Ferns are non flowering Vascular Plants with roots, stems and complex leaves. There are about 12,000 Species of Ferns which come in a variety of shapes and sizes. Ferns first appear in the fossil record 360 million years ago in the Carboniferous Period but many of the current families and Species did not appear until roughly 145 million years ago in the late Cretaceous Period. They are one of the earliest Vascular Plant forms on Earth. about 9000 Species. The spores are rich in lipids, protein and calories and some vertebrates so eat these. The European Woodmouse (Apodemus sylvaticus) has been found to eat the spores of Culcita macrocarpa and the Bullfinch (Pyrrhula murina) and the Short-Tailed Bat (Mystaina tuberculata) also eat Fern spores. A few of the Species that are most popular include the Boston Fern found in a shipment of other Fern Species coming from Boston, the Tropical Sword Fern, Maidenhair Fern, often used to treat respiratory illnesses, and the Christmas Fern, which is dark green and resembles an evergreen. There isn't much economic importance of Ferns part from some being used as Ornamental Plants, food source and for remediating contaminated soils. Some of the other useful Ferns are Tree Ferns (families Dicksoniaceae and Cyatheaceae) whose wood is used in construction. Starchy Pith was formerly eaten by the Maori's and other native groups. The dense root systems are widely used as a substrate for growing Orchids; many populations of Tree Ferns are destroyed for this purpose. Dense golden hair covers the base of the leaf stalks and buds in many Species and is exported as “pulu” for mattress and pillow stuffing and for packing material. A large number of Fern Species are used medicinally by local populations, especially in the tropics.
Kingdom - Plantae. Division - Pteridophyta.
Classification according to Smith System (Alan Smith)
1. Class Psilotopsida - There are about about 92 Species.
i) Order Ophioglossales.
Family Ophioglossaceae (incl. Botrychiaceae, Helminthostachyaceae).
ii) Order Psilotales
Family Psilotaceae (incl. Tmesipteridaceae).
2) Class Equisetopsida [=Sphenopsida] - There are about 15 Species.
i) Order Equisetales
3) Class Marattiopsida - There are about 150 Species.
4) Class Pteridopsida [=Filicopsida, Polypodiopsida] - There are about 9000 Species.
i) Order Osmundales
ii) Order Hymenophyllales
Family Hymenophyllaceae (incl. Trichomanaceae).
iii) Order Gleicheniales
Family Gleicheniaceae (incl. Dicranopteridaceae, Stromatopteridaceae).
Family Dipteridaceae (incl. Cheiropleuriaceae).
iv) Order Schizaeales
Family Anemiaceae (incl. Mohriaceae).
v) Order Salviniales
Family Marsileaceae (incl. Pilulariaceae).
Family Salviniaceae (incl. Azollaceae).
vi) Order Cyatheales
Family Cyatheaceae (incl. Alsophilaceae, Hymenophyllopsidaceae).
Family Dicksoniaceae (incl. Lophosoriaceae).
vii) Order Polypodiales
Adiantum lunulatum from Family Pteridaceae.
Family Lindsaeaceae (incl. Cystodiaceae, Lonchitidaceae).
Family Dennstaedtiaceae (incl. Hypolepidaceae, Monachosoraceae, Pteridiaceae).
Family Pteridaceae (incl. Pellaeaceae, Adiantaceae, Ceratopteridaceae, Cryptogrammaceae).
Family Woodsiaceae (incl. Athyriaceae, Cystopteridaceae).
Family Blechnaceae (incl. Stenochlaenaceae).
Family Dryopteridaceae (incl. Aspidiaceae, Bolbitidaceae, Elaphoglossaceae, Hypodematiaceae, Peranemataceae).
Family Lomariopsidaceae (incl. Nephrolepis).
Family Polypodiaceae (incl. Drynariaceae, Grammitidaceae, Gymnogrammitidaceae, Loxogrammaceae, Platyceriaceae, Pleurisoriopsidaceae).
Life Cycle of Ferns
Ferns have a life cycle referred to as 'Alternation of Generations; characterized by a Diploid Sporophytic and a Haploid Gametophytic Phase.
First, Sporangia (structure containing spores) on the underside of the Fern leaf release Haploid Spores.
A Spore develops into a Haploid Gametophyte, which produces gametes (sperm or eggs) through Mitosis.
The sperm swim to the egg and fertilization occurs within the Archegonium (female sex organ), resulting in a Diploid Zygote. (The sperm have flagella and require a film of water on the Plant to swim to the egg, so ferns cannot reproduce without moisture).
The zygote develops into the Sporophyte, growing out of the Gametophyte so rapidly that it tears the Gametophyte apart and kills it. The mature Diploid Sporophyte is a large, leafy Fern Plant. It produces spores through Meiosis and the cycle begins again.
Characteristics and Physical Features of Ferns
Fern Structure - Fern structure includes various body parts.
Stems - Fern Stems are mostly often underground creeping Rhizome(shoots above and roots below), sometimes an above-ground creeping Stolon (production of new plants from buds) like Polypodiaceae or an above-ground erect semi-woody Trunk like Cyatheaceae.
Leaf - Fern Leaves are often known as Fronds. They are known as Fronds not because because of differences in structure but because of the historical division between people who study Ferns and people who study Seed Plants. New leaves of Ferns typically expand by the unrolling of a tight spiral called a Crozier or Fiddlehead. This uncurling of the leaf is called 'Circinate Vernation'. The Fronds often consist of leaflets referred to as Pinnae. On the underside of a Fern leaf, there maybe brown dots called Sori, which are actually clusters of Sporangia. Spongia are structures on just one specialized portion of a special leaf or on some or all of the leaves.
There are 3 Types of Fern Leaves:
1. Trophophyll : It is a type of leaf that does not produce spores but produces only sugars by photosynthesis.
2. Sporophyll : It is a type of leaf that produces spores. These leaves are similar to the scales of Pine Cones or to stamens and pistil in Gymnosperms and Angiosperms. But the Sporophylls of Ferns are typically not very specialized and look similar to Trophophylls, producing sugars by photosynthesis just as the Trophophylls do.
3. Brophophyll : It is type of leaf that produces abnormally large amounts of spores. Their leaves are also larger than the other leaves but bear a resemblance to Trophophylls.
Roots - The roots of Ferns are underground and non-photosynthetic in structure. They take up water and nutrients from soil. They are always fibrous and are structurally very similar to the roots of Seed Plants.
Prothallus - Fern Prothallus is a green, photosynthetic structure that is one cell thick, usually heart or kidney shaped, 3–10 mm long and 2–8 mm broad.
The Prothallus produces gametes by means of:
Antheridia : Small spherical structures that produce flagellate sperm.
Archegonia : A flask-shaped structure that produces a single egg at the bottom, reached by the sperm by swimming down the neck.
Rhizoids - Ferns have root-like structures which consist of single greatly elongated cells. Water and mineral salts are absorbed over the whole structure. Rhizoids anchor the Prothallus to the soil.
Vascular Plants - Ferns are Vascular Plants as they have a system of woody tubes that transport water and add strength and rigidity to the plants. It also helps the plant to grow large in size. For the exposed part of the Fern to retain moisture, the Fern contains a microscopic waterproof layer called the Cuticle.
Epiphytes - Ferns are Plant that derives moisture and nutrients from the air and rain. They usually grows on another Plant but are not parasitic. They grow on the trunks and limbs of Trees. About a quarter to a third of all Fern Species are Epiphytes.
Reproduction via Spores - The Sporangia produce microscopic spores, which are released at the appropriate season and scattered by the wind. Those that find the right conditions develop into a small sexual generation plant called a Prothallia. Each of these short-lived intermediate Plants produce a few eggs and/or many sperm cells. The sperm require a film of water on the plants to swim over to the flask-like structures that hold the eggs. This is one reason why Ferns live in damp, shaded areas where condensation from fog and dew is common. The fertilized egg develops into a new spore-producing Fern, living off the Prothallia until it develops enough roots and fronds to survive on its own, at which time the Prothallia dies off.
Absence of Flowers and Seeds - Ferns do not bear Flowers and and as a result never produce seeds. They have Spores instead by which they reproduce.
Homosporous - Most Ferns are Homosporous. Each Fern has Spore of one shape and size which develops into one kind of Gametophyte. The Spore is usually 30 to 50 micrometres in length or diameter, although some reach more than 100 micrometres. A few Fern families, however, have Dimorphic Spores, small ones (microspores) and large ones (megaspores).
Preference for Moist Habitats - Ferns are delicate plants which grow in areas which are moist. They prefer sheltered areas on the Forest floor, near streams and other sources of permanent moisture. However there are a few exceptions, like the Desert Ferns have evolved several strategies to thrive in the warm, dry climates by curling up when dry and reviving during rainy periods.
Size - The sizes of Ferns vary considerably among the different Species. Tree Ferns of the Cyatheaceae family are the largest Ferns. They are tropical plants which can grow 60 ft (18 m) or more in height and have fronds 15 ft (5 m) or more in length. In contrast, Species in the Genus Azolla, a group of free-floating aquatic Ferns, have very simple Fronds (compound leaf) which are less than 0.2 in (0.5 cm) in diameter.
Geographical Range and Habitat
Ferns are found in a wide variety of Habitats. They can be found in Forests, Mountains, Deserts, Wetlands, Grasslands etc. However they are common in Moist Shady Forests, in Mountains amongst crevices in rock faces, in Acid Wetlands including Bogs and Swamps; and Tropical Forest Trees. However Ferns are the most abundant in the Tropics. Ferns are generally considered as 'Specialists in Marginal Habitats', often thriving where various other Plants are unable to survive. There are very few Species in Arctic and Antarctic regions.
History and Evolution of Ferns
The Ferns are one of the ancient vascular plants, some of them as old as the Carboniferous Period (about 359 million years ago) and perhaps older. Several extinct groups of the Carboniferous Period and the Permian Period (299 million to 251 million years ago) that followed - Coenopteridaceae, Anachoropteridaceae, Tedeliaceae, Sermayaceae, and Tempskyaceae - represent related lines of evolution, but there are no intermediate examples to show close ties with any of the modern families of Ferns. By the time of the Triassic Period (beginning 251 million years ago), some of the modern Fern families were well established, and there are fossil records of the families Osmundaceae, Equisetaceae, Marattiaceae, Schizaeaceae, Matoniaceae, Dipteridaceae, Cyatheaceae, Marsileaceae, and Salviniaceae. However, according to most estimates, the families that contain the bulk of the modern Fern Species did not diversify until the Cretaceous Period (145.5 million to 65.5 million years ago.
The sex organs of Ferns are of two types:
1. Antheridium - It is the sperm-producing organ and consists of a jacket of sterile cells with sperm-producing cells inside. Antheridia may be sunken (as in the families Ophioglossaceae and Marattiaceae) or protruding. They vary in size from those with hundreds of sperm to those with only 12 or so.
2. Archegonium - It is the egg-producing organ, and contains one gamete (sex cell), which is always located in the lower, more or less dilated portion of the Archegonium, the Venter. The upper part of the Archegonium, the neck, consists of four rows of cells containing central neck cells. The uppermost of the neck cells are the neck canal cells; the lowest cell is the ventral canal cell, which is situated just above the egg.
Fertilization is attained by the ejection of sperm from Antheridia. The sperm swim through free water toward simple organic acids released at the opening of the Archegonium, the neck of which spreads apart at the apex, permitting the neck cells to be extruded and the sperm to swim in and penetrate the egg. The sperm are made up almost entirely of nuclear material, but their surface is provided with spiral bands of cilia—hairlike organs that effect locomotion. When the egg is fertilized, the base of the neck closes, and the embryo develops within the expanding Venter. Within the Archegonial Venter the zygote undergoes characteristic cell divisions to form the embryo, which remains encapsulated in the Gametophyte until it breaks out and becomes an independent Plant.
Ferns also produce asexual spores which are reproductive cells of Ferns. The spore-bearing sacs called Sporangia are borne in clusters, which are called Sori. The Sori are microscopic spores which first appear as small brown streaks or dots located on the underside of the leaves or Frond's. The Sori works much the same way as a seed in that when the wind blows or if something were to brush up against the Plant, the Sori would drop to the ground, which then begins the process of Fern reproduction. The Spores are capable of growing into new Plants if they settle in a suitable environment.