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Like the Animal Kingdom, the Plant Kingdom is also a major part of the Linnaean system of classification that includes organisms like trees, bushes and grasses. The plants in this kingdom can be called autotrophs because they perform photosynthesis to provide food for themselves. These organisms also provide oxygen for humans and animals to survive.
As scientists learn more about the evolutionary relationships between all the various creatures on this planet, we keep having to change our classifications systems to accommodate this new information that comes in. However, there's one group that seemed pretty solid and that's the plant kingdom. I'm going to take you through the various characteristics that define what is a plant and what is not. Then I'll show you the four major groups that make up today's modern plants.
Now the first of the plant characteristics is that they are multicellular. They have lot of cells in them as opposed to things like cyanobacteria, the blue-green algae which are singe-celled. Now, plants are all photosynthetic and they use a chemical called chlorophyll to absorb light that gives plants their characteristic green color. Now the cells of plants are surrounded by a cell wall made out of a chemical called cellulose. Other kinds of photosynthetic creatures like the algae, they may use other kinds of chemicals to make their cell walls like silicon dioxide or other chemicals. Alternation of generation is a weird little thing that plants do where they'll have one generation that is diploid followed by the next generation is haploid; which means one generation will have two copies of every chromosome. Well, their children will have one copy of every chromosome. It's kind of like if a plant was a human, it's like if the human mother never actually, she gave birth to something that looked very different from herself which would then give birth to a new human. Now you may not have seen this different generations because most of the plants actually will keep one of the generations within or dependent on the previous generation as if a woman never actually released her child, instead she just allowed that child to stay inside of her and then ultimately that child had her grandchild who then came out; freaky weird plants, yeah.
Now, one of the key things that groups all the plants together is the fact that they're adapted to life on land. They evolved from algae that were in water, but they took advantage of the some of the things that are on land like more carbondioxide in the air and more light. But they had to deal with the fact that life on land requires that you figure how not to dry out. Now one of the first adaptations that the plants have is a layer on top of them, a waxy cuticle and that wax helps prevent water from just evaporating from their surfaces. Most of the plants have specialized structures called roots. Roots are structures that are designed to absorb water from the soil. Now the mosses don't really have roots, they have typically something called a rhizoid. "Rhize" is a root well, that means ironically root and "oid" means resemble. So the rhizoids are mainly just there to anchor them to the ground. They do not have special adaptations to allow them to suck in the water from the soil.
Vascular tissues is something that many of the taller plants have that allows them to transport water and nutrients from deep in the soil to the top of their bush. Again, the mosses which are typically much shorter don't have that vascular tissue. Another thing that the plants have is special adaptations to keep their offsprings, their embryos from drying out and the sea plants are the best example of this.
I'm going to start through the four major groups of plants that are around today. The first of which, are the mosses. Let me get this up here and here is the moss that you might have seen growing on a rock or on the side of a tree and you can see here there's teeny tiny. Now you occasionally see these little things sticking up. That's actually that diploid generation that is the child of the green leafy mat that you're most familiar with and it all release spores that afloat to the air, land some place and allow them to grow more of the green leafy mat.
Let's take a look at the ferns. The ferns have this stereotypical feather-like leaf which gives them their scientific name; pteridophyta or other names depending on which text books you look up the name in. You often also see these weird little curly cued up fiddle heads; they're are called. And on the under side of their leaves they'll have specialized structures called sori which release spores to give rise to a new fern.
The next group after this is the gymnosperms which means literally naked seeds. Now the gymnosperms your most familiar with are the pine trees and other cone-bearing plants and here you can see some teeny tiny little cones about to grow and those are specialized structures that they use to do to sexual reproduction. The larger female cones have the egg inside while the smaller pollen cones release the pollen that is spread by wind and then it will land within the female cone and fertilize the egg there. The female cone will protect the seed and ultimately release the seed to drift in the air.
The last of the major plant groups are the angiosperms commonly called the flowering plants and they're the most recently developed. May take the advantage of the fact that there are these things called animals on land and what they do is that they have the flower and that's to attract some kind of animal like an insect or whatever to come along and feed off of the nectar or other things produced by the plant while it sticks pollen on the back of the bee or whatever and then that bee flies to another similar flower and pollinates that other flower. Then they take that fertilized egg that's inside of the flower and that develops into the fruit with seeds inside, the fruit being a bribe to some animal again to come along, "Come here! Eat the fruit," but the seed is designed to survive passing through digestive system and 24 to 48 hours later you deposit that seed somewhere else in a nice steaming pile of fertilizer ready to give birth to a new plant again.