and wider. The thing that you have to assume happens is that cambium also gives rise to more cambium, so it goes from a one-celled thick layer to a multi-celled thick layer, because otherwise the cambium would end up getting split up and be in between xylem and phloem, and that would never work. But this shows how lateral growth--now imagine the analogy with the nail in the tree, imagine if you put a nail into the tree say right here, within a few years that nail would start to get covered up as the tree grows outward over the nail.
There's more. Let's take a look at these steps and how this is going to happen. Let's take a look at the top of a branch. Now why the top? Well, the top of a branch is where you have the primary growth. This is the primary stem. And what do you see? You see in nice diagrammatic form the vascular bundles spread around, no ring of cambium. But then as we move on we start to see something else very interesting happening. Moving on, what I mean is we're going to older tissue. Remember, you can go down a branch and be temporally distributed as well as biologically distributed. And so this is older tissue down here. And by now look what you have formed. You have formed a ring of cambium and some other very cool things have happened. And by the time we get way down deep, we still have that ring of cambium, but somehow this thing is getting thicker and thicker and thicker.
Let's take a look at the cellular events that correspond to these three diagrams, and we'll start with our earliest section, that is at this point as simple as you know it to be. We have pith in the middle of the woody stem. Not a lot, but a little bit of pith. But then what we want to see is our primary vascular tissue. So this is what it looks like after primary growth. So if it's a maple tree it's maybe in its first month of growth before it really starts secondary growth. What do we have? We have an inner layer of xylem. We have a cambium ring or beginning to form a ring, and we have phloem on the outside, and the epidermal layer outside protecting it.
Let's see what happens next. As we move down into the later stages, we start to get what is termed "secondary growth." In secondary growth, here's what's going to happen. The purple was the primary growth from before. So once again, the purple is the xylem. But look what's happened. There's a new layer that's appeared, and that new layer is the secondary xylem. So how did we make secondary xylem? You know the answer to that. Cambium divides and forms xylem, right? But the cambium stays there. We still have cambium. So here's the thing. Let's go through the steps in the lateral growth of a stem. So the number one thing that happens is we're making new xylem and phloem. So step one in the lateral growth of a plant--new xylem and phloem. So we get our secondary xylem and out there what I have in as blue is the primary phloem, and there's the secondary phloem. Do you see why? Think about it. The primary phloem was next to the cambium before. The primary phloem used to be here where the white is, but the cambium gave rise to new phloem, pushing the blue out and making the secondary phloem there. So we have secondary phloem, we have primary phloem, because it's now outside, we have our cambium, we have our secondary xylem, and we have our primary xylem left behind in the center, stuck to the outside.
And now we have this strange new yellow layer coming up right up here, and before I had shown you the yellow layer in the primary xylem as cambium. Did the cambium slide out there? No. Guess what? We have two cambium layers. I'm going to feed this to you a little at a time here. It turns out this cambium layer, which is going to be your lateral meristem, which is going to cause growth out like this, is going to give rise to xylem and phloem. But stay tuned to this one. This layer here is called "cork cambium," and it arises once you start your secondary growth. So you're saying, "What's its function?" Just hang on to that question. If you're asking that question it means you're thinking and I like that.
Let's move on then to what happens next. Well, as the secondary xylem and phloem begins to grow, you have this effect of the potential for starvation. In other words, what can happen is as this thing grows out and the phloem is getting pushed out further and further, you need to somehow channel materials to the inside of the stem, and so you start to form these things called "vascular rays," which you're seeing happening right in here. And these rays, their function is to take the phloem materials and bring them to the interior of the stem. So these vascular rays are forming, if you think about it, in a direction perpendicular to the growth of the lateral stem. So the lateral stem is going out this way, the apical stem is going up this way, and meanwhile these things are going across into the center of the tree, and so they form these literal ray-like things. See the rays in this picture? So there are the rays, and they're going across like that.
So what's the second thing that's going to happen? The phloem is going to form the vascular rays. Let's talk about this cork cambium as we go to the third layer of what's going to be going on here. What's going to happen next is we are going to form a cork cambium, and the cork cambium is going to form, because as this thing pushes outward, guess what's going to happen? The epidermis is going to get destroyed. It's going to be pushed off, it's going to die, it's going to fall off. And so the epidermal cells are going to fall off. So what you need to do is form a new type of tissue. It's a type of parenchyma taken from the cortex, and it becomes meristem. So it's undifferentiated parenchyma that becomes meristem, and guess what it does? It makes a substance called "periderm" or bark or cork, as you know it. Highly waterproof, tough material, loads of lignin in it, that's going to form this outside thing called bark. And so the phloem continues to push outward, and the phloem dies, and that becomes part of the periderm, too. So if you said to me, "What is the periderm made out of?" I'd have to say squashed phloem, cork cambium, and cork, as it is squashed outward.
That's great stuff. One last story. What's up with this counting the rings of a tree to tell how old it is? Can you really do that? Yeah. Watch this. When is it the wettest time of the year in most places in America? It's wettest in the spring, and your spring xylem that is made is going to conduct the most water. So spring xylem is going to be really big. There's the primary xylem. But then as the year moves on it gets smaller and smaller and smaller, structure follows function. Now remember, we have our cambium here. Now, the winter goes into a state of dormancy and what's going to happen next? What's going to happen next is you're going to make spring xylem again. And what' going to happen next? It's going to go dormant. And then you're going to make spring xylem again. Guess what? Rings on trees in the tropics don't have this well-differentiated tissue that you can tell the rings as well, so this particular tree would be one, two, three years old. And so that's the way we tell the rings on a tree. We tell them by the way the phloem grows and the xylem grows inward. By the way, what is xylem? Do you guys know a common name for xylem? It's called wood. That's exactly what the wood is, because as we go from the outside to the inside of this tree, we see this kind of area right in here which is merely pith, but we see spring xylem, small xylem, spring xylem, small xylem, spring xylem, small xylem. This is a three-year old tree. There's the phloem, there's the rays coming down, and there's the periderm. And you thought plants were boring.
Plant Systems and Homeostasis
Plant Development
Secondary Growth: Lateral Meristems and Secondary Vascular Tissue Page [1 of 2]

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