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Seedless Vascular Plants

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Biology

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.

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 & 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 (Fig. 1). The quillworts and silver mosses do not have strobili, but instead have spores on their “microphylls”.

Seedless Vascular Plants Photograph of club moss Source: pixabay,com | StudySmarterFigure 1: Club moss in its diploid sporophyte form, showing the strobili and microphylls. Source: pixabay.com, edited.

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. Horsetails have “euphylls”, or true leaves that have been reduced, meaning 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).

Seedless Vascular Plants Photograph of fern and horsetail plants Source: pixabay.com | StudySmarterFigure 2: The top photo shows the underside of a fern leaf with sori; the bottom photo shows a horsetail plant. Source: pixabay.com, edited.

Still, the common factor linking the club mosses, spike mosses, quillworts, ferns, and horsetails is that they all predate the evolution of the seed. These lineages instead disperse their gametophyte generation by means of spores.

During the Carboniferous period, club mosses and horsetails reached up to 100 ft tall. That means they would have towered over even some of the woody trees we see in our forests today! Being 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 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 & 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.

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 (Fig. 3).

  1. The mature haploid gametophyte stage has both male and female sex organs- or antheridium and archegonium, respectively.

  2. The antheridium and archegonium both produce sperm and eggs via mitosis, as they are already haploid.

  3. The sperm must swim from the antheridium to the archegonium to fertilize the egg, meaning the fern depends on water for fertilization.

  4. Once fertilization happens, the zygote will grow into the independent diploid sporophyte.

  5. The diploid sporophyte has sporangia, which is where the spores are produced via meiosis.

  6. 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.

Seedless Vascular Plants Fern life cycle Source: Sigel et al. 2018, Frontiers in Plant Science | StudySmarterFigure 3: Fern life cycle showing diploid stages in green and haploid stages in brown. Source: Sigel et al. 2018, via Frontiers in Plant Science.

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.

Seedless Vascular Plants - Key takeaways

  • 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.
  • Seedless vascular plants include the monilophytes (ferns and horsetails) and lycophytes (clubmosses, spike mosses, and quillworts).
  • Seedless vascular plants have a dominant, more prevalent diploid sporophyte generation. They also have a reduced but independent gametophyte generation.
  • Ferns and other seedless vascular plants still rely on water for reproduction (for the sperm to swim to the egg).
  • The monilophytes have true leaves because they have multiple veins and are branched. The lycophytes have "microphylls" which have only a single vein running through them.
  • The seedless vascular plants have true roots and stems because of the presence of a vascular system.

Seedless Vascular Plants

Seedless vascular plants include the lycophytes and the monilophytes. The lycophytes include the:

  • Clubmosses

  • Spike mosses

  • and quillworts.

The monilophytes include the:

  • ferns 

  • and horsetails.

The seedless vascular plants include the two phyla: 

  • Lycophyta- clubmosses, quillworts, and spike mosses
  • Monilophyta- ferns and horsetails.


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.

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. 

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. 



Final Seedless Vascular Plants Quiz

Question

Which of the following are groups of seedless vascular plants?

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Answer

Monilophytes

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Question

Seedless vascular plants do NOT require water for reproduction.

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Answer

False

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Question

Seedless vascular plants are the early vascular plants, predating the gymnosperms and angiosperms.

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Answer

True

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Question

Seedless vascular plants have:

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Answer

Vascular systems for food, water, and nutrient transport.

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Question

The _____ _________ generation is the dominant, noticeable one in seedless vascular plants.

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Answer

Diploid sporophyte

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Question

The lycophytes (clubmosses, etc.) have _______, which 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.  

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Answer

Microphylls

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Question

What are strobili? 

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Answer

Spore-producing structures that clubmosses have. They produce haploid spores via meiosis. 

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Question

Euphylls are true leaves that have multiple, branched ____ running through them.

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Answer

Veins 

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Question

What are sori?

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Answer

Clusters of the sporangia, spore-producing structures, that are found on the underside of spores. 

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Question

What is the common factor linking the club mosses, spike mosses, quillworts, ferns, and horsetails?

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Answer

They all predate the evolution of the seed and are seedless vascular plants?

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Question

Why are roots an important adaptation?

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Answer

They allow vascular plants to stabilize in the soil.

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Question

Most seedless vascular plants are homosporous, meaning they produce only one kind of spore.

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Answer

True

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Question

Homospory means that a plant produces only one kind of spore, and the spore grows into a haploid gametophyte that has both ______ and ______ sex organs.

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Answer

Male and female

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Question

Heterospory means that a plant produces two kinds of spores, the ______ and the ______.

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Answer

Megaspore and the microspore

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Question

In heterosporous plants, the megaspore becomes a plant with only (1) _____sex organs and the microspore becomes a plant with only (2)_____ sex organs. 

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Answer

1. Female

2. Male

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