Biology: Primary Growth: Root Growth & Development

Root differences between monocot and eudicot

10/29/2012

~ sdeniz

Explains clear and straight to the point the root differences between monocot and eudicot.

Good overview of the root system

01/29/2011

~ Balasaheb

But explanation for protoxylem and metaxylem is missing. also the secondary xylem.

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Let's talk about how the embryonic plant starts to differentiate at the level of the root. So let's start below ground and eventually we'll work our way up to the stems. Let's talk about roots. There's a root. It's a root perhaps like you've never seen a root before because we've sectioned this root in what we call a longitudinal section. So we've cut it open and we're looking inside. So what we see here are some things that may seem familiar up here, and the other region down here that seems just a bit bizarre, but as you'll see, it all fits with what you already know about plant differentiation and growth. Let's see what you know.
Think about a root. A root has to push its way through the soil. And remember that there is a material called "apical meristem" on all roots and on the apex, the tips of all plants. It's the place a plant grows from. And this particular root has a zone right here where there is meristem. And one of the types of meristem there is called "apical meristem." And here's the thing. If you are considering the fact that the meristem is a very fragile tissue, and the root is growing through the dirt, that should get damaged, and the answer is, it would except there is a cap called a "root cap," and that root cap is a group of cells that are literally hardened off so it can push its way through and actually lubricate--in addition it also will produce some secretions so that the root can grow through the soil and literally snake its way in an ever-widening and downward growth, until, of course, it gets to layers where it can't penetrate, like rocks, or there's no more water gradient to grow along.
But the other thing I want you to think about is how do all of these primary structures in the root form? How do we get this green thing? How do we get these blue things? I haven't told you what they are yet--and this pink region right here. Well, there's more to this. The top part of this region is where you're going to form your three layers of pre-differentiating tissue. So what we have are literally, in this case, a very clear delineation of our three layers. And let's just go over those one more time. So right here is where you're going to start to form your procambium, and that would be in the central part of that cylinder. Again, it's tough to make the transition from a slice to a reality. But you're going to have your procambium there. You know about procambium? You're going to have your ground meristem, and then last, but not least, you're going to have the protoderm that's going to give rise to your dermal materials.
Now, why is this so hard to visualize? Well, remember what's going to happen as the root continues to grow downward or outward, these are going to then have time to differentiate, and so they're going to start to turn into things--things like root hairs, things like xylem and phloem, things like sclerenchyma and collenchyma--supportive tissues. And that's going to happen up here. So as much as it is physical, it's temporal, too. So if this root is growing in this direction, this portion of the root is older, and therefore it has had time to differentiate. And so we actually add zones to this root. What you're going to get is a place where cells are going to elongate. We're going to call that the zone of elongation. Look what's happening to the cells. They're small down here; they're longer up here. So we're going to have the zone of elongation, and then we're going to have the maturation zone. And obviously this isn't like borderline, but we're going to get differentiation, too, where things are going to mature into the cells they're supposed to occur. So at any given point you can kill a root, section it, and say, "Wow, look. These cells haven't elongated yet." "Oh, look, but they're just starting to elongate and they certainly haven't differentiated yet, and they're not going to differentiate until further down when they're older." So that works, and that's why you don't have any root hairs down here, because they haven't differentiated yet.
Okay, so we elongate and we mature. What are we elongating from and what are we maturing from and what does it look like if you cut this in another direction? Well, if you cut it in another direction we get an interesting thing. That was kind of a generic monocot/dicot root. Let's take a look at the differences between the roots and how the procambium, ground meristem, and the protoderm eventually will give rise to new tissues.
Let's start with the monocot root. Why do I want to contrast these two? I want to tell you that structure and function is everything. Remember the monocot roots had fibrous roots? Long, thin roots? That was what monocots did? Well, there's not going to be a lot of storage in a monocot root. Keep that in mind. There's going to be some. But wait until we see a dicot root. Structure follows function. Let's take a look at a monocot root. A monocot root in cross-section is going to look something like this. There's going to be out here the dermal tissue. That's going to be the epidermis, and that came from protoderm. That's easy. We have another dermis, if you will, which is going to be in the center, and that's going to be called the "endodermis." But what I really want to talk to you about is the differentiation of some of these other tissues.
Remember ground meristem? Ground meristem was going to give rise to storage tissue, things like parenchyma. Well, guess what? A monocot root has storage out here, and it has storage on the interior, too. And then it also has storage in the center, too. And in the center it has pith, which is merely parenchyma cells.
So this part is called the cortex, which is outside the endoderm, and the inside is called the pith. So there's the pith. And where's the xylem and phloem? Where's the transport going to happen? And now it gets very interesting, because we're going to see that the xylem--I'm going to go to a close-up of the endoderm. So let me draw the endoderm. There's the endoderm. And remember, we've got all this pith in the center. But the xylem cells, the things that are going to conduct water, are going to be spread around the interior of this, kind of like this. That's going to be the xylem cells. So we have the xylem cells surrounding the pith, and then the phloem cells are going to be outside of that, so we've got to get some phloem cells out here. So what are looking like two differentiated regions of xylem and phloem. And it's true. Now remember, what are these? These are all procambium in origin--these xylem and phloem. So within the central cylinder we have some ground meristem that gives rise to pith, and we have some procambium that is going to give rise to xylem and phloem.
Let's move to dicots. Dicots are very cool, too, and you'll see the differences. Now, what does a dicot root do? They have a taproot so they're going to do a lot of storage, aren't they? Yeah. So what do you think they're not going to have? They're going to sacrifice one of those storage regions. And the one they sacrifice is going to be the pith. So if we look at a dicot root--once again, we obviously have the outside, we have the epidermis, but let's go to the inside. And we're going to have this thing here. Now, I drew this in the monocot root, too. Let's get a little more specific here. These central cylinders are sometimes referred to as the stele. In the dicot there's no pith, so it's going to be all procambium, it's going to be all xylem and phloem. And why I want to really center on this is there's a real good story to be told about the stele. The stele on monocots and dicots is waterproof. It's actually what is called--the outside of this thing, the endodermis is suberinized. You're going to hear that term "suberinized" a lot. It has a waxy coating for waterproofing, and you'll see how important that is later when we go through water transport.
So remember what this outside is called? That's called the endodermis. Now comes the great stuff. I want to go inside the stele. Inside the stele, inside the endodermis is a layer of cells that monocots don't have. This is important. Why is this so important? This is called the pericycle. This is important because it is undifferentiated tissue. It's meristem. See why it's important? Not yet? What can meristem do? Divide. Therefore, what can a dicot root do? Get thicker. So this can get thicker. This can actually grow. Remember, this is the endoderm here. So this can actually make new tissue. That's how you get this whole idea of much more room--all the storage is out here and no storage in here.
Well then, where's the xylem and phloem? The xylem forms a cross, sometimes referred to as the "xylem cross," like so. And the phloem is found within the arms of the xylem, right in there. So that's xylem. Xylem, phloem. And here's the thing. What you have with this pericycle is the ability to grow wide. And now I want to do one thing. I want you to go and get a carrot. Now, hopefully you had one sitting right next to your screen. I want you to bite that carrot or cut it. I want you to look inside that carrot. What do you see? Do you see a round thing? Do you see something round inside of it that looks like that? Congratulations. You're looking at your first stele.
Plant Systems and Homeostasis
Plant development
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