(Laying the ground work now to make you guys really suffer, later, since many of you seemed cheered at the idea of more plant posts. One of these days I'll do a bio of Linneaus. He was raving.)
Now. The term 'plant' is, in fact, a sketchy one, and people argue about just what it means. If you ask ME, it's about some living critter that uses photosynthesis to 'eat', so anything with a chloroplast (the cellular organ that does photosynthesis) is a plant, and anything without a chloroplast is... something else. But according to the three kingdom system of taxonomy, that's not the case. Of course, not all biologists use the three kingdom system any more. Some use a five kingdom system. Some use a six kingdom system. And I heard about some wild-ass molecular biologist who developed a twenty-two kingdom system a few years back (oddly - ha - all but two or three kingdoms were composed of microbes), but I don't think anyone but wild-ass microbiologists use that one. These days, nobody really knows what to do, but they all do DNA testing and argue with each other. So I'm falling back on my personal definition "If it's got chlorophyll, it's a plant." (They used to classify mushrooms as plants. They must have been DOING mushrooms for that to seem right.)
The following groupings are - mostly - general, so there can be a bit of overlap. (Can too. Tell me where a slime mold fits, hmmm? Lichens? SPONGES?) But it'll do well enough for laypeople, especially considering my textbooks are all in boxes. (I love you guys, but I'm not digging out those boxes.) I'm also putting them in the rough order in which they evolved/first appeared. Again, there's overlap and times on a lot of them are approximate, but you'll get the idea.
Algae and related: Nuclei enclosed in membranes and chlorophyll and blah blah. These are simple plants; the ones that are not single cells, often function as clumps of single cells together. Unlike more advanced plants, where cells form specialized organs like leaves and flowers and whatall, algaes are just a buncha cells, hanging out. Sometimes alone, sometimes together in clumps. That's what makes them algae. These suckers have been dated in the fossil record back a billion and a half years and more. Most people think they go back much further, but there's no fossil record to support it; all the rock we have now has been recycled in volacoes since then. That's how damn old they are. As with so much else, biologists are still arguing over exactly how the algaes and their close relatives (like sea weed and coral reefs) are related. Personally, I think algae is so complicated it needs its own kingdom. Don't say THAT too loudly in a biology department. Someone will jump you.
Mosses/bryophytes (they're now giving liverworts and hornworts their own divisions, and I'm skipping them or I'll be here all damn night): This is another type of plant that goes back so far, no one's quite sure how old it is. And also like algae, it lacks a vascular system. (Remember biology class where everyone was obsessed over spinal columns? Well in the plant world, it's vascular systems. Those are best known to knitters as bast fibers; it's like a circulatory system for the plant, dedicated cells that do nothing but move nutrients and water around. Xylem and phloem in really complex plants.) Mosses, don't have those. Another thing that makes mosses freaky is, they're backward. I'll skip the diploid and haploid stuff, but in genetic terms, mosses spend most of their lives as egg and sperm. They don't have seeds, they have spores, which are... oh, forget it, just believe me when I say they're freaky, and insanely complex genetically, for how simple they are, on a cellular level.
Ferns and horsetails, AKA seedless vascular plants: Which are just what they sound like. Plants that don't reproduce with seeds and flowers, but are complex enough to have cells dedicated to moving stuff around - a vascular system. Without hitting my textbooks, I do believe ferns win the prize as the most genetically fucked up organisms in the plant kingdom. Possibly the world, but plants for sure. I forget which one, but some fern has 400+ chromosomes. (Humans have 46 - 22 pairs and some stragglers.) They reproduce by, well, a spore grows into a gametophyte, which grows eggs AND sperm, which fertilize each other and produces little baby ferns that honest-to-god remind me of tadpoles, that then take root and grow into fern, ferns. Told you they were complicated. They're also old. Show up in the fossil record around 410 million years ago.
Conifers: (We're now firmly into the realm of stuff you can identify. I think.) Coniferous trees go back about 360 million years. They're defined as vascular plants (again with the vascular stuff) that reproduce with cones. Mostly they're trees, but there are some shrubs in there too. (Quick. Define 'shrub'. QUICK! DARE YOU! Presidential slurs do not count!) It's thought that conifers evolved as a response to the fern life cycle needing so much water; conifers don't need any standing water to reproduce, unlike ferns. Instead, they're wind-pollinated. (Think about the shape of a pine cone, and imagine it needing to catch as much air as possible to reproduce the tree. Clever, no?) Most of the trees have male and female cones, one to release pollen and one to catch it and turn it into seeds. Some species have entirely separate male and female trees. The cones and seeds can take years to mature from little bud all the way through to producing seeds. Pine nuts, so popular in Italian food, are seeds from the cones of a specific species of coniferous tree. And no, they aren't domesticated.
Ginkgo trees go in here somewhere, but they are a blog post unto themselves. I refuse to take all night at this, and I don't want anyone's eyes to glaze over (well, worse than they are now). But ginkgoes are genetically unrelated to, well, anything we know of, and they go back about 280 million years. In China they are a symbol of longevity - I always wonder how the ancient Chinese KNEW THAT STUFF (if you're an ancient Chinese person, e-mail me and let me know).
Cycads (known as dinosaur trees at MY house): These are trees that are in many ways similar to conifers. Vascular tissue, cones, etc. The only difference is in structure, which is similar to palm trees, but not. (I do believe - again, not digging out the text books - the way to differentiate between a cycad and a palm is, palms are hollow in the middle, cycads are not.) Cycads have things that look much like pine cones growing up out of the centers of the leaves, which then pollinate and produce seeds much like the conifers do. These plants go back 280 million years, easy, and have a really unusual global distribution that may support plate tectonics and continental drift. Cool trees, kinda freaky looking, and I respect that.
Angiosperms, AKA "flowering plants": You know, the ones in your gardens. The reproductive cycle that you guys ALL studied in school, with the pollen and the ova and pistils and stamens and all that, allows for a HUGE genetic variance that then produces hugely varied flowering plants. And here we are at the end (ish) of our lesson, at a group of plants that have only been around about 140 million years, and yet have taken over most of the planet. Nearly all our agricultural output is done with angiosperm plants. Many of those have flowers so small you can't see them, but they're in there. You know when your yard gets overgrown and the grass grows little light yellow-green tufts at the ends? Those are flowers. Yup. Angiosperms also, not surprisingly, have the most well-developed vascular systems, and to my knowledge, all the commercial vascular bast fibers used today come from, you guessed it, flowering plants.
I'll skip the further distinctions of monocots and dicots. For today. (A friend of mine, a fellow plant freak, has taken to calling my only child 'the monotot'. We crack ourselves up.) I'm also skipping chloroplasts, but they need their own post. They're like nothing you've seen, this side of a bad science fiction movie. And possibly an annotated list of my favorite plants. And I don't think I've properly sung the praises of the humble potato... or the breadfruit tree... or the three sisters... and bromeliads - are they angiosperms or gymnosperms...