34.3: Seedless Vascular Plants

Seedless Vascular Plants Were the First Tall Plants on Earth

Today, seedless vascular plants are represented by monilophytes and lycophytes. Ferns—the most common seedless vascular plants—are monilophytes. Whisk ferns (and their relatives) and horsetails are also monilophytes. Lycophytes include club mosses, spikemosses, and quillworts—none of which are true mosses.

Unlike nonvascular plants, vascular plants—including seedless vascular plants—have an extensive network of vascular tissue comprised of xylem and phloem. Most seedless vascular plants also have true roots and leaves. Furthermore, the life cycles of seedless vascular plants are dominated by diploid spore-producing sporophytes, rather than gametophytes.

However, like nonvascular plants, seedless vascular plants reproduce with spores rather than seeds. Seedless vascular plants are also typically more reproductively successful in moist environments because their sperm require a film of water to reach the eggs.

The Life Cycle of Seedless Vascular Plants

Like animals, seedless vascular plants (and other plants) alternate between meiosis and fertilization during reproduction. Meiosis is a cell division process that produces haploid cells—which contain one complete set of chromosomes—from a diploid cell—which contains two complete sets of chromosomes. Fertilization, by contrast, produces a diploid cell called a zygote through the fusion of haploid cells called gametes—sperm and eggs.

In most animals, only the diploid stage is multicellular, and gametes are the only haploid cells. Plants, however, alternate between haploid and diploid stages that are both multicellular; this is called alternation of generations. Alternation of generations is a feature of all sexually reproducing plants, but the relative size and prominence of the haploid and diploid stages differ among plants.

In seedless vascular plants (as well as seed plants), the diploid stage of the life cycle—the sporophyte—is dominant. For example, what most people recognize as a fern is the large, independent fern sporophyte. Sporophytes produce haploid cells called spores through meiosis.

A spore can germinate and develop into a gametophyte—the haploid stage of the life cycle—through mitosis. Gametophytes produce egg and sperm cells through mitosis (unlike animals, which produce gametes through meiosis). Most seedless vascular plants produce one type of spore that gives rise to a bisexual gametophyte. The gametophytes are smaller and less structurally complex than the sporophytes, but they can photosynthesize and do not depend on the sporophyte for nourishment or protection.

Egg and sperm cells fuse through fertilization, forming a diploid zygote. The zygote divides through mitosis to generate the familiar, fronded fern sporophyte—continuing the cycle.

There are three major groupings covering the plant life on Earth—nonvascular plants, seedless vascular plants, and seed plants.

The seedless vascular plants were the first to evolve specialized vascular systems—an adaptation that helped them become the first tall plants on Earth. Today, seedless vascular plants are represented by lycophytes and monilophytes. 

Lycophytes include clubmosses, spikemosses, and quillworts. Notably, none of the lycophytes are true mosses, which are nonvascular plants. Monilophytes include ferns, horsetails, and whisk ferns and their relatives.

Like all plants, seedless vascular plants display an alternation of generations in their life cycle, as shown here using a fern plant as an example. 

This means that they spend part of their life cycle as a haploid gametophyte, and the other part as a diploid sporophyte. 

Like nonvascular plants, seedless vascular plants reproduce using spores, rather than seeds. The spores are haploid, and are dispersed by structures called sori, clustered on the underside of the leaves. 

The sori themselves contain many sporangia. Upon reaching maturity, these sporangia open, dispersing the haploid spores. The spores then grow via mitosis to form the haploid gametophyte. 

At the gametophyte stage - which is typically very small and found on or just below the soil surface - haploid gametes are formed by mitosis. A single gametophyte is bisexual and develops two different structures - the antheridia and archegonia - that produce gametes in male and female forms respectively.

Like the nonvascular plants, the male sperm gamete is flagellated and requires water to travel to the female gamete, following a chemical attractant to find the egg.

Because the gametes in a single gametophyte will be genetically identical due to their haploid origin, crosses typically occur between different gametophytes. Ferns can prevent any self-fertilization by having their antheridia and archegonia mature at different times. 

Finally, the fertilized egg will grow a new diploid sporophyte from the diploid zygote of the gametophyte, completing the life cycle.

Like seed plants, seedless vascular plants have life cycles dominated by sporophytes. However, unlike either of the other major plant lineages, their smaller gametophytes can live independently—meaning they do not provide nourishment to the sporophyte, or require it from the sporophyte.

Arguably the key feature of seedless vascular plants is their specialized network of vascular tissue, akin to that of the seed plants. This adaptation allowed them to transport water, nutrients, and other organic materials, and to attain greater sizes—which distinguished them from their nonvascular relatives.