In this lesson, we’ll discover the characteristics, classification, and life cycle of the group Pteridophyta, which includes plant species like ferns and horsetails.
Ferns and Their Relatives
What do you picture when you think of ancient forest-scapes, or even modern forests? It’s hard to think of a forest without a lush, green abundance of ferns. But what makes ferns and their relatives so special?The group pteridophyta, the ‘wing plants’, is a diverse bunch of related plants. At one point, pteridophyta was considered its own phylum, although now they are considered a group of disparate relatives with separate common ancestors. That makes pteridophyta a paraphyletic group, one containing many phyla.
This group includes ferns, horsetails, clubmosses, spikemosses, and quillworts. Let’s talk about how they’re related, as well as how they survive and thrive.
When the first vascular plants successfully rooted themselves into the early Earth’s soil 400 million years ago, they looked a little like pteridophytes. Vascular plants are plants that have food-transporting phloem and water-transporting xylem tissues. Most pteridophytes have vascular tissue in their stems and roots.
Pteridophytes are seedless plants. That means they have to pass on their genes to the next generation without using cones, fruits, or any other form of seed. Instead of seeds, ferns produce spore capsules, or sporangia, on the undersides of their green leaves or on specialized, non-green leaves called sporophylls.
Sometimes ferns can catapult their spores several meters away using the spring-like structure of these sporangia.Some pteridophytes are further united by the presence of a single leaf per vein, an arrangement known as a microphyll. Clubmosses, quillworts, and spikemosses have this simple leaf arrangement.
True ferns, horsetails, and whisk ferns all share larger, more complex branching leaves, known as megaphylls. Ferns and their kin share this megaphyll trait with other modern vascular plants such as cone-bearing plants and flowering plants.
To tease out the classification of pteridophytes, we’ve got to mention each of the key groups. They are the lycophytes (clubmosses, quillworts, and spikemosses), the whisk ferns, the horsetails, and the true ferns, each one with its own unique subset of characteristics.
The division known as lycophytes, or clubmosses, are relics of an ancient world where giant woody lycophytes thrived in the swamps of the Carboniferous Period 300 million years ago. Some ancient lycophytes stood more than 40 meters tall. That’s as tall as many water storage towers.These tall clubmosses became extinct and formed some of the coal we mine today. The living lycophytes are much smaller and thrive in the tropics or on forest floors. Even though they are called clubmosses, remember that lycophytes are not true mosses.
Quillworts and spikemosses are also specialized types of lycophytes, though they are in their own class separate from the clubmoss class. These seedless plants look a little like a bottle brush, and are among the oldest vascular plants.
Whisk Ferns, Horsetails, and True Ferns
Whisk ferns, horsetails, and true ferns are united by branching megaphyll leaves. The whisk ferns, in the genus Psilotum, are a kind of living fossil that harks back to the first vascular land plants 400 million years ago. They appear to be genetically most closely related to true ferns.Horsetails, or sphenophytes, have a brushy appearance and are often found along marsh edges and streams. The stem itself is the major place where photosynthesis occurs.
True ferns grew alongside the big tree-like lycophytes and the horsetails of the Carboniferous Period 300 million years ago and are by far the most diverse pteridophyte group today. They are often found in the tropics, but can grow in temperate forests and are even known from some arid habitats.
Life Cycle of Pteridophyta
The life cycle of pteridophytes is probably best exemplified by the general life cycle of the fern.
Ferns have a life cycle that alternates from a mature adult to an immature fern. We use the term sporophyte for the mature adult, and gametophyte for the immature baby fern. When you think of a sporophyte, think of the part of the fern that you can see. The leaves and stems of a fern comprise the adult sporophyte, but you would have to get down on your hands and knees and search in the soil to find the tiny, growing gametophyte.When the sporangia release spores, the spores land on the soil.
Each spore develops into a tiny, heart-shaped gametophyte. This young little fern can sustain itself through photosynthesis. The gametophyte is haploid, meaning it has only one copy of its chromosomes.Next, the gametophyte matures enough to produce male and female sex organs – the antheridium is the male organ that releases sperm, and the archegonium, or female organ, holds an egg that becomes fertilized by a different fern’s sperm. Moisture from rain moves the sperm to the egg.
These two sex organs mature at different rates, so a single gametophyte isn’t fertilizing itself.Once the gametophyte’s egg is fertilized, the egg develops into a new sporophyte, which has the two full copies of the chromosome, which we denote as diploid, or 2n in the diagram. The young fern then grows from the archegonium of its parent. This is almost as if our children grew out of a piece of our shed skin!
Pteridophytes are a diverse paraphyletic group of seedless, vascular plants that have an alternation of generations between a sporophyte and a gametophyte. The major groups include lycophytes (clubmosses, quillworts, and spikemosses), which has a long and fascinating fossil history, with some examples of ancient clubmosses growing up to 40 meters tall. Other groups include true ferns, horsetails and whisk ferns. These relationships are based on ever-changing genetic studies of the kinship of these vascular plants.
Some pteridophytes, such as ferns, bear their spores on the underside of their green leaves in clusters called sporangia. Other pteridophytes hold their spores along their stems or on non-green areas known as sporophylls.The alternation of generations allows a haploid gametophyte to mature into a diploid sporophyte. The mature sporophyte therefore has a full pair of chromosomes.