Biology: Angiosperm Embryogenesis: Dicots&Monocots

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Let's take a look at the branches of the angiosperms. Now, you know, one of the things that has happened over the course of plant evolution is a diversity in terms of function. Remember something about function--it is always a result of structure, or vice verse. Structure and function always go together. So as we start talking about how plants develop, remember the goal. The goal is survival. All of the structures that develop in a plant embryo are eventually going to get to the point where we're talking plant tissues. When we're talking plant tissues, we're talking about adaptations for survival. So keep that in mind. It always comes back to that. If your structures are not going to get you through the day, you're going to die.
So let's go to angiosperms. Let's do a generic thing first. Let's talk about the two divisions of the angiosperms. Remember what the angiosperms are? The angiosperms are the flowering plants. If this were a botany course we'd talk gymnosperms, we'd go back to some of the ferns and mosses, but right now what I want to give you guys is a sampling, and we're going to really concentrate on the angiosperms and try to take a look at the dicots and the monocots, because that's the two divisions of angiosperms--dicots and monocots.
Why do I feel this is important? Well, besides the fact that it's biology, and you're probably going to have a test on it, and we're hoping that's not the reason why you love biology, but the point here is this. Look around you. You live in a world of plants. You don't live in a world of people. You live in a world of plants. If you go out and run around on a football field, there's more plants on that football field than there are probably people in your town or city, and so you literally live in a world of plants. And we tend to be ignorant of plants for some reason because they just kind of sit there. They're generally no danger to us unless they're poison ivy. We go to our grocery stores and we find plants wrapped and plastic and most of us assume that's how they grew. The point is, an understanding of plants, besides the fact that plants are unbelievably cool and exciting, the whole understanding of plants is absolutely essential to the understanding of ecology and ecological relationships and agronomy, because, let's face it, you depend on plants for food. So I feel very strongly for plants' rights, and we need to learn plants. Let's learn some plants.
Dicots and monocots--perhaps right now, sitting in your room, there are some dicots and monocots and you may not know the difference. The most obvious way to find a dicot versus a monocot is by the way the veins are arranged. Look at a leaf. Dicots have kind of net-like leaves or veins, so the veins are in a netlike formation, kind of like your typical--think of a maple leaf, whereas monocots have parallel veins. If you have hanging from your ceiling a spider plant, with its grass-like leaves. In fact, grass is a monocot. Corn is a monocot. Anything with long, thin leaves, because they're parallel.
Two, flower arrangements. Did you ever hear of a flower called "trillium"? Trillium has three petals. It has everything in threes. We won't get into the whole structure of trillium. Monocots have flowers in multiple of three. They have structures called "sepals," there are multiples of three--flowers in multiples of three. I don't mean like you get either three flowers or nine flowers, but the parts of the flower are multiples of three. On the other hand, dicots are not--they're four or five. Go pick a flower and take a look at it. Pick a tulip. Pick a daffodil. You tell me whether it's a monocot or dicot. Just look at the flower.
Wait, there's more. The veins we talked about, but even in the stem the veins are arranged differently. I'm going to show you a cross section of a--this is probably the easiest way to show you this. So I'm looking at a stem, and I've cut it off, and now we're looking down inside of it. In a monocot they have these veins called "vascular bundles," and we'll see why they're called bundles later, but they're arranged in what seems to be, at least microscopically, at first glance, in this random order. On the other hand, in dicots, the veins, the bundles--these are called "vascular bundles," or veins--on the other hand, in a dicot, the veins are around the outside. Now, all of these are leading to a discussion of how do they get this way? Embryogenesis.
Just one more thing--two more things. Tap root. Dicots have a tap root, which is a big old fat root. Monocots tend to have fibrous roots. And now you're saying, "Well, why do we call them dicots and monocots?" and that's going to give me the answer to this next thing. There are things called "seed leaves," which we're about to talk about called cotyledons. A dicot has--you guessed it--two cotyledons. Seed leaves. Monocots have one. There's so much more than seed leaves, which brings me to the real thing I want to talk about--the early development of a plant.
Let's take a quick reminder of what happened. Remember, in angiosperms there was this very unique form of fertilization called "double fertilization." Two sperm nuclei came down, one fertilized the cell nucleus and that formed the zygote, and the other one will fertilize the polar nuclei, which will form a triploid endosperm, a triploid 3N endosperm. And it's that endosperm that is going to be so important in the development of the embryo. So watch this. This is very exciting.
So let's talk about the early divisions of the plant. The very first thing that happens to a plant is--and I want to talk about the baby itself, the embryo itself--is you get one division, and then immediately we are going to have a dichotomy of function. The first division is going to give two cells that are going to develop into two different parts. On the one end is what we're going to call the "terminal cell," and the terminal cell is going to end up the baby plant, the embryo. I know you're not used to thinking of plants having babies, but let's get real here; this is an embryonic plant. The basal cell is going to end up forming something called a "suspensor." You can see this suspensor forming right through here, right through here, and right through here, and the suspensor is going to literally hold it into the seed and maybe in some, hook up to the endosperm to suck up nutrients because that's the function of the endosperm. Remember that. The endosperm is to store nutrients in its big old fat cells. So one function of the suspensor, besides suspending it in the seed and holding it in place, is in some plants to suck up some of the nutrients.
The thing we really want to talk about, though, is not the suspensor--that's all we need to know about the suspensor. I want to talk about the embryo. What starts to happen is very interesting. You get another division and you can see these start to form--wow, a structure, and it becomes this heart-shaped structure which will eventually--and we're taking kind of a giant leap from here to here, but you can see that that enfolding continues to occur. We get this structure here, which is the actual embryo of the plant, and that's where some very exciting stuff begins to happen.
So at this point, I guess I'll call this the pro-embryo. It's just before the embryonic stage, and I want to show these buds. These buds are what are going to be important, because those are going to form into what are called "cotyledons," so therefore this particular one is a dicot because it has two cotyledons. Now, where's the plant? The plant is actually going to be this thing, literally, right in here. These are the cotyledons.
Let's talk about what cotyledons do and what they're used for. So obviously if this is a dicot it has two cotyledons; a monocot will only have one cotyledon, which will be literally this entire structure right here. So what happens is, in this case, and here's the embryo, the cotyledon is going to attach to this and feed it as it grows. It's going to be completely absorbed, and you're going to have this embryo grow upward. Let me show you a couple of things here.
There are two different forms of dicot. Look at these. Do you see a difference? I think you do. After this lecture I want you to go and find if you have any peanuts, and break open a peanut and look inside of it. You'll see an embryo inside of that peanut. Yes, you will, and it will look just like this. I want to show you the function of cotyledons here. In the case like the peanut what's going to happen is that the endosperm of the peanut--remember that food storage thing--is going to be completely absorbed by the cotyledons. What you consider the inside of the seed is really the cotyledon and it's attached roughly here. So this thing is attached to this big thing we've got in beige here, and this is the cotyledon.
Well, then, what's everything else in here? If that's the cotyledon, this must be the plant. And watch this. The part above the cotyledon is called the "epicotyl," above the cotyledon, and the part below the cotyledon is called the "hypocotyl." And guess what's going to happen? The epicotyl is going to turn into the upper part of the plant. Does that look suspiciously like a leaf? That's because it is. It's the plumule and that's going to end up the leaves. Well, what about this? Well, the hypocotyls is going to end up becoming the lower stem and root to the point that even below the hypocotyl there is a portion called the "radical," which is going to eventually become the root. So that's root precursor.
On the other hand, very briefly let me show you this one. This particular one, the endosperm is not absorbed. The purple is the endosperm, and the cotyledons are very thin. And as I showed you before in the monocot, the endosperm is not absorbed, and the cotyledon also remains thin.
Well, what is this all about? So what? Where do we go from here? When we are done with this structure inside of a seed we have this dried out, dormant embryo. Why is it dormant? No water. It has lost approximately 95 percent of its water. What does it look like? Well, in the case of, say, a peanut, it literally has this little embryo plant that looks something like this with a plumule up on top. That's attached to a cotyledon like so and literally attached here, too. That's why that picture looked like this. But when you open that seed, that embryo can grow out. The key is, where's the plant? The plant, as you're going to see... Where's the growth going to occur? The growth is going to occur in two very important sections--one of those sections is right here, and the other section is right there. What are those two sections? They're called "meristem," and that is what's going to give you your adult plant.
The Evolution of Life on Earth
Angiosperms
Embryogenesis in Angiosperms: Dicots and Monocots Page [3 of 3]