Seedless Vascular Plants
The vast majority of plant life today is made up of flowering plants that reproduce using seeds. They are classified as vascular plants because they contain tissue that distributes water and nutrients to all parts of the plant.
Seedless vascular plants also have a vascular structure, roots, stems and leaves, but they reproduce in a very different way, using spores. Millions of years ago, seedless vascular plants were the primary form of plant life, but today are only represented by a small group of plants that include club mosses, scouring rushes, horsetails and ferns.
Spore to Gametophyte
When conditions are favorable for growth, spores are released by mature plants and dispersed into the air. Because of their tiny size they can be carried great distances by the wind but unlike seeds, spores have very little protection against adverse conditions.
A seed can remain dormant for long periods of time, protected by a hard outer shell. Not so with a spore. The spore must come in contact with moist soil in order to develop into a gametophyte.
Gametophyte to Sporophyte
Unlike flowering plants where fertilization takes place inside the flower, and the resulting zygote is protected until it forms an embryo inside a seed, the fertilization of a seedless vascular plant is quite different.
Under ideal conditions, the spore develops into a gametophyte either underground or on the surface of the ground. This is a very small structure that contains the reproductive parts of the plant. The eggs and sperm develop separately. The sperm must swim to the eggs in order for fertilization to occur.
Due to this limitation, seedless vascular plants moist conditions to develop. A zygote develops from which the young sporophyte grows. The young sporophyte is initially dependent on the gametophyte but as it matures it develops rhizomes and the gametophyte dies.
Leaves begin to emerge from the rhizome and the plant begins to resemble its mature form. Modified leaves called sporophyll form. These bear sporangia, small capsule-like structures that contain spores. When conditions are right the spores are released and the process begins again