Ferns

The ferns (class Filicopsida) first appeared in the Devonian period, more than 350 million years ago, and have flourished ever since, particularly in forests. They are a large group of plants, containing approximately 10,000 species, which now inhabit a variety of environments. Most are ground-dwelling, preferring damp, shady habitats, but some are epiphytic, growing attached to the stems or leaves of other plants; others can survive in more exposed situations, and a few have an entirely aquatic life style.

Ferns vary widely also in their appearance, ranging from the giant, treelike ferns, which can reach a height of 65 feet (20 meters), to tiny, mosslike species that grow on rocks.

They all belong to the division Pterophyta and are, therefore, closely related to club mosses, horsetails, and quillworts. Like them, ferns have alternating sporophyte (asexual) and gametophyte (sexual) generations. The class is roughly divided into the “primitive” eusporangiates, such as adder’s tongue (Ophiog/ossom vulgatum) and moonwort (Botrychium lunaria), and the “higher” lep-tosporangiates (which contain the most common ferns, such as maidenhair fern, Adiantum sp.); the subclass between, Osmundidae, shares features of both groups. The class Os-munda includes the complex-leaved royal fern, Osmunda regalis. (Remember that there are several classification systems; the system used here may differ slightly from others.)

A young fern, the sporo-phyte generation of its reproductive cycle, develops from a prothallus, the ga-metophyte generation. The new fern is nourished by the photosynthesizing prothallus, but when it can pho-tosynthesize for itself, the prothallus dies.

Structure

In most ferns the stem is usually a short, thick stock, as in the common male fern (Dryopteris filix-mas), or a long rhizome, as in bracken (Pteridium aquilinum) and epiphytic ferns. The stock grows almost completely buried in the ground, with an ascending growing point above ground surrounded by a close spiral of leaves, resembling a whorl, and often forming a basketlike tuft. Roots usually grow from the backs of the leaf-bases and on the lower surface of rhizomes, which grow horizontally above or below the ground, bearing single leaves at intervals along the upper surface.

The growing point of rhizomes is surrounded not by leaves, but by scales or hairs, which are usually brown and sometimes glandular.

The leaves (fronds) of most ferns are spirally arranged and known as megaphylls. When they are in bud, in most species, they are rolled up like a shepherd’s crook, or crazier. Like the stems, they are normally covered (at least when young) with scales or hairs. They contain chlorophyll and photosynthesize.

Small adjustable pores called stomata, through which gas exchange takes place, are located in the epidermis of the leaf, and water and nutrients in solution are carried through the leaves and stems by xylem and phloem tissue.

Spore production

Some ferns are able to reproduce vegetatively (asexually) by means of creeping rhizomes or by bulbils produced on the leaves, but propagation is usually by spores formed in sporangia. In most species, the sporangia are located on the lower surface of ordinary leaves; others bear them on the axis of the leaves or at their tips. Still others have separate fertile leaves, such as the royal fern.

The sporangia on most species of ferns are borne on the underside of the fronds, grouped into sori. On buckler fern, each sorus is partly covered by a kidney-shaped indusium. On lady fern the indusium forms a flap, whereas on holly fern it is reduced, forming a dimpled disk which barely covers the sorus. The sori of common polypody have no indusium. The sporangia of maidenhair fern develop on the margins of the leaves and are protected by the leaf curling over them. Those of tree ferns are contained in cuplike receptacles sometimes covered by an indusium which splits to uncover the sporangia.

Usually, many sporangia are grouped together to form a sorus, many sori occurring on each leaf or leaf-segment. Each sorus is normally situated on a cushion-shaped structure, called the receptacle, directly above a vein that supplies it with nourishment. In some species, such as common polypody (Polypodium vulgare), the sori are not protected, but in most ferns each sorus has a cover (the indusium), which is most commonly linear, kidney-shaped (reniform), or mushroomshaped (peltate).

The sporangium is usually stalked and has a head that is often shaped like a biconvex lens, with a ring or annulus of thickened cells almost completely surrounding the margin. Thin-walled cells occupy the remainder of the margin, flanking a weak point called the stomium, which eventually ruptures to release the spores. The sporangia contain a large number of spores; Christensenia, for example, releases about 7,000 spores from each sporangium, and adder’s tongue more than 15,000. These spores contain chlorophyll.

The sporangium of a fern splits at a point called the stomium when the spores are ripe. The surrounding ring of cells (the annulus) shrinks, pulling the cap back. When the tension reaches a maximum point, the cap jerks back to its original position, shooting the spores into the air.

Sexual reproduction

If a spore lands in favorable conditions it germinates a few days after it is released, forming a tiny green prothallium (the gametophyte). Hairlike roots (rhizoids), which form on the lower surface of the prothallium, supply the plant with water and nutrients, and in some species the prothallia photosynthesize. Most prothallia, however, are also mycorrhizal.

On the underside of this cushion of cells are the female reproductive organs (arche-gonia), each one consisting of an egg cell and a narrow, cylindrical neck that projects from the surface of the prothallium and is filled with neck canal cells. The male reproductive organs (antheridia) are normally located on the same prothallium, toward its base. Inside the spherical wall of each antheridium is a cavity containing sperm cells, each producing a single sperm with several long whiplike hairs (cilia or flagella) with which it propels itself through water.

Because the only way the sperm can reach the archegonia is by swimming, fertilization takes place only when the prothallium is covered with a film of water. This moisture causes the antheridia to split open, releasing the sperm cells whose walls dissolve, setting the sperm free. Simultaneously, the canal cells of the archegonia disintegrate, releasing a chemical that attracts the sperm and leaves the neck open. The sperm travel up the neck canal where one enters the egg and fuses with the nucleus.

Following fertilization, the resulting zygote begins its development attached to the prothallium and is nourished by it. But when it has formed its first root stem and leaf and is able to photosynthesize and thus manufacture food itself, the prothallium dies.

Most ferns are termed homosporous, which means that their spores are all alike. But the water ferns (those belonging to the orders Marsileales and Salviniales) are termed het-erosporous, which means that they produce two different types of spores from different sporangia—micro- and megasporangia—on the same plant. In these ferns, the tiny microspores germinate to form the male prothallia, which bear antheridia, and the larger megaspores give rise to female prothallia, with archegonia.

Tree ferns (families Cyath-eaceae and Dicksoniaceae) are found in mountain forests in tropical areas such as the South Pacific, Malaysia, and parts of Australasia. They can reach 65 feet (20 meters) in height and usually have unbranched stems that end in a crown of leaves. They first appeared in the Jurassic period about 170 million years ago.