Ferns The Emergence of Roots and Stems

In the previous tutorial we discussed lycophytes, which were the first vascular plants on earth. But these still did not closely resemble the modern plants we are most familiar with, like trees. The next type of vascular plants to evolve were finally of more familiar form, and those were ferns. Ferns are more advanced vascular plants than those we’ve discussed previously, namely because they are the first to have true roots and stems. Fern stems bear leaf-like structures called megaphylls, which are essentially larger versions of the microphylls grown by lycophytes. The fern group is large and incredibly diverse, including variety of ferns and scouring rushes or horsetails. Most fern species grow as small or medium-sized herbaceous plants, meaning no woody stems above the ground, but larger tree ferns can be up to 25 meters tall. The smaller herbaceous ferns can grow as epiphytes, which again are plants that grow on other plants, and they can also be epipetric, which is term for plants that grow on rocks. The oldest ferns appeared during the Carboniferous period, about 360 million years ago, but most modern fern groups appeared during the Cretaceous period, about 145 million years ago. In ferns, like lycophytes, the diploid sporophyte is the dominant generation, or the generation that we’re most aware of seeing. If you look at the back or undersides of the megaphylls on fern sporophyte, you’ll often see small brown or black dots in neat rows. These brown dots are sori, or groups of sporangia where the haploid spores are produced through meiosis. Spores are released from the sporangia, where they fall to the ground and germinate as new haploid gametophytes. Not much is known about fern gametophytes in the wild because they are very small and ephemeral, meaning they don’t live very long. Most of what scientists know about fern gametophytes, also known as prothallia, comes from studying them in laboratory setting. The haploid gametophyte attaches to the soil with rhizoids. As with the other gametophytes we’ve discussed, fern gametophyte bears both reproductive heads and can self-fertilize. Just as with previous plant groups, antheridial heads produce sperm cells through mitosis, and archegonial heads produce egg cells, also through mitosis. When there is sufficient water in the environment, the sperm cells can swim to the archegonial heads and fertilize the egg cells. Once an egg cell has been fertilized, it germinates into new diploid sporophyte directly from the gametophyte. Once the sporophyte is ready, the gametophyte dies off. The fern sporophyte generation is readily identified by the characteristic “fiddlehead” of its curved stem before the megaphylls unfurl. Many people eat fiddleheads, but it’s important to properly identify the fern species first because some are poisonous. Once established, many fern sporophytes are perennial, meaning they will grow in the same spot year after year without having to alternate with the gametophyte generation. Additionally, many fern sporophytes are able to engage in different form of reproduction called vegetative reproduction, which entails propagating new plants via rhizomes growing in the soil. This reproductive strategy has caused few species, like bracken fern, to become aggressive weeds in some disturbed areas. And with that, we’ve covered both nonvascular plants and the simpler vascular plants. Now it’s time to discuss the more complicated and more recognizable types of vascular plants, so let’s keep moving forward in the evolutionary history of plants.