Ferns
The ferns first appeared about 400 million years ago and have flourished ever since, particularly in forests. A fern is a member of a group of roughly 12,000 species of vascular plants that reproduce via spores and have neither seeds nor flowers. They differ from mosses by being vascular.
Ferns are usually found in border habitats that other plants find hard to survive in. They grow in rock crevices, mountains, deserts and most are ground-dwelling, preferring damp, shady habitats but some are epiphytic, growing attached to the stems or leaves of other plants.
Ferns vary widely also in their appearance, ranging from the giant ferns, which can reach a height of 40 feet (12 metres), to tiny moss like species that grow on rocks.
In most ferns, the stem is usually a short, thick stock, as in the common male fern or a long rhizome as in epiphytic ferns. The stock grows almost completely buried in the ground, with an ascending growing point above ground surrounded by a close spiral of leaves, resembling a whorl, and often forming a basketlike tuft. Ferns have fibrous roots. They are generally considered non-woody plants but some giant ferns can be considered semi-woody. Roots usually grow from the backs of the leaf bases and on the lower surface of rhizomes which grow horizontally above or below the ground.
Ferns have green leaves that provide energy through photosynthesis. New leaves grow out from fiddleheads. They have leaves that produce spores called sporophylls. The leaves (fronds) of most ferns are spirally arranged and known as megaphylls. When they are in bud, in most species, they are rolled up. Like the stems they are normally covered (at least when young) with scales or hairs. Stomata are located in the epidermis of the leaf, and water and nutrients in solution are carried through the leaves and stems by xylem and phloem tissue.
Fern’s Life cycle
Ferns are vascular plants differing from lycophytes by having true leaves (megaphylls), which are often pinnate. They differ from seed plants (gymnosperms and angiosperms) in their mode of reproduction—lacking flowers and seeds. Like all other vascular plants, they have a life cycle referred to as alternation of generations, characterized by alternating diploid sporophytic and haploid gametophytic phases. The diploid sporophyte has 2n paired chromosomes, where n varies from species to species. The haploid gametophyte has n unpaired chromosomes, i.e. half the number of the sporophyte. The gametophyte of ferns is a free-living organism, whereas the gametophyte of the gymnosperms and angiosperms is dependent on the sporophyte.
Life cycle of a typical fern:
1. A diploid sporophyte phase produces haploid spores by meiosis (a process of cell division which reduces the number of chromosomes by a half).
2. A spore grows into a haploid gametophyte by mitosis (a process of cell division which maintains the number of chromosomes). The gametophyte typically consists of a photosynthetic prothallus.
3. The gametophyte produces gametes (often both sperm and eggs on the same prothallus) by mitosis.
4. A mobile, flagellate sperm fertilizes an egg that remains attached to the prothallus.
5. The fertilized egg is now a diploid zygote and grows by mitosis into a diploid sporophyte (the typical "fern" plant).