Plant Evolution and Diversity
Topics in Depth
The Theme of Common Mistakes in Plant Evolution and Diversity
It is tempting to call algae plants because they do photosynthesize and are like plants in so many ways. Drill it in that noggin of yours right now: algae are not plants. Seaweeds in the ocean are algae. Green pond scum is algae. But, mosses, even though they live in moist environments and don’t make seeds, are plants. It all comes down to whether or not the organism makes an embryo and protects it somewhere on the plant-like body.
It is also tempting to say things like, "Shmoop, charophytes and land plants are closely related, so land plants evolved from charophytes." Resist temptation! It is better to say that charophytes and land plants share a common ancestor. After all, charophytes have not stayed the same for the last billion years. They have evolved during that time, too.
Plant Colonization of Land
Bryophytes are similar to early land plants. They are not the same as early land plants. Though they share similar traits as early land plants, the bryophytes living today have been evolving for hundreds of thousands of years. They are called primitive because they did not evolve the traits of "higher" plants, like advanced vascular systems or flowers. Just because they are "primitive" does not mean they stopped evolving. Their primitive traits are a testament to their hardiness and the ability of that plant type to exist for hundreds of thousands of years.
In science books, you may occasionally come across the term blue-green algae and say to yourself, what the heck are they talking about? Blue-green algae is an outdated name for cyanobacteria. You might see it if your textbook is old, or if you are talking to your grandparents or something. Cyanobacteria, the first beings to oxygenate the atmosphere, are bacteria, not algae.
Gaps in the fossil record do not disprove evolution. Sometimes, there are not fossils available from every time period, or they may not show every transitional state. This occurrence does not mean that evolution did not happen. Fossils are only created in specific conditions, and there are many, many dead plants that did not leave fossils behind. Additionally, not every fossil as been found, so there are still lots of discoveries to make.
Artistic recreations of past landscapes are guesses and not the last authority on what those landscapes looked like. They are educated guesses based on scientific evidence. If you find a time machine and travel back 300 million years, don’t be surprised if the plant and algae look a little bit different than what’s in the current travel brochure.
Levels of Organization
Closely related organisms do not always look alike. Even species within the same genus can look very different but are still closely related. Try doing an image search of the genus Euphorbia, for example.
Make no mistake, just because two plants look alike does not mean they are closely related. Euphorbias and cacti both have spines but are in different families. Mangrove trees are adapted to live in swampy conditions and so have a lot of similar traits, but they come from many different families.
Every single organism has a species name, but it is always called by the genus + species name, and sometimes, by a subspecies name. Dogs are Canis lupus, but so are wolves, so familiaris is added to the species name for dogs: Canis lupus familiaris. Canis is the genus, but it also makes part of the species name. No one would know what you were talking about if you just called them "lupus" or "lupus familiaris."
Gametophyte vs. gamete. A gametophyte is a plant that makes gametes. Gametes are sperm and eggs. The sperm and egg join together to form a zygote, which grows into another plant.
Sporophyte vs. spore. A sporophyte is a plant that makes spores. Spores are single-celled reproductive units that can grow into new plants all by themselves, without any extra help. Sporophytes make spores, but spores grow into gametophytes. Gametophytes make gametes, which form zygotes and then grow into sporophytes.
Archegonia vs. antheridia. Archegonia and antheridia are structures that make gametes, so you find them on the gametophytes. An archegonium makes eggs, and an antheridium makes sperm.
Seedless Vascular Plants
We told you that fern leaves can be called fronds, sporophylls, and fiddleheads. Is there a difference? When do we use each term?
Frond can refer to any fern leaf. Because they look different from the leaves we have on many other plants, they have a special name, similar to how we call pine leaves "needles." Sporophyll is the scientific word used for leaves that have or produce spores—the word is basically telling you, "these are leaves that have spores on them." Usually, spores are only found during certain times of the year. Fiddlehead is used to talk about new fronds that have not unfurled yet. Just like a square is a type of rectangle, a frond is a type of sporophyll. A fiddlehead is a type of frond (a very young one), so it is therefore also a sporophyll.
Gametophyte vs. sporophyte. You know that gametophytes make gametes, and sporophytes make spores…and then what? The plant life cycle alternates between the sporophyte generation and the gametophyte generation. In bryophytes, the gametophyte is the independent, I-don’t-need-anybody-looking-out-for-me, generation. The sporophyte is the little brother that still needs to stand on someone’s shoulders to see the parade. In bryophytes, the sporophyte only grows on top of the gametophyte…on its shoulders, so to speak. In ferns, the sporophyte grows on top of the gametophyte but the sporophyte is much bigger. The ferns you know and love are sporophytes; the fern gametophytes only live a few days.
Xylem and phloem. In plants with vascular tissue—that is, most plants, except bryophytes—the transport tissue is called xylem and phloem. Xylem carries water around, and phloem carries sugars made during photosynthesis. You can remember which is which by the P’s: Phloem is for Photosynthesis.
Spore vs. seed. Ferns and bryophytes make spores, but gymnosperms and angiosperms make seeds. What is the difference? Spores are single-celled packets of genetic material. They need to start growing soon after being released from their parent plant because they are each only one cell and have no protection from the environment they are in. Seeds do have protection because they each have a seed coat. This coat allows them to lie dormant for a while and start growing when conditions are right. They also have extra nutrients that the embryo can use when it starts growing into a little plant.
Megaspores vs. microspores. Megaspores are female spores, and big ones at that. Microspores are male spores, and…tiny. Megaspores grow into female gametophytes that produce female gametes (eggs). Microspores grow into male gametophytes that produce male gametes (sperm).
Gymnosperm gametophytes. In bryophytes and ferns, gametophytes are visible to the naked eye and are separate plants from the sporophyte generation. In gymnosperms, the gametophytes are so tiny that you can only see them with a microscope. The female gametophyte stays on the sporophyte, and the male gametophyte is inside the pollen grain.
Double fertilization is a strange and scary thing. The main thing to remember is that it produces both an embryo and an endosperm, and the endosperm is important for nourishing the embryo.
Alternation of generations in angiosperms. Though it is not obvious, angiosperms still have alternation of generations like the rest of the plant kingdom. The tricky part is that the gametophytes are tiny. The male gametophyte is still part of the pollen grain, like it was in gymnosperms. The female gametophyte is the site of double fertilization, which produces both an embryo and an endosperm.
Pollination. The word pollination refers to the transfer of pollen. Plants are usually animal or wind pollinated. Most, but not all, angiosperms are animal pollinated. Some notable exceptions are grasses, which are wind pollinated. Plants that have only one type of pollinator are called specialists, and plants that are not picky about their pollinators are generalists. You can say the same thing about the pollinators: if they only visit one type of plant, they are specialists, and if they visit more than one, they are generalists.
Convergent evolution. Two plants can look similar even if they are not closely related. Why does this happen? Natural selection favors plant characteristics that help the plant survive in its environment. Since deserts are dry, desert plants often have traits that help them minimize water loss and survive dry conditions. The plants converge on a particular trait that helps in their habitat. Convergent evolution also happens in other environments, such as mountaintops and tropical forests.
Vicariance. This happens when the former range of a species splits into two and results in the evolution of two different species. If a plant inhabited a big area, but a volcano in the middle erupted and wiped out every plant living in the middle of the range, the plants living on the two sides of the range might be separated from each other. Over many generations, these two sides could evolve into separate species.
Adaptive radiation. You know that saying, the apple doesn’t fall far from the tree? It means that kids usually act like their parents. In an adaptive radiation, the apples do fall far from the tree. The kids and their offspring all go off and act very differently from their parents and from each other, resulting in new species. This occurrence does not happen overnight; it takes many generations.
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