Seedless Vascular Plants

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.