Ferns
Ferns are a diverse group of plants that are unranked at the division level in some taxonomies. Formerly, the group was designated as division Pteridophtya, but their phylogenetic relationships remain unresolved. Although they have a worldwide distribution, ferns are more common in tropical and subtropical regions. They range in size and complexity from small floating aquatic plants less than 2 cm (0.8 inch) long to tall tree ferns 20 metres (65 feet) high. Tropical tree ferns possess erect columnar trunks and large compound (divided) leaves more than 5 metres (about 16 feet) long. As a group, ferns are either terrestrial or epiphytic (growing upon another plant). Fern stems never become woody (composed of secondary tissue containing lignin), because all tissues of the plant body originate at the stem apex.
Ferns do photosynthesize to produce their own food, as they are plants that contain chlorophyll and have specialized cells for this process, similar to other plants. Ferns are primary producers that require sunlight, water, and soil to survive, using their green, leaf-like structures called fronds to capture light for photosynthesis.
Common Features
- Vascular Ferns are vascular plants: they have specialized transport bundles that conduct water and nutrients from and to the roots
- Herbaceous perennials Extant ferns are herbaceous perennials and most lack woody growth. When woody growth is present, it is found in the stem.
- Reproduction Ferns differ from spermatophytes in that they reproduce by spores rather than having flowers and producing seeds. However, they also differ from spore-producing bryophytes in that, like seed plants, they are polysporangiophytes, that is, their sporophytes branch and produce many sporangia. Also unlike bryophytes, fern sporophytes are free-living and only briefly dependent on the maternal gametophyte.
Importance
Ferns are not of major economic importance, but some are used for food, medicine, as biofertilizer, as ornamental plants, and for remediating contaminated soil. They have indeed been the subject of research for their ability to remove some chemical pollutants from the atmosphere.
Some fern species, such as bracken (Pteridium aquilinum) and water fern (Azolla filiculoides), are noxious weeds worldwide. Some fern genera, such as Azolla, can fix nitrogen and make a significant input to the nitrogen nutrition of rice paddies. They also play certain roles in, well, folklore.
Coal consists of the remains of primitive plants, including ferns.
(From Wikipedia)
Class Polypodiopsida
Ferns of the class Polypodiopsida typically possess a rhizome (horizontal stem) that grows partially underground; the deeply divided fronds (leaves) and the roots grow out of the rhizome. Fronds are characteristically coiled in the bud (fiddleheads) and uncurl in a type of leaf development called circinate vernation. Fern leaves are either whole or variously divided. The leaf types are differentiated into rachis (axis of a compound leaf), pinnae (primary divisions), and pinnules (ultimate segments of a pinna). Fern leaves often have prominent epidermal hairs and large chaffy scales. Venation of fern leaves is usually open dichotomous (forking into two equal parts).
Each frond is a potential sporophyll (spore-bearing leaf) and as such can bear structures that are associated with reproduction. When growth conditions are favourable, a series of brown patches appear on the undersurface of the sporophylls. Each one of the patches (called a sorus) is composed of many sporangia, or spore cases, which are joined by a stalk to the sporophyll. The spore case is flattened, with a layer of sterile, or nonfertile, cells surrounding the spore mother cells. Each spore mother cell divides by reduction division (meiosis) to produce haploid spores, which are shed in a way characteristic to the ferns.
Each fern spore has the potential to grow into a green heart-shaped independent gametophyte plant (prothallus) capable of photosynthesis. In contrast to bryophytes, in which the sporophyte is nutritionally dependent on the gametophyte during its entire existence, the fern sporophyte is dependent on the gametophyte for nutrition only during the early phase of its development; thereafter, the fern sporophyte is free-living. In some ferns the sexes are separate, meaning a gametophyte will bear only male or female sex organs. Other species have gametophytes bearing both sex organs. Features important in the identification of ferns include such aspects of the mature sporophyte plant as differences in the stem, frond, sporophyll, sporangium, and position of the sporangium and the absence or presence, as well as the shape, of the indusium (a membranous outgrowth of the leaf) covering the sporangia.
Class Psilotopsida
Psilotopsida (whisk ferns) is a class represented by two living genera (Psilotum and Tmesipteris) and several species that are restricted to the subtropics. This unusual group of small herbaceous plants is characterized by a leafless and rootless body possessing a stem that exhibits a primitive dichotomous type of branching: it forks into equal halves. The photosynthetic function is assumed by the stem, and the underground rhizome anchors the plant. The vascular tissue is organized into a poorly developed central cylinder in the stem.
The family Ophioglossaceae, comprising four genera and some 80 species, is sometimes placed in the class Psilotopsida, though the taxonomy of the group is contentious.
Class Equisetopsida
Equisetopsida (also called horsetails and scouring rushes) is a class represented by a single living genus (Equisetum). It has a worldwide distribution but occurs in greater variety in the Northern Hemisphere. Like the lycopods, this group was a diverse and prominent group of vascular plants during the Carboniferous Period, when some genera attained great size in the coal-forming swamp forests. Known as sphenophytes, these plants are differentiated into stem, leaf (microphylls), and root. Green aerial stems have longitudinal ridges and furrows extending the length of the internodes, and stems are jointed (articulated). Surface cells are characteristically filled with silica. Branches, when they occur, are borne in whorls at the node, as are the scale leaves. Sporangia are borne in terminal strobili. Equisetopsida had its origin in the Devonian Period (419.2 million to 358.9 million years ago).
Class Marattiopsida
Known as giant ferns, the class Marrattiopsida comprises a single extant family with four genera and some 150 species of large tropical and subtropical ferns with stout erect stems. The leaves (fronds) may be very large, some reaching 4.5 metres (15 feet) or more in length. The Marattiaceae generally are considered to be one of the most primitive families of ferns still living.