Seedless Vascular Plants

Seedless Vascular Plants

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If you were to travel back 300 million years, you'd be surprised by the forests. They didn't have the same kinds of trees as we do now. The forests were filled with nonvascular plants and early vascular plants, which we call seedless vascular plants. Some examples include ferns and clubmosses.

We still see these seedless vascular plants today, but they're not as popular as the ones that produce seeds, like conifers and flowering plants. Unlike seed-producing plants, seedless vascular plants don't create seeds. Instead, they have a separate gametophyte generation through spores. Unlike nonvascular plants, though, seedless vascular plants do have a vascular system that helps them move water, food, and minerals.

Seedless vascular plants are an important part of our history and the evolution of plants. It's fascinating to see how plants have changed over time, and how they've adapted to different environments.

The definition of a seedless vascular plant

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.

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 are a group of plants that include quillworts, spike mosses, and club mosses. These plants are different from nonvascular mosses because they have vascular systems. The lycophytes are different from the monilophytes in that their leaf-like structures are called “microphylls”, which means “small leaf” in Greek. These “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 (Fig. 1). Quillworts and spike mosses, on the other hand, do not have strobili, but instead have spores on their “microphylls”.

Club moss in its diploid sporophyte form, showing the strobili and microphylls
Club moss in its diploid sporophyte form, showing the strobili and microphylls

The monilophytes

The monilophytes are a group of plants that are separated from the lycophytes because they have “euphylls” or true leaves, which are the plant parts we typically think of as leaves today. These “euphylls” are broad and have multiple veins running through them. The common names of plants in this group that you may recognize are the ferns and the horsetails.

Ferns have broad leaves and spore-bearing structures called sori located underneath their leaves. Horsetails, on the other hand, have “euphylls” or true leaves that have been reduced. This means that they are thin and not broad like fern leaves. Horsetail leaves are arranged at points on the stem in a “whorl” or circle (Fig. 2).

The top photo shows the underside of a fern leaf with sori; the bottom photo shows a horsetail plant

Despite their differences, the club mosses, spike mosses, quillworts, ferns, and horsetails all share the common characteristic of predating the evolution of the seed. These lineages disperse their gametophyte generation by means of spores.

During the Carboniferous period, club mosses and horsetails were able to grow up to 100 ft tall. This means that they would have towered over even some of the woody trees we see in our forests today! As the earlier vascular plants, they could grow tall with support from their vascular tissue and had little competition from seed plants, which were still evolving.

Seedless vascular plant characteristics

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 was a significant adaptation that led to the development 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 the sink (where they are used).

The evolution of vascular tissue allowed plants to grow more efficiently by transporting water and nutrients throughout the plant's body. With the development of vascular tissue, plants could grow taller and compete for resources more effectively. This innovation was a major step in the evolution of terrestrial plants, allowing them to thrive in a wide range of environments. Today, vascular plants are the dominant form of terrestrial vegetation and have played a crucial role in shaping the Earth's ecosystems.

True roots, stems, and leaves

The development of the vascular system in seedless vascular plant lineages was a significant event that introduced true roots, stems, and leaves. This innovation revolutionized the way plants interacted with the landscape, allowing them to grow bigger and colonize new parts of the land.

With the introduction of true roots, plants could go deeper into the soil, provide stability, and absorb water and nutrients more efficiently. Mycorrhizal connections between the roots and fungi further increased the surface area in soil, allowing for faster absorption of water and nutrients.

The vascular tissue allowed for the transport of water, nutrients, and sugars throughout the plant body. The stems became a central part of the plant body, allowing for larger growth and support.

Microphylls, small leaf-like structures with a single vein of vascular tissue, were the first leaf-like structures to evolve in vascular plants. Lycophytes, such as club mosses, have microphylls. Euphylls, true leaves with multiple veins and photosynthetic tissue, exist in ferns, horsetails, and other vascular plants. These leaves allowed for more efficient photosynthesis and contributed to the success and diversity of 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.

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 involves several distinct stages. The mature haploid gametophyte stage has both male and female sex organs, antheridium and archegonium respectively. Both organs produce sperm and eggs via mitosis, as they are already haploid. Water is necessary for fertilization, as the sperm must swim from the antheridium to the archegonium to fertilize the egg. Once fertilization occurs, the zygote will develop into an independent diploid sporophyte that has sporangia, where spores are produced via meiosis. Sori, clusters of sporangia, are found on the underside of fern leaves. When the sori mature, they will release spores, and the life cycle will restart.

Fern life cycle showing diploid stages in green and haploid stages in brown

In summary, seedless vascular plants are a group of early land plants that have vascular systems but lack seeds, and instead, disperse spores for their haploid gametophyte stage. They include the monilophytes (ferns and horsetails) and lycophytes (clubmosses, spike mosses, and quillworts). Seedless vascular plants have a dominant, more prevalent diploid sporophyte generation and a reduced but independent gametophyte generation. Although they have true roots, stems, and leaves, they still rely on water for reproduction. The monilophytes have true leaves, while the lycophytes have "microphylls." Most seedless vascular plants are homosporous, but some are heterosporous, which is common in seed-producing vascular plants. The adaptation of heterospory in the seedless vascular plants was an important step in the evolution and diversification of plants.

Seedless Vascular Plants

What are 4 types of seedless vascular plants? 

Seedless vascular plants include the lycophytes and the monilophytes. The lycophytes include the: Clubmosses Spike mossesand quillworts. The monilophytes include the: ferns and horsetails.

What are the three phyla of seedless vascular plants? 

The seedless vascular plants include the two phyla: Lycophyta- clubmosses, quillworts, and spike mosses Monilophyta- ferns and horsetails.

How do seedless vascular plants reproduce? 

Seedless vascular plants reproduce the diploid sporophyte generation sexually via sperm and egg. The sperm is produced in the antheridium on the haploid gametophyte via mitosis. The egg is produced in the archegonium of the haploid gametophyte, also via mitosis. The sperm still relies on water to swim to the egg in seedless vascular plants. The haploid gametophyte grows from spores, that are produced in the sporangia (spore-producing structures) of the sporophyte. Spores are produced via meiosis. Heterospory, which is when two types of spores are produced that make separate male and female gametophytes, evolved in some species of seedless vascular plants. Most species, however, are homosporous and produce only one kind of spore that produces a gametophyte with both male and female sex organs.

What are seedless vascular plants? 

Seedless vascular plants are a group of early land plants that have vascular systems but lack seeds, and instead, disperse spores for their haploid gametophyte stage. They include ferns, horsetails, club mosses, spike mosses, and quillworts. 

Why are seedless vascular plants important? 

Seedless vascular plants are the earliest vascular plants, meaning scientists like to study their evolution to understand more about plant evolution over time. Additionally, after nonvascular plants, seedless vascular plants are usually some of the first to occupy land during a succession event, making the soil more hospitable to other plant and animal life.

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