Ferns evolved alongside the dinosaurs and appeared long before cone-bearing gymnosperms and flowering angiosperms. This ancient plant family does not reproduce through seeds, instead depending on a spore-based life cycle that has been successful for millions of years. Spores evolved too, eventually leading to seed-bearing plants, but the vast majority of the fern species remain homosporous.
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Homosporous ferns produce only one type of spore. Each spore contains half the genetic code of the parent plant. Spores that land in a moist area develop into a tiny, flat, heart-shaped prothallus. A prothallus is the reproductive stage of the fern's life cycle and, although short-lived, is entirely independent of the parent fern. The prothallus forms both male sperm along the outer edge and female eggs near the notch. The sperm mature at a different rate than the eggs, and swim through available moisture to reach a mature egg on a different prothallus. The resulting embryonic fern eventually continues the cycle as it slowly matures.
Plants that produce two types of spores are heterosporous. While most ferns are homosporous, a few aquatic ferns are heterosporous and form separate male and female spores. The female spores are much larger than the male spores and are formed in their own specialised areas of the plant. Although the male spores are smaller, they are more numerous and also develop in specialised structures. As the spores mature, the female spore develops only into an egg and the male spores form only sperm. The spores are sexually differentiated, rather than a single spore developing into a single reproductive structure containing both male and female parts.
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Heterospores signalled an important evolutionary step for plants. The larger female spores contain stored nutrients, allowing the embryonic plant to develop rapidly rather than restricting growth based on available nutrients. The smaller male spores travel farther, cross-fertilising over a wide area and spreading the plant's genetic material. These differences led to seed development; larger female ovum are able to cope with wet or dry conditions before fertilisation, and the male pollen is able to drift on the wind for miles.
Pteridophytes, or spore-producing plants, include mosses and ferns. There are thousands of species in this group, but very few are heterosporous.. Heterosporous aquatic ferns (Salvinia spp.) are not native to North America and often form invasive mats on the water's surface. Water clover (Marsilea sp.) occurs throughout the world and produces hardened, spore-containing capsules that closely resemble seeds. Water clover, in terms of reproduction, is the most advanced member of the fern family.