Welcome back to our series on plant diversity and evolution. What do all these plants have in common? The pines, the palms, the ginkgos, the cycads, all the grasses, any shrubbery you can think of, the deciduous trees, and any flowering plant. They're all seed plants. Now how did we get to this point in our discussion on plants?
We began our discussion on plants saying we could organize plants based on some common questions. The first question we asked was, does the plant have vascular tissue? And if the answer was no, we talked about the bryophytes, and we used moss as an example. If the answer was yes, then we had vascular plants.
We talked about the evolutionary advantage of vascular tissue, a system of conduction that allows plants to grow large and large. as we can move materials efficiently throughout the plant body with the xylem and phloem. We also said that vascular tissue gives us structural support, so plants can grow upwards. So, lots of plants are vascular.
So the next question we have to ask ourselves is, is does the plant have seeds, release seeds, or does it release spores? If the answer is spores, we talked about the spore-bearing or seedless vascular plants. We use ferns as our example.
Now we're headed down this branch. Seeds. The seed plants. So what evolutionary adaptation do seed plants have that we haven't seen up to now?
Well, duh, the obvious one is that we have seeds. But maybe less obvious is that all seed plants have pollen and seed plants are heterosporous. So we're going to discuss what each of these things means and the evolutionary significance of them.
Let's start by talking about the evolutionary significance of seeds. Seeds have a thick protective seed coat, and this seed coat is resistant to harsh environmental conditions. It can withstand hot and cold and dry, and maybe even the enzymes of the intestinal system of an animal that it might pass through. Let's look inside the seed.
Inside the seed, we have the baby plant, or the embryo. And surrounding the embryo is nutritive tissue. This is... This is going to provide energy to sustain the embryo.
Now what's the significance of this? This thick seed coat and this nutritive tissue surrounding the embryo allow for a dormant stage for the plant. In other words, when the embryo is formed, it doesn't have to grow right away.
If conditions are poor, it can stay inside the seed and sustain life for very long periods of time, even years if under the right conditions. What does this do for plants from evolutionary? standpoint? Well, it opens up a much larger range of habitat.
By having this dormant stage, plants can live in areas where the environmental condition could vary greatly and they can reproduce and survive in those areas instead of having to live in areas where the embryo has to be able to grow right away. In other words, areas where the environmental conditions are relatively stable. So there's a huge evolutionary upside to having seeds. Now let's discuss the significance of pollen.
Now we all know what pollen is. Some of us have pretty bad allergies to pollen, or do we really know what it is? Besides giving us allergies, what's the significance of pollen?
Well, let's look at pollen up close. It turns out that pollen is the male gametophyte. Now we're going to have to go back to our alternation of generation life cycles and look at the life cycles of pollen. plants a little closer to remind ourselves what we mean by gamedophyte. And in the video that follows this one we're going to go into detail on the plant, the seed plant life cycles.
But in this video we're just going to allude to it so hang on. These highly motile pollen grains can transfer between plants via wind, we have wind borne pollen, water, water can move pollen, and biotic vectors which is a fancy way of saying pollinate. like this bee that's covered in pollen.
As it goes from one flower to the next, it can transfer those pollen grains. So what's the evolutionary significance? What advantage do seed plants have by having pollen? Well, it allows for reproduction that does not rely on water to transport the gametes.
If you recall from our discussions on moss reproduction and fern reproduction, they required water for the sperm to get to the egg, and thus, they were limited in habitats so having pollen opens up a much broader range of habitat for seed plants you don't see mosses or ferns growing in these types of arid environments because there's not enough moisture for the reproductive process to occur. But with pollen being moved the way it does, plants were able to spread out into a much broader range of habitat on the planet. Now let's talk about what we mean by heterospores.
We said that seed plants are heterospores. Well, what does homospores mean? Homo means the same, hetero means different. In a homospores organism, all the spores are alike. So when protists, fungus, mosses, or ferns make spores, well, each of the spores is the same.
But in heterosporous organisms, we have two types of spores. We have microspores and megaspores. Now, remembering your alternation of generation life cycle, your generic plant life cycle, what makes spores?
Do you remember? Pause the video and write it down. Spores come from sporophytes.
Now, what do spores become? Pause the video and write down what spores become. Well, spores become gametophytes. But we have microspores and megaspores. The microspores become the male gametophyte.
Therefore, the megaspore, that's right, the female gametophyte. Remember, the male gametophyte is the pollen. And what do gametophytes make?
Well, gametophytes make gametes. And the female gametophyte will make egg. and the male gametophyte will make sperm.
And this gets easy now. Sperm and egg are used in fertilization to make the zygote, which grows into the new sporophyte. So the only difference between homospores and heterospores is we have two distinct types of spores making a distinct female gametophyte and male gametophyte. We'll elaborate much more on this and show the differences between the types of seed plant reproduction in the next video, so come back for that one. So where do we go from here?
Now that we have seed plants, let's talk about the different types. The next question we need to ask is, do we have flowers or not? If it's a seed plant without flowers, we can call it a gymnosperm. If it has flowers, an angiosperm. Then we'll distinguish within the angiosperms two different groups based on the number of cotyledons, and we'll talk about what that means in just a moment.
If you have one cotyledon, you're a monocot. If you have two cotyledons, you're a dicot. So let's talk about the difference first between gymnosperms and angiosperms. Now, again, we said that gymnosperms were the non-flowering plants and angiosperms were the flowering plants. The word gymnosperm, gymno or gymnos means naked and sperm means seed.
So the literal translation is naked seed. And angiosperm means covered seed. So the question is, what is the seed covered with? I want you to think about that. Pause the video and think about what things cover seeds.
What do angiosperms have that gymnosperms don't? What'd you get? Did you really think about it? Have you ever seen this? Fruit.
Seeds are covered by fruit. Angiosperms, flying plants, make fruits. Where in the gymnosperms, the seeds are uncovered or naked. Now some examples of the gymnosperms are the conifers, the cygids, and the ginkgos.
And the angiosperms are pretty much all the other seed plants. Shrubs, grasses, trees, all your other flying plants that are not these over here. Okay, how about the difference within the angiosperms?
Because lots of our plants are flying plants. What's the difference between the monocots and the dicots? Well, let's take a look at it. The first is what the name comes from, the number of cotyledons.
And cotyledons are embryonic leaves. If you look inside the seed, in the monocots, there's a single embryonic leaf, and in the dicot, two, one cotyledon or two cotyledons. Well, What if you can't see inside the seed?
Can you tell angiosperm apart from aginosperm? Another way to do it is to look at the flowers and count flower parts. In the monocots, the flower parts are in multiples of 3. So if we count the petals here, we count 6. That's a multiple of 3. In the dicots, we have multiples of 4 or 5. We count the petals and we count 5 here.
What if you have 12? Then you're in a problem area. It's a multiple of three or four. Well, there's another way to tell monocots from dicots.
We can look at the pattern of veins in the leaves. When we look at the leaf of a monocot, we see that the veins in the leaf run parallel. You can kind of see them here in a different color here.
They run parallel. Whereas in the dicot leaf, it's net-like. They intersect. So we have all these branches.
It looks like a net thrown over this leaf. You can see the veins there. So it's a different pattern. So it's an easy way to tell a monocot from a dicot.
We can also look down the stem. If we cut the stem and look down the barrel, we see a different organization in the vascular bundles, the xylem and phloem. We see them here, these vascular bundles.
In the monocot, they're scattered throughout the stem. stem. But in the dicot stem, the vascular bundles are in a neat ring around the outside.
You can see them in a ring. You can follow it around there. So there's a difference in the organization of the vascular bundles. Well we have, I think, one more.
Yes, the pollen grains. The number of pores or furrows in the pollen grains. In the monocot, they have a single pore or furrow. But in the dicot pollen grains, we see three pores or three furrows. So there we go, a comparison contrast between the monocots and the dicots, all within the angiosperm.
We don't use the term monocot or dicot if we're not already in the grouping angiosperm. So that ends our introduction to seed plants. Check back for the next video where we're going to go into details on the reproductive cycles of gymnosperms and angiosperms.
I'll put a link to that down here. If you have any questions, leave them for me in the comments section below the video and I hope you like it. learned something.