If you were to travel back in time 300 million years, you wouldn't be standing in any kind of forest you've ever seen before. In fact, the forests of the Carboniferous period were dominated by nonvascular plants and early vascular plants, known as the seedless vascular plants (e.g., ferns, clubmosses, and more).
We still find these seedless vascular plants today, but now they are overshadowed by their seed-producing counterparts (e.g., conifers, flowering plants, etc.). Unlike their seed-producing counterparts, seedless vascular plants do not produce seeds, but rather have an independent gametophyte generation through the production of spores.
Unlike nonvascular plants, however, seedless vascular plants contain a vascular system that supports them in the transport of water, food, and minerals.
What are seedless vascular plants?
Seedless vascular plants are a group of plants that have vascular systems and use spores to disperse their haploid gametophyte stage. They include the lycophytes (e.g., clubmosses, spike mosses, and quillworts) and monilophytes (e.g., ferns and horsetails).
Seedless vascular plants were the early vascular plants, predating the gymnosperms and angiosperms. They were the dominant species in ancient forests, consisting of nonvascular mosses and seedless ferns, horsetails, and club mosses.
Characteristics of seedless vascular plants
Seedless vascular plants are early vascular plants that contain a number of adaptations that helped them survive life on land. You will notice that a lot of the characteristics that developed in the seedless vascular plants are not shared with nonvascular plants.
Vascular tissue: a novel adaptation
The development of the tracheid, a type of elongated cell that makes up the xylem, in early land plants led to the adaptation of vascular tissue. Xylem tissue contains tracheid cells fortified by lignin, a strong protein, that provides support and structure to vascular plants. The vascular tissue includes the xylem, which transports water, and the phloem, which transports sugars from the source (where they are made) to sink (where they are used).
True roots, stems, and leaves
With the development of the vascular system in the seedless vascular plant lineages came the introduction of true roots, stems, and leaves. This revolutionized the way plants interacted with the landscape, allowing them to grow bigger than they ever could before and colonize new parts of the land.
Roots and stems
True roots appeared after the introduction of vascular tissue. These roots can go deeper into the soil, provide stability, and absorb water and nutrients. Most roots have mycorrhizal connections, meaning they are connected to fungi, in which they exchange sugars for nutrients the fungi extract from the soil. Mycorrhizae and the extensive root systems of vascular plants allow them to increase the surface area in soil, meaning they can absorb water and nutrients faster.
The vascular tissue allowed the transport of the water from the roots to the stems to the leaves for photosynthesis. Additionally, it allowed for the transport of sugars produced in photosynthesis to the roots and other parts that can't make food. The adaptation of the vascular stem allowed for the stem to be a central part of the plant body that could grow to larger proportions.
Leaves
Microphylls are small leaf-like structures, with only a single vein of vascular tissue running through them. Lycophytes (e.g., club mosses) have these microphylls. These are thought to be the first leaf-like structures that evolved in vascular plants.
Euphylls are the true leaves. They contain multiple veins and photosynthetic tissue in between the veins. Euphylls exist in the ferns, horsetails and other vascular plants.
A dominant sporophyte generation
Unlike the nonvascular plants, the early vascular plants developed a dominant diploid sporophyte generation, independent of the haploid gametophyte. Seedless vascular plants also have a haploid gametophyte generation, but it is independent and reduced in size compared to nonvascular plants.
Seedless vascular plants: common names and examples
Seedless vascular plants are mainly split into two groups, the lycophytes and the monilophytes. These aren’t common names, however, and might be a little confusing to remember. Below we go over what each of these names means and some examples of seedless vascular plants.
The lycophytes
The lycophytes represent the quillworts, spike mosses, and club mosses. Although these have the word “moss” in them, these are actually not true nonvascular mosses, because they have vascular systems. The lycophytes differ from the monilophytes in that their leaf-like structures are called “microphylls”, meaning “small leaf” in Greek. The “microphylls” are not considered true leaves because they only have a single vein of vascular tissue and the veins are not branched like the “true leaves” that monilophytes have.
Club mosses have cone-like structures called strobili where they produce the spores that will become haploid gametophytes. The quillworts and silver mosses do not have strobili, but instead have spores on their “microphylls”.
The monilophytes
The monilophytes are separated from the lycophytes because they have “euphylls” or true leaves, the plant parts we particularly think of as leaves today. These “euphylls” are broad and have multiple veins running through them. The common names you may recognize of plants in this group are the ferns and the horsetails.
Ferns have broad leaves and spore-bearing structures called sori located underneath their leaves.
The life cycle of seedless vascular plants
The seedless vascular plants go through an alternation of generations just as the nonvascular plants and other vascular plants do. The diploid sporophyte, however, is the more prevalent, noticeable generation. Both the diploid sporophyte and haploid gametophyte are independent of each other in the seedless vascular plant.
Fern life cycle
The life cycle of a fern, for example, follows these steps.
The mature haploid gametophyte stage has both male and female sex organs- or antheridium and archegonium, respectively.
The antheridium and archegonium both produce sperm and eggs via mitosis, as they are already haploid.
The sperm must swim from the antheridium to the archegonium to fertilize the egg, meaning the fern depends on water for fertilization.
Once fertilization happens, the zygote will grow into the independent diploid sporophyte.
The diploid sporophyte has sporangia, which is where the spores are produced via meiosis.
On the fern, the underside of the leaves have clusters known as sori, which are groups of sporangia. The sori will release spores when they mature, and the cycle will restart.
Notice that in the fern life cycle, although the gametophyte is reduced and the sporophyte is more prevalent, the sperm still relies on water to reach the egg in the archegonium. This means that ferns and other seedless vascular plants must live in damp environments to reproduce.
Homospory versus heterospory
Most seedless vascular plants are homosporous, which means they produce only one type of spore, and that spore will grow into a gametophyte that has both male and female sex organs. However, some are heterosporous, which means they make two different kinds of spores: megaspores and microspores. The megaspores become a gametophyte bearing only female sex organs. Microspores develop into a male gametophyte with only male sex organs.
Although heterospory is not common in all seedless vascular plants, it is common in seed-producing vascular plants. Evolutionary biologists believe the adaptation of heterospory in the seedless vascular plants was an important step in the evolution and diversification of plants, as many seed-producing plants contain this adaptation.
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