Ferns (Polypodiopsida or Polypodiophyta) are a group of vascular plants that reproduce via spores and have neither seeds nor flowers. They reproduce in two phases, a short-lived gametophyte that reproduces sexually to develop a spore-bearing leaf structure. Some ferns produce coiled fiddleheads, which expand into fronds. The group includes over 10,500 extant species. The Polypodiopsida consist of both the leptosporangiate (Polypodiidae) and eusporangiate ferns, the latter group including horsetails, whisk ferns, marattioid ferns and ophioglossoid ferns. The fern crown group is estimated to have originated c. 423 million years ago (mya), during the late Silurian period and the rapid radiation of land plants, but Polypodiales, the group that makes up 80% of living fern diversity, did not appear and diversify until the Cretaceous (c. 143 to 66 mya), contemporaneous with the heightened diversification of flowering plants. Some fern species, such as bracken (Pteridium aquilinum) and water fern (Azolla filiculoides), are significant weeds worldwide, and brackens are carcinogenic. Ferns are not of major economic importance, but some are used as food, medicine, and ornamental plants. They also play a role in human culture.
Description
The life cycle of a fern occurs in two stages (as in clubmosses and horsetails). Firstly, each active spore germinates into a short-lived gamete-producing gametophyte, known as the prothallus, which is anchored to the ground by rhizoids. Secondly, a sporophyte, which is free-living and only briefly dependent on the maternal gametophyte, develops spores on fertile fronds. As in all vascular plants, the sporophyte is the dominant phase of the alternation of generations expressed by all land plants. Ferns differ from seed plants in their reproduction by spore dispersal and lack of flowers and seeds. Ferns differ from spore-bearing lycophytes by having true leaves, which are often pinnate. In more detail, after spores settle on the soil, they germinate to form initial rhizoids and protonemata, which develop into a free-living haploid gametophyte by mitosis (a process of cell division which maintains the number of chromosomes). Using mitosis, the gametophyte (the prothallus) produces spherical antheridia containing male gametophytes (i.e. sperm or antherozoids) and archegonia, which release a single oosphere (egg cell). The flagellate sperm swims into the archegonium and fertilizes the egg, which remains attached to the prothallus as it grows by mitosis into a diploid zygote that develops into the sporophyte, while the separate prothallus persists briefly. Finally, the sporophyte undergoes a diploid phase in which it produces haploid spores by meiosis (a process of cell division which reduces the number of chromosomes by a half), and a mature frond releases spores from the sori (clusters of spore-enclosing sporangia) on its underside. 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. Sometimes a gametophyte can give rise to sporophyte traits like roots or sporangia independently from the sporophyte.
Gametophyte The gametophytes of ferns are very different from those of seed plants (gymnosperms and angiosperms). They are free-living and resemble liverworts, whereas those of seed plants develop within the spore wall and are dependent on the parent sporophyte for their nutrition. A fern gametophyte typically consists of:
Prothallus: A green, photosynthetic structure, the initial growth of which is planar in one cell layer. It resembles a heart or kidney in shape and measures 3–10 millimetres (1⁄8–3⁄8 inch) by 2–8 mm. The prothallus produces gametes by means of: Antheridia: Small spherical structures that produce flagellate antherozoids. Archegonia: A flask-shaped structure that produces a single egg at one end, which the male gametophyte reaches by swimming down the neck. Rhizoids: root-like structures (not true roots) that consist of single greatly elongated cells that absorb water and mineral salts over the whole structure. Rhizoids anchor the prothallus to the soil.
Sporophyte
Extant ferns are herbaceous perennial plants and most lack woody growth. When woody growth is present, it is found in the stem. Their foliage may be deciduous or evergreen, and some are semi-evergreen depending on the climate. Like the sporophytes of seed plants, those of ferns consist of stems, leaves, and roots. Ferns differ from spermatophytes in that they reproduce by spores rather than by seeds. However, they also differ from spore-producing bryophytes in that, like seed plants, they are polysporangiophytes, their sporophytes branching and producing many sporangia. Unlike those of bryophytes, fern sporophytes are free-living and only briefly dependent on the maternal gametophyte. The green, photosynthetic part of the plant is technically a megaphyll (more complex than the microphylls of clubmosses) and in ferns, it is often called a frond. In leptosporangiate ferns, new leaves typically expand by the unrolling of a tight spiral called a crozier or fiddlehead into fronds. This uncurling of the leaf is termed circinate vernation. In some families, such as the Blechnaceae, the leaves are divided into two types, sporophylls or fertile fronds that produce spores and trophophylls or sterile fronds that do not. Fern spores are borne in sporangia which are usually clustered to form sori. The sporangia may be covered with a protective coating called an indusium. The arrangement of the sporangia is important in classification. In monomorphic ferns, the fertile and sterile leaves look morphologically the same, and both are able to photosynthesize. In hemidimorphic ferns, just a portion of the fertile leaf is different from the sterile leaves. In dimorphic (holomorphic) ferns, the two types of leaves are morphologically distinct. The fertile leaves are much narrower than the sterile leaves, and may have no green tissue at all, as in the Blechnaceae and Lomariopsidaceae.
… excerpt ends here. Continue reading the full article.
