FERNS – beautiful plants of shade and damp

A fern is a photosynthetic green plant that grows in soil or epiphytically on other k_plants, with a few species restricted to freshwater habitats. The basic construction is of roots, rhizomes and leaves (called fronds), interconnected by a complex vascular system comparable to that found in flowering plants. Reproduction is by means of spores, with the life cycle having two distinct phases—a sexual gametophyte and an asexual sporophyte.

Structure and life cycle

The fronds of most ferns arise from the rhizome, though in some species they grow from creeping stems. When young they are tightly curled and commonly called croziers or fiddleheads. As they mature they unfurl and enlarge, the size increase being due to both cell division and cell enlargement. The shape varies from simple strap-shaped as in the hart’s-tongue fern (Phyllitis scolopendrium) to complex and highly divided as in bracken (Pteridium aquili-num). The divisions of the more complex fronds are called pinnae.

Most ferns are small, few exceeding 2m (6.5ft) in height. There are, however, a few that grow considerably taller, up to 25m (85ft) high; these are mostly the tree-ferns. Often they have ‘trunks’ consisting of old frond bases with a rosette of fronds at the crown, and superficially they resemble palm trees. The few others that grow to a large size are the climbing ferns.

Some aquatic species have a diameter of less than 1 cm (0.4m) and are often confused with duckweeds (Lemna).

The fern plants that are commonly seen are the sporophytes, so called because they bear spores which develop in and are dispersed from spore-cases called sporangia that are located on the lower surface of some fronds. The fronds bearing sporangia are termed fertile, whereas those without are sterile. The sporangia usually occur in round or elongate clusters called sori, but in some species they are evenly distributed over part or all of the underside of the frond. When ripe, the sporangia split, and the spores contained therein are released. To the naked eye these spores appear as dust, each being no more than about 50 microns diameter, but when viewed through the microscope surface patterning is often apparent, usually spines, pores or ridges.

Damp conditions are required for the spores to germinate; they grow and form the gametophyte or prothallus which is normally shortlived. Most prothalli are thin, heart-shaped and less than icm (0.4m) long; a few are much-branched and threadlike. They mostly grow on rocks, soil or other plants and are photosynthetic, but some are subterranean and only grow after having formed a symbiotic relationship with a fungus. On the surface of the gametophyte, microscopic sex organs are formed, the male antheridia and the female archegonia, and it is in these that the sex cells or gametes develop. The male sperms swim to and one fertilizes the female egg to form a zygote; a film of water over the surface of the prothallus is essential for this process. The zygote divides and grows to form a new sporophyte plant, living parasitically on the gametophyte until established.

This sporophyte/ganletophyte cycle is called alternation of generations; it allows an intermixing of genetical material to take place which results in variation in the characteristics of the individuals of a species, the basis upon which natural selection and evolution operates.

Habitat

There are about 10,000 fern species found throughout the world, the majority of which thrive only in moist or wet regions not subjected to freezing temperatures: about 30 percent of the total grow in tropical rain forests, and about 40 percent in subtropical and montane rain forests. Comparatively few grow in cool and dry regions.

Gremmitis fasciala and G. siibpinnatifide

Tropical rain forests are fairly dark, wet and hot, and provide a variety of habitats for ferns. Those at ground level are usually large and produce masses of spores. Their gametophytes find it difficult to become established on the rain-soaked mud, and in order to overcome this problem many of the sporophytes have developed methods of vegetative reproduction, such as bulbils or plantlets on the fronds which droop to the ground; these stand less chance of being smothered by mud than do the gametophytes. More sporophytes of these plants are produced vegetatively than sexually. A few species rooting in the ground climb up the trees and expose their fronds to bright sunlight. However, most ferns found in the tropical rain forest are epiphytic; a few grow on the dimly lighted tree trunks and ground vegetation, but the bulk live in the canopy.

The subtropical and montane rain forests are cooler and brighter than tropical rain forests and are more consistently wet: both epiphytic and soil-growing species are common. In temperate coniferous and deciduous forests there are relatively few species, and these are predominantly terrestrial or grow on rocks. There are many unshaded areas where ferns grow— grasslands, tundra and deserts. These offer only a few usable habitats—crevices, ravines or in the shade of boulders—and consequently few species grow in these regions.

Most fern species are adapted to, and grow in dim damp places, the ideal habitat for the game-tophyte generation, but some grow in and colonize open spaces and thrive in sunny conditions. In order to resist desiccation various mechanisms have evolved, the simplest being to allow the frond to shrivel as happens in some Asplenium species. The most common adaptation is the development of thick fleshy leaves that contain more water. However, more complex techniques are found in other species— often a barrier of scales, hairs or roots around parts of the plant, which help to retain moisture, or, in other cases possessing fronds that are easily lost under adverse conditions coupled with the ability to rapidly grow replacements when water is available again; this is frequently shown by species of Davallia.

Some species, e.g. Salvinia and Azolla, have overcome all water-supply problems by being adapted to an aquatic habitat; these are free-floating, forming carpets on the water surface with the roots hanging down. The fronds are small, up to about 3cm (I.2in) long, but they can grow and spread very rapidly, often blocking waterways where the water is slow flowing or still. The upper surface of the fronds is covered with specialized hairs that repel water and ensure that the plant will not sink.

Some ferns have evolved special adaptations. The species of the epiphytic genus Platycerium have two distinct frond forms; the normal sterile and fertile types, and specialized basal ones which trap rotting vegetation and provide a mineral supply from the forming humus. Two other epiphytic genera, Solanopteris and Lecanop-teris, form symbiotic relationships with ants which live in the swollen rhizomes.

Classification

In order to distinguish the different species of ferns, a variety of characters need to be studied, these include external structure, habit, vascular system, scales, hairs, position and structure of sori, sporangium development, spore structure and chromosome number.

Often studied with the ferns are the plants called the fernallies. These are not true ferns, but are more nearly related to the ferns than they are to any other group of plants. They include Equisetum (the horsetails), Lycopodium, Phylloglossum and Selaginella (the clubmosses), and Stylitcs and Isoetes (the quillworts). The horsetails have ridged stems which in some species branch several times to give a brushlike or horsetail appearance. The leaves are reduced to papery scales which occur at the nodes, they contain no chlorophyll; the stems undertake all photosynthesis. The fertile shoots have a small cone or strobilus at the apex which contains the spores. Clubmosses have branching stems with rows of green leaves, and grow in a creeping fashion on the soil or epiphytically on trees. Sporangia are found at the base of leaves at the apices of some growing shoots. The quillworts grow submerged in water or almost buried in moist soil and appear as bunches of plain straight green stems.

Fossil evidence shows that the ferns are a very old group of plants. The earliest remains are from the Lower Devonian period about 300 million years ago, these fragments are thought to be of an extinct group of fernallies. True ferns did not appear for another 50 million years, until the Upper Devonian and Lower Carboniferous period. At this time many of the species were large and formed trees; with the horsetails and clubmosses they were the dominant plants and their remains form today’s fossil fuels.

Ferns and man

Most tern species today have a restricted distribution, though they tend to be more widely distributed than flowering plant species. Probably the most widespread fern is bracken, Pteridium aqitilinum: in temperate areas it is a weed often encroaching on pastureland where it is difficult to eradicate. In tropical areas one fern weed that is a problem is Dicranopteris which forms impenetrable thickets.

Many people consider ferns to be of little value to man other than being ornamental, but in some areas they form part of the local diet, and of more importance is evidence that chemicals found in some of them may prove useful as drugs and in insect control.

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