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| Ferns |
Ferns are among the most recognizable members of the phylum Pterophyta, which are primitive, nonflowering, vascular plants that primarily reproduce by spores and occur in many variations, complicating classification.
Approximately twelve thousand extant species of fern are classified in the phylum Pterophyta. These seedless plants display a diversity of physical and reproductive characteristics that separate them taxonomically. They have leaves containing branching veins known as megaphylls. Fossils from the Devonian period, about 395 million years ago, include some structures resembling Pterophyta.
These plants are believed to have been the source for gymnosperms. Most early fernlike plants that evolved in a variety of forms during the next period, the Carboniferous (approximately 345 million to 280million years ago), which is often referred to as the age of the ferns, became extinct afterward.
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Evaluating the many fossils from this period containing fernlike structures, paleobotanists have identified Archaeopteris, which they call the primitive fern, and Protopteridium, labeled the first fern, which they hypothesize are the ancestors of modern ferns. Botanists have linked two existing fern genera as possible descendants of Carboniferous ferns.
Some fossilized leaves thought to be ferns had characteristics indicating that they had seeds and were not ferns. Seed ferns, or pteridosperms, were trees and vineswith fronds butwere not directly related to true ferns.
Before the seed ferns became extinct in the Cretaceous period, about 136 million years ago, they were the predecessors to angiosperms. Based on anatomical comparisons, modern ferns and seed ferns may have descended from the same ancestors. Eleven modern fern families are present in fossils from the Mesozoic era, approximately 245 million years ago.
Although many modern ferns are morphologically similar to one another, they deviate in expression of specific traits. Theories differ about the evolutionary origins and development of ferns, specifically their stems and leaves.
Many botanists designate the family Schizeaceae, or the curly grass, in phylum Pterophyta as the evolutionary origin of ferns. Types of curly grass also belong to family Thelypteridaceae.
Tropical ferns are classified in family Dicksoniaceae. Other familiar families in phylum Pterophyta include Adiantaceae, which represents the maidenhair ferns; Hymenophyllaceae, which are the filmy ferns; Blechnaceae, the deer ferns, with reddish leaves; and Cyathaceae, the arborescent tree ferns.
Spleenworts are in family Aspleniaceae. Brackens in family Dennstaeditaceae are further classified into the tribes Dennstaediteae, Lindsaeeae, and Monachosorae. The family Dryopteridaceae is divided into six tribes.
Floating Pterophyta plants belong to the families Salviniaceae and Azollaceae, the latter of which is considered to display the plant world’s most complex reproductive system. Marsileaceae is the water clover family. Variants of Pterophyta that grow on forest floors belong to the family Marattiaceae. The royal ferns (family Osmundaceae) exhibit some primitive characteristics such as stipular leaf bases.
Family Ophioglossaceae, which includes adder’s-tongue, are not closely related to extant ferns and express primitive traits such as high-spore-yielding sporangia, indicating an origin among the progymnosperms. The epiphytic family Polypodiaceae, divided by tribes, also is isolated from most Pterophyta ferns and is considered most closely related to the family Gleicheniaceae.
Pteridium, the bracken, is the most familiar fern worldwide and is among the earth’s six most common and oldest plants. Classification of ferns fluctuates according to different authorities. Many known groups of ferns have not been fully examined, while other species are being discovered in previously uncharted places.
Some researchers consider such plants as whiskferns (Psilotophyta), horsetails (Sphenophyta), and club mosses (Lycophyta) to be closely related to ferns, but most botanists stress that ferns differ morphologically from those plants, although their life cycles are alike.
Reproduction
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| Fern spores |
Pterophytes such as ferns exist as two alternating forms during their sexual reproductive cycle. Initially, in the gametophyte phase, ferns are a stem known as the rhizome, from which roots and leaves, called fronds, grow as the fern matures into an adult sporophyte. The sporophyte exists separately from the gametophyte, which usually dies as soon as the sporophyte’s root sinks into soil.
Growing on or near the soil surface, rhizomes, the most common stem form, can be as thin as threads or as thick as ropes and sometimes have hollows that house ants that scientists believe to be living in symbiosis with the fern, protecting it from insect predators. The roots also have differing characteristics of quantity, width, length, and texture that aid in classification.
Ferns do not flower or create seeds; instead, they produce brown sacs, or sporangia, on the bottom surfaces of their fronds which contain single-cell spores. These spores vary in size and have tetrahedral or oval shapes.
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Spore arrangement aids fern classification. Spores are distributed by wind to germinate and form a small, heart-shaped prothallus which has sexual organs. The prothallus is tiny, averaging 8 millimeters (0.3 inch), and often is not visible before it dies.
Water is necessary for fertilization to occur, opening the sex organs by swelling. Sperm from the antheridium fertilizes eggs, located in the archegoniumon either the same or a different prothallus, which protects the resulting embryos until they mature.
Embryos renew the cycle by maturing as rhizomes, which form fronds and roots to spread ferns to different areas. While spores are haploid (with one set of chromosomes), fern cells have two sets of chromosomes, one set acquired from the egg and one from sperm. Ferns create genetically identical clones in colonies.
Most fern reproduction is vegetative (asexual, without gametophytes) and occurs during the sporophytic stage, inwhich rhizomes produce fern clones or fragments are distributed bywind, water, or insects. The walking ferns, members of the species Camptosorus rhizophyllus and Camptosorus sibiricus, grow from sprouts emerging where parent leaves touch soil.As a result, large groups of identical ferns can be formed.
Sexual reproduction’s role is to introduce ferns to new habitats and geographical areas. Genetic changes that occur because of meiotic cell division before the production of spores result in subtle variations.
Structure and Distribution
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| Fern illustration |
Most Pterophyta tissues, specifically those in ferns, are created near the top of the fronds, with the most mature cells being located at the base. Some fern genera have deviations in leaf structure. Fern leaves, known as the pteridophyll, have a fiddle-head, called the crosier, from which they unroll, instead of unfolding, as do other plants’ leaves.
Ferns are vascular, and most have pinnate leaves with leaflets extending from a central rib. Other types of leaves include palmate ferns, in which the leaflets emerge from one place, and staghorn ferns, with dichotomous leaves.
Leaves can be simple or compound, depending on whether they are segmented. Such appearances help identify species, especially when they are closely related. Leaves and their bases protect the stems, which have a surface consisting of one layer of flat epidermal cells with wide outer walls. Sometimes stems are covered with leaf armor, which is formed by the hardened remains of leaf bases.
This armor thickens stems and can store food in addition to shielding ferns from harm. Scales and hairs are also often present on stems, guarding them and providing identifiable traits for classification. Internally, stems have vascular tissues called steles.
Fronds are of varying shapes and sizes, ranging from 1 millimeter to 30 meters (0.04 inch to 100 feet). Some leaves appear feathery, while others look solid. They exhibit different shades of green and textures, such as glossy or leathery. Fern height also ranges broadly, from a few millimeters (0.1 inch) to as tall as 10 to 25 meters (30 to 80 feet) for some tropical tree ferns.
Habitats
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| Fern habitat |
Pterophyta habitats are numerous. The greatest number of species live in tropical rain forests. Most ferns are terrestrial, while some ferns are vines; others are epiphytes, wrapping around trees; and some float on bodies of water. Ferns prefer damp, warm environments but can also grow in arid and cold settings.
Because less moisture and lower temperature occur at higher latitudes than in tropical and subtropical zones, fewer native ferns are found at the higher latitudes, although some species can live in the polar regions of the Arctic and Antarctic.
Epipteric ferns grow in rocky landscapes, with different species preferring acidic or alkaline stones and others thriving in marshes, bogs, forests, and fields. Several ferns grow at high elevations, living on volcanoes and mountains. Ferns adapt to arid, sunny, and salty conditions by developing harder tissues, waxy surfaces, hair or scale coverings, and altered life cycles.
Species are occasionally introduced into areas where they are not indigenous and then thrive to become common. Hybrids also occur between species within genera, adding to classification confusion. Rarely, hybridization between genera happens.
Because they are primarily sterile, hybrids reproduce with vegetative propagation or apogamy, whereby spores with the same number of chromosomes as parent cells generate gametophytes that bud a sporophyte without undergoing fertilization.
Some ferns, including the bracken, are regarded as weeds because they cover fields and bodies of water, blocking light and oxygen necessary for other organisms to survive.
Uses
Economically, Pterophyta are not as significant as other plants. Nevertheless, some ferns species are edible, with crosiers being considered delicacies. Other ferns are used medicinally.
By contrast, Osmunda and Pteridium ferns are considered to be carcinogenic. The aquatic mosquito fern (Azolla) hosts Anabaena azollae, which converts nitrogen for use by plants such as rice, enhancing production in rice paddies and other fields.
Animals often root for fern rhizomes, which store starches. The braken fern Pteris vittata absorbs arsenic, a carcinogenic heavy metal, from soil. By removing this toxin, ferns can restore contaminated areas into viable agricultural, industrial, and recreational sites.
The most important economic use of Pterophyta is as ornamental garden plants and houseplants. Fern fronds are used in cut-flower arrangments as ornamental greenery. Botanical collectors have identified rare ferns, for which they pay high prices; there is even an underground economy of smuggling ferns illegally from protected areas.
Masses of fern roots are used to cultivate such epiphytic greenhouse plants as orchids. In the Middle Ages, some people believed that at midnight on June 24, St. John’s Day, ferns would produce blossoms which contained magical seeds.
