Seedless Vascular Plants

Seedless vascular plants are one of the three major plant groups on Earth, along with nonvascular plants and seed plants.

They were among the first plants to develop specialized vascular tissues, which allowed them to grow taller than seedless nonvascular plants.

These plants share traits with both nonvascular plants, such as mosses, and seed plants, such as conifers and flowering plants.

Modern seedless vascular plants include two major lineages: lycophytes and monilophytes.

Lycophytes include clubmosses, spikemosses, and quillworts, although none of these are true mosses, which are nonvascular plants. Monilophytes include ferns, horsetails, whisk ferns, and other members.

Like all plants, they show alternation of generations, cycling between a haploid gametophyte and a diploid sporophyte.

These plants reproduce through spores, like nonvascular plants, instead of forming seeds like seed plants.

For example, in ferns, the dominant stage is the sporophyte. On the undersides of the leaves, there are often many sori.

Each sorus is a cluster of sporangia where haploid spores are produced. When the sporangia mature, they open and release spores.

After dispersal, each spore undergoes mitosis and forms a small, free-living haploid gametophyte.

Some seedless vascular plants have bisexual gametophytes that contain both antheridia, the male reproductive organs, and archegonia, the female reproductive organs. Antheridia produce haploid sperm, and archegonia produce haploid eggs through mitosis.

Because the sperm are flagellated, fertilization still depends on water, as it does in nonvascular plants. The sperm swim by following chemical signals released by the egg.

Since both gametes come from the same haploid gametophyte, self-fertilization can reduce genetic variation. Many ferns reduce self-fertilization by having male and female structures mature at different times.

Finally, the fertilized egg develops into a new diploid sporophyte, completing the life cycle.

Seedless vascular plants have life cycles dominated by sporophytes, like seed plants. However, unlike those of the other major plant groups, their smaller gametophytes can live independently and do not nourish the sporophyte or depend on it for nourishment.

The key feature of seedless vascular plants is their specialized vascular tissue system, which is similar to that of seed plants. This adaptation allows them to transport water, nutrients, and other organic materials and to grow larger than nonvascular plants.