Plant Life Cycles and Reproduction

Life Cycle of Pteridophytes

Ferns have a life cycle dependent on spore production.
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.

A fertilized egg, or zygote, grows through the process of mitosis (cell division), producing roots, stems, and a new sporophyte. Embryonic sporophytes are initially tightly curled structures called fiddleheads that unfurl as they grow into fronds (leaf-like structures). The mature frond is the sporophyte, which contains multiple clusters of sporangia, sacs that hold asexual spores. Spores form by meiosis and are released into the air, and the life cycle continues.
The life cycle of ferns is dominated by the sporophyte stage. The sporophyte is diploid, meaning it has two copies of all chromosomes (genetic material). Cells from the sporophyte form spores, which are haploid (one copy of chromosomes). These grow into the haploid gametophyte. The gametophyte contains female structures (archegonia) and male structures (antheridia). The joining of the male sex cells (sperm) and female sex cells (eggs) is called fertilization. The genetic material from both sperm and ovule combine to form a diploid zygote, which then grows into the sporophyte.