Pteridophyta include ferns, horsetails, and whisk ferns. The Pteridophyte group has a fossil record dating back to the Devonian period, a period of time during the Paleozoic Era, lasting from about 416 to 359 million years ago. Today, there are about 13,000 species in 35 plant families and 568 genera. Many, but not all, epiphytes are also pteridophytes. Epiphytes live on other plants, usually in tropical ecosystems, and get water from the air or from rainfall. Common epiphytes that are pteridophytes include spike moss (not a true moss), quillworts, and club moss (also not a true moss). In addition, a number of pteridophytes are aquatic, including salvinia, water ferns, water velvet, and water sprites.Pteridophytes are similar to bryophytes in that they are seedless, flowerless plants that depend on the production of spores to propagate. They undergo photosynthesis, so they are autotrophs, meaning they make their own food. Pteridophytes differ from bryophytes, however, in that they are vascular plants with structures for transporting water from roots, through stems, and into leaves. Unlike mosses, ferns and their closely related species are homosporous, having only one type of spore.
The life cycle of pteridophytes is a continuous reproductive process that is dominated by the sporophyte (sexual) stage of the alternation of generations. Fern spores are catapulted into the air, and the spores develop into heart-shaped haploid gametophytes that contain both male and female sex organs. As the young gametophyte matures, the sex organs become active. In ferns, the male reproductive organ is the antheridium, which produces and releases sperm. The female reproductive organ is the archegonium, at the base of which lies the egg. Fern reproduction requires water for the sperm to swim to the archegonium and fertilize the egg. The presence of water is essential for maintaining genetically healthy sporophytes. Sperm, released in large numbers from many gametophytes, swim through the same waters that contain the archegonium. Thus, sperm cells from the same gametophyte do not necessarily fertilize that gametophyte's egg. Without this opportunity for cross-fertilization, inbreeding might lead to a rise of harmful recessive alleles.