This Introduction to Ferns (and other Pteridophytes) is based on a chapter from the book "A World of Ferns", by Josephine M. Camus, A. Clive Jermy & Barry A. Thomas, Natural History Museum Publications, London, in paperback 1991 - ISBN 0 565 01120 0; in hardback 1992 - ISBN 0 565 01128 6. The book contains over a hundred pages filled with superb photographs of pteridophytes in much better resolution than can be achieved on these WWW pages. It is available from bookshops.

In most cases, larger images can be viewed by selecting the images or captions on this page.

Belorussian translation


AN INTRODUCTION TO FERNS

A bird's-eye view of the crown of a tree-fern, Dicksonia antarctica. - photo: T. C. T. Edwards


WF008T - Cnemidaria horrida

A crozier lying across part of an unfurled frond of Cnemidaria horrida, a tree-fern of the American tropics. The sori, groups of sporangia, are borne close to the margin of the frond segments. - photo: T. Lemieux

WF008B2 - Adiantum

Part of a frond of a maidenhair fern, Adiantum, showing the mature sporangia pushing up the membranous flap (indusium) that protected them in the early stages of development. - photo: J. M. Hobday

Ferns and related plants are found all over the world, from sea-level to high mountains. They are descended from some of the oldest plants of the earth's history, being found as fossils dating back nearly 400 million years. Although most are found in the rain forests, both tropical and temperate, their habitats range from sea-sprayed cliffs through freshwater rivers and lakes to the semi-deserts of arid climates. Just a few examples of the 12000 different species are shown in the following pages, with glimpses of their varied lifestyles, habitats and usefulness to man.

Man has long been familiar with the knowledge that most conspicuous plants have flowers that, after pollination by wind or insects, produce seeds which are often tasty to eat. For many centuries, the reproduction of ferns and related plants was a complete mystery because they do not have flowers. The problem of the absence of flowers was solved by the belief that ferns produced very minute, invisible, short-lived flowers during midsummer's eve, a night credited from the time of the prehistoric Druid religion in northern Europe with mystical powers. The 'seed' was thus naturally invisible at first, and became visible only as time passed. This 'seed' is now known to be the spores, each a dust-like cell so small that 25 lined up just equal a millimetre in length. It was late in the eighteenth century before the basic details of the complicated reproductive process were known.

The leaves, or fronds, of ferns span an amazing range of shapes and sizes, and their diversity is illustrated throughout this book. Some are undivided, others are so finely dissected that they resemble lace; and they range in length from a few millimetres to over seven metres long. All but the moonworts and grape ferns have fronds that, in the bud stage, are tightly coiled into the familiar fern crozier that is usually protected by a covering of scales or hairs. The first fronds to appear in a new season's growth are purely vegetative; fronds unfurling later have the sporangia (spore capsules). Sporangia-bearing fronds may be otherwise identical to vegetative fronds, or may have narrow pinnae (leaf segments) that do not extend beyond the sporangial zone. Sometimes only part of the frond is fertile.

Sporangia form on the underside of the frond, being pale at first and darkening as the spores mature within them. The sporangia are usually arranged in finite patches (sori) that lie along veins or over vein endings. Sori may be scattered over the lower pinna surface or occur only at the margin. They may be linear, curved, oval or round. In many ferns, each sorus is protected at least in the early stages of development by membranous tissue called an indusium, whilst others have evolved a cup-like structure to enclose the sorus. The sori of other genera lack any protection and are often described as naked.

The great majority of ferns have sporangia that are stalked capsules with walls only one cell thick. This wall is composed of different types of cells, the most obvious of which is a row of thickened cells that act almost like a spring as the sporangium matures and dries out, causing the sporangium to rip wide apart. The spores are forcefully flung out as the upper part of the sporangium immediately flicks backs, almost closing the capsule again. Details of the sorus, indusium, sporangium and spores are used, together with the dissection of the fronds and the anatomy of the whole plant, to distinguish the many different kinds of ferns and group them into an ascending hierarchy of species, genera, families and orders.

 


WF009TL4.JPG - Dryopteris filix-mas

This Dryopteris sp. has round sori, covered by kidney-shaped indusia, scattered over the lower surface of the frond segments.- photo: E. Hennipman

WF009TR2.JPG - Cyrtomium

A section through a sorus of Cyrtomium, a holly fem. The developing sporangia radiate out from the receptacle directly under the vein (both are stained dark). Here the protective indusium is borne on a central 'pillar'. - photo: E. Hennipman

In species of Davallia the sori are in pockets along the margin of the frond segments. Golden sporangia are protruding from the mouths of the pockets. - photo: E. Hennipman

WF009B6.JPG - Davallia

 


WF010T.JPG

A spore of Psomiocarpa apiifolia with an elaborate outer layer. - photo: P. J. Edwards

WF010BL2.JPG - archegonia

Necks of archegonia, the reproductive structures that each contain one egg-cell, protruding from the lower surface of a bracken prothallus. - photo: E Sheffield

Most ferns of the order Filicales have 64 spores in each sporangium, though some have as few as 32 or 16. Ferns of the orders Ophioglossales and Marattiales have several thousand. Ovoid spores are common, but some species produce spherical ones. The surface of spores is often highly decorated, and the pattern of flanges, wings or spines may be diagnostic of a species. They may be carried only a few centimetres or several hundred kilometres by air currents. If they settle on a suitably damp surface, they germinate to produce a tongue of cells that in most ferns soon develops into an approximately heart-shaped pad of delicate green tissue called a prothallus or gametophyte. This tiny plant, less than 1 cm long, has root-like hairs called rhizoids to anchor it to soil, bark or rock. It lives in the same way as the much more conspicuous plant that produced the spores by absorbing water-borne minerals through the rhizoids and using the energy of sunlight to make its food substances (photosynthesis). The prothallus is several cells thick in the centre, but only one cell thick elsewhere. The reproductive organs, antheridia and archegonia, develop on its lower surface. Numerous antherozoids (sperm) are produced in the antheridia and they swim through the covering film of water to reach the archegonia, each of which contains a single egg cell. Antheridia are formed before the archegonia to increase the chance of fertilization between different prothalli and so keep a high level of potential genetic variation. This also gives the possibility of fertilization by sperm from another species of the same genus, with the result that a hybrid fern is formed. Such a fern generally produces imperfect spores and therefore cannot reproduce sexually. Recent studies of chromosomes in several genera of ferns have shown that their evolutionary history is a complicated network of hybridization. The first sporeling leaves to appear are very simple and often quite unlike the fronds characteristic of the mature fern. Successive leaves show a sequential modification until the final degree of dissection and architecture has been reached. Studying this progressive change can be helpful in working out the relationships between species of a genus.

WF010BR4.JPG - antheridium


An antheridium, the reproductive structure that produces sperm, on the lower surface of a royal fern prothallus.- photo: E Sheffield

 


Some prothalli, like this one of a species of ribbon fern (Vittaria minima), produce clusters of cells called gemmae which break off to form new prothalli.- photo: D. R. Farrar WF011T4.JPG - Vittaria minima

A classically heart-shaped fern prothallus or gametophyte.- photo: G J Proper

WF011BL3.JPG - prothallus

A sporeling fern, still attached to the now redundant prothallus, showing sequential changes in leaves from juvenile stages towards the adult frond form. - photo: G J Proper

WF011BR3.JPG - sporeling

 


WF012T2.JPG - Lycopodium

The fertile zone of a clubmoss (Lycopodium species) showing the sporangia at the base of the leaves. - photo: J W Wallace

WF012B3.JPG - Equisetum silvaticum

The 'cones' of Equisetum silvaticum, a common horsetail. The polygonal sporangiophores each bear many sporangia. - photo: C N Page

All ferns and their more primitive relatives have basically the same method of sexual reproduction taking place in a free-living gametophyte. The clubmosses, quillworts and horsetails are a rather curious assemblage of plants. They are traditionally grouped with ferns because they are more closely related to that group than to any other group of living plants. Clubmosses and quillworts differ from ferns in one very marked respect: they only have one sporangium per leaf, and this is always at the base of the leaf. Whilst the sporangia-bearing leaves (sporophylls) of some clubmosses are borne in a terminal spike, others have less conspicuous zones of sporophylls and vegetative leaves alternating along the stem. Quillworts, members of the genus Isoetes, grow at the bottom of mountain lakes or on seasonally damp land. Their presence is often overlooked because, as the illustrations later in the book show, they are easily mistaken for vegetative states of other aquatic plants. Some clubmosses, which are often all grouped in the one genus Lycopodium, produce spores which form bisexual prothalli as in the ferns. Quillworts and other clubmosses (Selaginella) bear two sorts of sporangia, containing either large or small spores which germinate to give female or male gametophytes respectively.

All species of horsetails belong to the genus Equisetum. They often form conspicuous stands in damp or wet places. One common species, Equisetum arvense, is cursed by many gardeners for its invasiveness. Although many horsetails look feathery, this effect is due to the whorled arrangement of the branches on the main stem rather than to the leaves which are vestigial, papery and fused into a sheath round the nodes of the stem. The sporangia of horsetails are produced at the margins of polygonal structures called sporangiophores which are grouped into a 'cone'. The cones are borne typically at the apex of the main shoot, but on the branch apices in some species. One group of horsetails produces the 'cone' on a very bizarre-looking, pale shoot that dies once the spores have been shed. The green vegetative shoots grow later. Spores are green and have four 'arms' (elators) that twist about when the humidity of the air changes, and it is this movement that helps to disperse them away from the parent plant.

 


WF0134.JPG - Equisetum arvense

Vegetative shoots of Equisetum arvense, showing the whorled branches. - photo: R Prelli


WF014T.JPG - Lycopodium

A bulbil of a clubmoss (Lycopodium). This plantlet is easily dislodged from the parent plant and can quickly grow into a full plant. - photo: J W Wallace

Some ferns, clubmosses and horsetails form large colonies from creeping stems. A single plant of bracken can cover a whole hillside. Attractive house and garden plants are often propagated from cuttings that are nurtured to produce roots. This is a way of increasing the population without sexual reproduction, and many plants are naturally able to produce such clones of themselves. A trampled horsetail will readily break up into fragments which can develop roots and establish themselves as new plants. If the fragments are swept away by a stream or river and settle elsewhere, a new population is formed. Some clubmosses produce small plantlets called bulbils on mini-branches between the leaves. These are easily dislodged by gusts of wind or passing animals and, if they land in a suitable place, quickly grow into a full plant. Some ferns also produce bulbils, either along the frond midrib or at the tip of a frond that arches over and touches the ground. These are less easily dislodged than those of clubmosses and may have to wait until the parent frond dies before they can become established as separate plants.

Bulbils at different stages of development on the frond of Diplazium proliferum. - photo: T. C. T. Edwards

WF014B6.JPG - Diplazium proliferum


WF015TL.JPG - Isoetes echinospora

The base of a fertile leaf of a quillwort, Isoetes echinospora, showing the sporangium bulging with megaspores. - photo: NHM

WF015TR.JPG - Lygodium

Part of the fertile frond of a climbing fern, Lygodium. Sporangia are borne on the little 'tassels' on the margins of the frond segments. - photo: C J Piggott

The pale fertile stems of the horsetail Equisetum arvense emerge in spring and die before the green vegetative ones appear. - photo: R Prelli

WF015B6.JPG - Equisetum arvense

Copyright © 1991, 1992 J. M. Camus, A. C. Jermy & B. A. Thomas (original work); ©1998,1999 British Pteridological Society (this WWW presentation).

This page was last updated on 25/5/2008.