Ferns

Fusion of the egg and sperm nuclei during fertilization leads to the formation of a zygote, with the unreduced number of chromosomes, usually two sets. The zygote develops into a sporophyte, the most familiar stage of the fern life cycle. As the sporophyte grows, the prothallus to which it is attached eventually decays. Most fern sporophytes in temperate North America are green and terrestrial.

As the sporophyte continues to grow, it eventually develops numerous structures with spores inside, referred to as sporangia. The sporangia form on the underside of fronds or on specialized fertile fronds, depending on the species. In many species, the sporangia develop in clusters referred to as sori (singular, sorus). The size, shape, and position of the sori are frequently used in species identification. As development proceeds, the sporangium dries out, releasing the many spores inside for dispersal into the environment.

Most ferns are homosporous, in that all their spores are identical and all spores develop into a gametophyte with antheridia and archegonia. However, some water ferns are heterosporous. In these species, separate male and female spores develop on the sporophyte. The smaller and more numerous male spores germinate and develop into male gametophytes with antheridia. The female spores germinate and develop into female gametophytes with archegonia.