An Introduction to the Structure of Ferns and their Allies
|Introduction||Habit, Lifeform||Stems, Rhizomes||Leaves, fronds||Sporophyte fertility||Cytology|
|. Life Cycle
|. Growth form
The ferns and their allies share a lot of commom morphlogy with the other vascular plants and in many cases the same descriptive terminology is used. However, there are some fundamental and significant differences of structure unique to the pteridophytes and a specialized terminology has evolved to descdribe these. The most obvious difference between the pteridophytes and the remainder of the vascular plants is that the ferns and their allies do not produce large floral or reproductive structures that give rise to seeds which eventually develop into the next generation of plants. Pteridophytes reproduce and disperse by means of microscopic spores, the structure and development of which is every bit as intricate and amazing as the flowers of the higher plants.
This outline covers the easily recognised features of the ferns and their alies and mentions many of the technical terms used to describe them.
The life cycle of pteridophytes involves two distinct and separate phases known as the gametophyte and sporophyte (these two phases also occur in the seed-plants, but because there they occur concurrently, they are less obvious). The sporophyte, is the most conspicuous phase, and is so-named because this is the stage that produces the spores; it is the sporophyte that most of us refer to when we describe or name a fern. Under favourable conditions, the spores shed from the sporophyte develop into the gametopyhte which is small, inconspicuous and short-lived. The purpose of the gametophyte is to produce the male and famale sex cells (gametes), the female of which, when fertilized, develops into a new sporophyte to continue the cycle. This regular process is known as the alternation of generations and involves an alternate doubling and halving of chromosome numbers at each phase.
It is interesting to compare the pteridophyte life cycle with that of another major group of spore-producing plants, the bryophytes or mosses and liverworts which also have alternating generations. In the bryophytes, the gametophyte is the dominant and most conspicuous stage and the sporophyte is retained as a relatively small. dependent appendage on the mature gametophyte.
A more detailed discussion of pteridophyte life cycle and reproduction is provided elsewhere.
The gametophyte is the sexual or haploid stage of the pteridophyte life-cycle and contains a single set of chromosomes. It develops from the spore produced on the sporophyte. This spore germinates and develops into a body called the prothallus. At maturity, regions of the prothallus develop into small separate sexual organs. Antheridia produce the male sex cells and archegonia produce the female sex cells. The female sex cells are usually sedentary whereas the male are usually more or less coiled and motile by means of two or more cilia and swim or are carried by water to the archegonia where fertilization takes place. After fertilization a new sporophyte develops from the fertilized cell and takes over as the gametophyte withers and dies.
This is the general pattern of behaviour but there are fundamental differences in a number of genera with dimorphic spores and an number of genera that abandon the haploid phase (apogamy).
Prothallus morphology. In most cases the prothallus is green and photosynthetic, developing on the surface of the ground or on moist rocks or bark. They lack vascular tissue, mostly they are thin and more or less heart-shaped, less often ribbon-like or filamentous, the central region is somewhat thickened and bears the antheridia, archegonia and rhizoids (filamentous root-like structures). Useful taxonomic characters are afforded by the overall shape, the presence or absence of various hairs, the appearance of the rhizoids and the arrangement of antheridia and archegonia. However because the gametophyte generation is so small, so short-lived and must be grown from spore for reliable identification, it is rarely used in taxonomic treatments.
A more detailed discussion of gametophyte, prothallus and reproduction is provided elsehwere.
In the genera of the Ophioglossaceae, Lycopodiaceae, Psilotaceae and some Schizaeaceae the propthallus is thick, non-photosynthetic, grows and lives in the dark, underground or in the detritis in the forks of trees, and is saprophytic, obtaining its nutrients from decaying vegetable matter with the assistance of an endophytic fungus. Water is still essential for the process of fertilization.
Heterosporous pteridophytes. Certain groups of pteridophytes (Selaginellaceae, Isoetaceae, Marsiliaceae, Azollaceae, Salviniaceae) produce haploid meiospores of different sizes (big = megaspores; small = microspores). The microspores develop into small microgametophytes and the megaspores develop into larger megagametophytes. Apart from the separation of the gametophytes at the sexual stage, the alternation of generations is essentially the same.
Apogamous pteridophytes. In some species of ferns and their allies, the sexual process is omitted and the spores produced on the sporophyte are diploid rather than the usual haploid. The resulting prothallus is thus diploid and a few sporophyte is produced vegetatively without the usual union of gametes.
The sporophyte is the asexual or diploid stage of the pteridophyte life-cycle. It is mostly large and conspicuous, always green and photosynthetic, long-live and produces the spores in special fruiting bodies; the spores are shed to produce the next gametophyte generation in the cycle. Because it is large, easily collected and preserved, and provides a vast array complex and distinctive characters for identification, the sporophyte generation is the foundation of all pteridophyte classification and taxonomy and the sporophyte alone is the fundamental unit of a modern pteridophyte specimen. All of the following discussion refers to the sporophyte generation or the fern plant as it is commonly known. It is convenient to consider the plant to be composed of three major parts: the stem which bears the roots, the leaves and the fertile parts with sporangia.
Pteridophytes exhibit a range of habits and life forms, and exist in most habitat types except the marine environment. The sporophytes range in size from a few millimetres to tens of metres tall and in mass from a few milligrams to many tens of kilograms. Pteridophyte habit reflects the enviroment and substrate in which they choose to grow and it is convenient to divide recognize three major classes: those that grow on the ground, those that grow on trees and those that grow in or on the water. However, it should be noted that a habit/habitat preference may span these classes. In particular, species that live low on trees, might be classed as epiphytes, subepiphytes or terrestrian, and plants that grow on the margins of water bodies might be considered aquatic or aquatic.
While this division of the overall structure of a plant based on where it grows might appear contrived and artificial, it should be noted that major families of pteridophytes are predominantly terrrestrial, predominantly epiphytic or predominantly aquatic, demonstrating a significant correlation of habit defined in this manner with other morphological characters.
Terrestrial pteridophytes have erect or creeping stems and the leaves are held more or less upright, either vertical or spreading and arching. Erect stems are generally unbranched, radial with a more or less terminal rosette of fronds, and may be stout and wooody (Osmunda, Todea), stout and fleshy (Marattia, Angiopteris) or arborescent (Cyathea, Dicksonia, Leptopteris). Creeping rhizomes have spaced or remote fronds, and the stem itself may be branched or unbranched, on the ground surface or subterranean. Such plants can be thicket-forming (Gleicheniaceae), and the fronds of some with particulaly long stems (Lycopodium volubile), stipes and rachises (Lygodium) can ascend into the crowns of small trees.
A number of species live on rocks or in rock crevise, with their roots going into bryphyte mats and trapped dirt and detritis. Their habit is often more akin to epiphytes that to other terrestrial species.
Epiphytic pteridophytes have their stems attached to or rooted on trunks sor branches of trees. A distinction should be made bewteen these and species such as Lygodium that have terrestrial rhizomes an long fronds that climb into trees.
Epiphytes can have compact or short to long-creeping stems. The stems can start on the tree or on the ground, but in any case are attached to the tree and are not reliant on the soil for moisture and nutrients. Leaves, fronds, or stems can be erect, arching or pendulous. The fronds of some species (Platycerium, Drynaria, Aglaomorpha and the bird's-nest Asplenium) are specially adapted to trap detritis in their bases.
Subepiphytes live low on the base of tree trunks, generally generally among bryophytes and decaying detritis. They may also occur on mossy ground and mossy rocks.
A number of ferns are truly aquatic, a much greater number are subaquatic or rheophytic (growing beside and periodically flooded by streams). Aquatic pteridophytes can have compact or shortly creeping stems and can be free-floating on the surface fo the water (Azolla, Salvinnia), completely submerged and rooted in the bottom sediment (Isoetes), rooted and emergent (Marsilia) or a combination of all three (Ceratopteris).
A more detailed account of aquatic ferns is available elsewhere.
The stem is the main lengthwise growing axis of the fern or fern ally and bears the roots or root-like organs for attachment to the substrate and acquisition of nutrients and water, and produces leaves or fronds in a more less regular manner. Once leaves or fronds are produced, they generally do not increase in diameter at that poing. for the most part, they are not photosynthetic but in some epiphytic climbing species there is some chlorophyll; the stems of Psilotaceae and Equisetaceae are phyllodes and have taken over the photosynsthetic function of the leaves which are reduced to small scale-like teeth.
Fern stems are often called rhizomes, but strictly speaking this term refers to those with a long-creeping or climbing habit. If the stem is short and compact and radially symmetric, either erect, prostrate or ascending, it is called a caudex or rootstock (or simply stock). In some cases the caudex is erect and tree-like with an apical radial tuft of leaves (Cyathea, Dicksonia, Leptopteris, some Blechnum, Thelypteridaceae) in which case it is often called a trunk. Creeping rhizomes can be either long-creeping with widley spaced fronds or short-creeping with fronds reletivley close together. The rhizomes can be radial or radially symmetric or dorsiventral with leaves or fronds produced on the dorsal or upper side and roots produced on ventral or lower side.
Overall, the rhizome or caudex may be unbranched (most compact rhizomes are unbranched) or branched in various ways. Creeping rhizomes may be unbranched or branched irregularly. Sometimes the branching is regularly dichotomous (or isotomous), sometimes with alternate branches reduced to a great or lesser extent (anisotomous. Sometimes the fork or branching is associated with the production of a leaf or frond in the axil. The rhizomes or caudices of some species that do not usually branch (e.g. tree ferns) may sometimes be induced to branch following physical damage. The branching patterns of the stem some groups of pteridophytes are quite distinctive and may be used taxonomically.
The rhizomes of ferns are very rarely naked and usually have some form of covering, especially on their growing tips, to provide protection from the extremes of temperature, dessication, predation and physical abrasion. This covering takes the form of epidermal hairs, bristles or scales, or a combination of any of these. The young developing fronds also bear these appendages in many cases and sometimes they are persistent on fully expanded leaves. Some rhizomes that do not produce epidermal appendages for protection achieve the same result by secreting a mucilaginous slime that engulfs the growing tip and the developing leaves (e.g. Plagiogyria). Sometimes the surface of the rhizome is glaucous with a bluish grey waxy sheen and sometimes with a dense white waxy covering; this often useful diagnostic character may be lost with collecting and preservation techniques involving excessive heat or solvents such as ethanol.
Hairs. Hairs may consist of a single cell (unicellular hairs) or single row of cells (multiseptate or multicellular hairs). Rhizome hairs are generally not glandular but those of Monachosorum and some other species are capable of secreting a mucilaginous slime. Hairs may be variously forked or branched but those on rhizomes generally are not. The length, density, colour, thickness and appearance of cells and the crosswalls are useful taxonomic characters. Some thick hairs can be quite stiff and bristle-like but are only composed of a single row of cells.
Bristles. Bristles are epidermal appendages resembling stiff, more or less rigid hairs. They are approximately circular in section and several to many cells thick at least at the base. Bristles may be mixed with hairs and grade into them. They are distinct from spines which are projection of the stem tissue and not epidermal structures.
Scales. Scales are flat plates of epidermal tissue one cell thick and several to many cells wide. Although microscopic, the details of orientation, shape, colour of the cells, and any hair-like or glandular appendages are often very important taxonomically. Mostly they are attached along their basal edge; those that are attached at their centre or at some point on the fact surface are described as peltate. In some families(Cyatheaceae, Grammitidaceae) the scales of some species are setiferous with short often discolourous hairs along their margins. Also in Cyatheaceae, the marginal cells of the scales of some species are of quite different structure, orientation and colour to the body of the scale and are described as flabellate or marginate. The scales of certain families (e.g. Vittariaceae, Aspleniaceae, some Polypodiaceae, Loxogramme, Rheopteris) are clathrate and have very dark and thickened cross walls and thin more or less transparent upper and lower walls; this is a very useful character.
Internal structure. The internal structure of the rhizome can provide useful diagnostic characters especially from the arrangement of the vascular bundles, leaf traces and the associated sclerified supporting tissue if any. Compact caudices are nearly always radial, so that the leaves and corresponding vascular tissure is evenly distributed around the stem. Elongate rhizomes may also be radial but are commonly dorsiventral, where the leaves and their traces are found on the upper surface, often in two rows, sometimes in three of more, and the roots on the lower surface.
The structure of the vascular tissue or stele has been used to separate certain groups of pteridophytes. The simplest form is the protostele, a solid vascular strand with internal xylem and an external strand of phloem; sometimes the central core of the protostele is composed of non-vascular parenchyma cells and this is termed a medullated protostele or equivalently an ectophloic siphonostele. A siphonostele is any uninterrupted stele with an undifferentiated centre and one with internal as well as external phloem is known as a amphiphloic siphonostele or a solenostele; this type of stele is most common in those ferns with long-creeping rhizomes and is usually dorsiventral. Leaf-gaps occur in such steles, usually above where the leaf traces diverge; the internal and external parenchyma are continuous through these gaps. Where the leaves are closely spaced, the leaf-gaps are so close that the vascular cylinder appears like a net-work and is called a dictyostele, each individual strand being called a meristele. This is a very common type of stem vasculature. In some families (e.g. Davalliaceae) the gaps in the network are not necessarily associated with leaf-gaps; such a stele has been described as a dissected solenostele (Holttum 1959). More complex stelar structure occur in some families (e.g. Matoniaceae) where there are several concentric series of steles. In some genera such as Stenochlaena there are additional vascular bundles outside the main system. In the climbing genera of the Lomariopsidaceae the leaf bases with their numerous vascular strands are decurrent along the rhizome for a considerable length and the resulting pattern of meristeles and leaf-traces can be very confusing. Where the stele is made up of a number of flat strands more or less parallel to each other it is known as a plectostele.
Internal Supporting structures. This often takes the form of dark, hard and durable, sclerified material variously distributed through the stem. In many cases the stem in encased in sclerified tissue and sometimes the entire caudex may become sclerified. Often the vascular bundles (stele, meristeles, leaf-traces) are encased in sclerified material and they are referred to as fibrovascular bundles. Sometimes separate dark fibrous strands are present inside and/or outside the stele. In Cystodium the caudex has a large, solid sclerified core. Many groups (e.g. Ophioglossaceae, Marattiaceae, some Vittariaceae) have little, if any, sclerified material and rely on tissue turgor to support the plant.
Roots. Pteridophyte roots are not produced as the downward proliferation of the stem but are mostly produced laterally, sometimes scattered along the length of the stem, and are thus adventitious. In some genera (e.g. Ceratopteris) the roots are also produced on the leaf bases. Sometimes these roots are simple, but mostly they are branched, sometimes dichotomously , mostly irregularly. In arborescent genera (Cyathea, Dicksonia etc.) the roots sometimes form a thick tangled mat that engulfs the lower trunk, becoming heavily sclerified and providing a considerable amount of physical support. Many epiphytic pteridophytes (Lycopodiaceae, Vittariaceae, many Aspleniaceae, Polypodiaceae etc.) produce a large dense mat of roots with many fine hairs that serves to trap and retain moisture and nutrients. Some reduced species of the Hymenophyllaceae seem to have forsaken roots for a covering of fine hairs that function as root hairs.
The root structures of Selaginella, known as rhizophores are borne along the stem, the upper ones often getting progressively longer and forming a series of supporting stilts; they branch dichotomously on contact with the ground and do not bear root hairs and there is some doubt whether they are true roots or specialized appendages of the stem.
Stolons. The stems of some species (e.g. Nephrolepis, some Blechnaceae) produce wide ranging runners or stolons that are capable of rooting and producing new plants at regular intervals. These are of stem origin and quite different to the proliferating fronds of other species (see under Bulbils below). The stems of some species of Selaginella produce runners from near their bases that ascend and root, becoming new plants.
The leaves of typical and true ferns are usually called fronds; those offern allies referred to as leaves. Although whole volumes of fern descriptions make no mention of "leaves", fern fronds are truly homologous with the leaves of flowering plants, being expanded vascular photosynthetic organs produced by the stem; the use of the term "leaves" or "fronds" is largely a matter of personal preference and in this work, 'frond' is used only for leaves of true ferns and not those of the fern allies.
The bewildering variety of leaf form or arrangement provide many of the most useful characters of pteridophyte taxonomy. The leaves of the fern allies (Psilotaceae, Isoetaceae, Selaginellaceae, Lycopodiaceae, Equesetaceae) are simple and undivided, not often toothed, sessile, with a single unbranched vein and are often small; they are called microphylls. Fern fronds are megaphylls and except in very reduced species, are large and complex, have many veins and are often lobed or variously divided, commonly with a distinct stalk or petiole. The microphyllous leaves of Psilotaceae and Equisetaceae are reduced to minute brown teeth and the photosynthetic function has been taken over by the green phyllodinous stems.
The expanded green portion of a leaf is called the lamina. A leaf where the lamina arises directly from the stem is described as sessile. The petiole or stalk of a fern frond is generally called the stipe (like 'leaf/frond', the use of this term in preference to stalk or petiole is a personal matter) and fronds that possess them are described as being stipitate or stalked.
The spore-producing organs are borne on the leaves and those that do so are called fertile and those without, sterile. The fertile fronds may be of markedly different size, shape or orientation to the sterile fronds and this is known as dimorphism (see later).
Phyllotaxy, Stipes and rhizome attachment. Pteridophyte leaves are nearly always alternate, spirally arranged or in two or more staggered rows along the stem. Often the fronds are clustered in a tuft at the end of the stem, sometimes in whorl-like clusters along the stem (some species of Oleandra) and in Equisetum the reduced leaves are in regular whorls. Leaves are generally produced ascending at an acute or obtuse angle towards the apex of the stem (whether it be vertical or pendulous) but, especially in some epiphytic ferns, they may arch and become pendulous.
The leaves may be produced one after the other in a more or less regular fashion, or a whorl leaves may develop more or less at the same time and appear to be synchronous. This is especially noticeable in some species of tree ferns (Cyathea, Dicksonis) and in the climbing members of Lomariopsidaceae (Lomariopsis, Lomagramma etc.) and the genera Plagiogyria and Leptopteris.
Stipe bases generally bear the same type of hairs or scales that cover the stem and are similarly very important taxonomically. In groups such as the Cyatheaceae the structure of the stipe and caudex scales is essential for identification. These scales and/or hairs perform the same protective function as those of the rhizome. In ferns, the young fronds develop by uncurling along the main axis from the base towards the tip; this type of vernation (leaf development) is known as circinnate vernation and is a distinctive characteristic of ferns; the uncurling frond is known as a fiddlehead or crosier. The fern allies have straight vernation. The stipes and young fronds of some ferns secrete mucilage as the frond is developing, and various structures have developed to improve gas exchange while the frond is in this tightly coiled position. Elongate areophores extend laterally from the main axis, usually at the base of pinnae (leaflets) and these or their remnants or scars are often evident in mature fronds (Plagiogyriaceae, Cyatheaceae, some Thelypteridaceae). In many genera there is a raised line of pale tissue, continuous or interrupted, running the length of the stipe; this pneumathode is also an aerophore and involved in gas exchange. In some groups (Angiopteris, Marattia, some Cyathea) the aerophores may be pale depressions. Numerous reduced pinnae are present along the stipes of many species (some Blechnum, common in Thelypteridaceae); these serve little purpose in mature fronds but are very prominent when the frond is coiled where they are active photosynthetically and where they overlap they provide considerable support and protection for the tender crosier.
Stipes may arise fairly abruptly from the stem or it may be decurrent and gradually merge in. There may be an obvious mark where the stipe joins the rhizome where the frond readily falls from as it gets old; stipes with such an articulation are said to be jointed or articulated. Such articulations may also occur at the base of pinnae (e.g. Nephrolepis, Goniophlebium, Drynaria, Aglaomorpha). In some cases (some Polypodiaceae, Elaphoglossum, Lomariopsis) the stipes are articulated to specialized outgrowths from the stem called phyllopodia.
Stipe vasculature. The vasculature of the stipe arises from the internal vasculature of the stem. The number and arrangement of vascular strands in the stipe is a very useful and important diagnostic and taxonomic character. This can usually be seen in a crude cross section made with a sharp knife, even without staining. Tectaria and allied genera have many vascular strands arranged more or less in a ring, similarly Davalliaceae and Lomariopsidaceae. Two unrelated families with elongate indusiate sori, Aspleniaceae and Athyriaceae have two vascular strands in the basal part of the stipe; the former unite upwards at their middles to form an X-shaped strand, the latter at their bases to form a U-shaped strand. The complex nature of the stipe vascular of Cystodium recently suggested the supposed relationship with Dicksonia was not as close as previously thought.
Fern fronds are either simple or compound and composed of several to many leaflets which may in turn by composed of several to many leaflets, and so on until the ultimate division is a simple leaflet or lobe. A simple lamina is continuous either side of the midrib, but may be shallowly or deeply dissected or lobed or unlobed and entire.
The branching of a fern frond is nearly always in the one plane (exceptions include the bottle-brush fronds of some species of Macroglena: Hymenophyllaceae and the horizontal twisting of the secondary branches of scandent species). This is usually the same plane as the stipe (sometimes there is an abrupt bend where the stipe meets the lamina e.g. Dipteris, Currania, some Tectaria) and perpendicular to the plane of the stem.
A frond axis is the central supporting tissue about which the lamina and/or subsequent axes are arranged. The main axis consists of the stipe (if present) and its continuation into the lamina of the frond; in simple fronds this is the midrib or costa, in compound fronds, the rachis.
Most commonly, the leaflets or secondary rachises are borne alternately (or at most subopposite) on either side of the main rachis. This is the pinnate condition and the leaflets are known as pinnae; the pinnae themselves may be similarly divided and the frond is described as being bipinnate and the pinna of each leaflet is known as a pinnule. Further division can result in tripinnate or even quadripinnate fronds; in each case the ultimate simple division is referred to as a pinnule. If the lamina is only partially incised towards the axis, the segment is described as being pinnatifid (the term pinnatisect describes very deep incisions, but this distinction is not often drawn). In a similar manner to pinnate division, it is possible to have bipinnatifid or tripinnatifid fronds. It is possible to have combinations of these forms of dissection and such structures as pinnate-bipinnatifid or bipinnate-pinnatifid etc. are common. Finely divided fronds are often described with the general term of decompound.
Fronds with opposite branches are not as common. Dipteris and Matonia could be considered to have fronds with two opposite branches. Usually this occurs when the growing tip of the frond remains dormant for a period as the pinnae develop and then resumes growth towards the next pair of pinnae. This can be seen in Gleicheniaceae, Lygodium and some Dennstaedtiaceae such as Histiopteris. In Lygodium and most Gleicheniaceae, the bud on the lateral branches remains dormant, resulting in a more or less equal (sometimes one side ma be suppressed) fork in the branching pattern. This is termed pseduodichotomy because of the presence of the central bud and should not be confused with the true dichotomous branch where the axis does not continue in a straight line but forks into two more or less equal parts. Examples of true dichotomy can be seen in Dipteris, some Gleicheniaceae and some Schizaeaceae. In some cases (e.g. Matonia) only the outer branch of each dichotomy forks again; this condition is known as pedate. It is easy to see how the pinnate condition can be derived from the dichotomous by the alternate suppression of the left and right forks. Fronds with a dormant terminal bud that are capable of indefinite growth are called indeterminate; most fronds are determinate and expand more or less at once to a limited size.
Those parts towards the apex or tip of the frond are described as being apical, whereas those parts towards the base are referred to as basal; similarly those parts directed towards the tip are described as acroscopic and those directed towards the base as basiscopic. The terms upper and lower can usually be applied without ambiguity to the surfaces of fern fronds, but in the case of pendulous or vertical fronds it is preferable to use equivalent but more precise terms adaxial (twoards the axis) and abaxial (away from the axis). The upper or adaxial surface develops facing the growing apex of the stem, the lower or abaxial surface faces away from the stem apex.
Anadromy, catadromy. In pinnately compound fronds the order in which the pinnules are produced has been found to be very important in the classification of some ferns (e.g Lastreopsis). If the basal acroscopic pinnule of a pinna is closer to the rachis than the basal basiscopic pinnule, the frond is called anadromous (Greek: up coursing). If the basal basiscopic pinnule is closer the frond is said to be catadromous (down coursing). Some groups exhibit both types of fronds but others consistently show one or the other type and it is here that this character is most useful.
Axis junctions. The structure of the rachises and their unions with other axes offer very useful characters in fern taxonomy. In some genera (e.g. Arthropteris, Nephrolepis, Stenochlaena, and some Dennstaedtia, Blechnum, Lomariopsidaceae and Polypodiaceae) there are distinct articulations at the base of pinnae and axes. Swollen or elongate aereophores may be present at the bases of pinnae (see above). The pinna- and pinnule-bases of Angiopteris and Marattia are swollen and turgid. Axes are often grooved on the adaxial surface and the grooves and ridges of the different order branches may be confluent or the ridge along the main rachis may not open to admit the groove of the smaller axis, the lamina may be decurrent (gradually merging) along the side of the rachis or along the ridge of the rachis groove. These characters seem to be consistent within genera or groups of genera and are frequently used for identification. The axes may be glabrous (naked) on either surface or bearing hairs and/or scales, the structure and abundance of which are diagnostically very useful.
Surface and texture of lamina and appendages. Pteridophyte leaves vary from thin and filmy and one cell thick (Hymenophyllaceae, Leptopteris) to many cells thick and fleshy (e.g. Ophioglossum, Angiopteris) or even stiff, coriaceous or leathery (e.g. Selliguea, Pyrrosia). False veins, also known as pseudoveins are present in some species. These are lines of tissue with the appearance of veins, usually not connected to true veins and not associated with any vascular tissure; they are diagnostically useful in Davallia, Angiopteris, Trichomanes sens. lat. and possibly other genera. The shape and arrangement of stomata and their supporting cells has proven to be very reliable in systematic studies and has been used taxonomically in some cases (e.g. Schizaea, Isoetes). The structure and orientation of the epidermal cells has been used in some cases (especially Hymenophyllaceae where they are so readily visible). In Vittatiaceae some epidermal cells are narrow and contain silica (spicular idioblasts); the phyllodinous (leaf analogue) stems of Equisetum also contain large crystals of silica. Either surface of the lamina may be glossy or dull, naked of with various type or hairs; if hairs are present, they are generally more abundant on the lower surface than on the upper surface. The hairs may be simple, binate (forked) or branched in a more complex manner, sometimes with glandular tips. Stellate hairs are particularly important taxonomically, consisting on numerous elongate cells radiating from a central point of attachment (e.g. Pyrrosia, Platycerium). Hairs may completely cover the lower surface of the lamina giving the frond a silky appearance beneath. The veins and constules of some species may bear scales beneath that are diagnostically important (e.g. Cyathea, Elaphoglossum). Like the rhizome scales they may be concolorous or clathrate; pale, deeply concave and blistered-looking scales are called bullate, those bearing hairs, ciliate, those with setae (stiff marginal bristle-like scales), setiferous or setose, and those with divergent free cells along the margin, fimbriate.
Glaucescence. A pale bluish grey waxy covering may be present on the lower surface of some species ans ins described as glaucous (e.g. Dipeteris). Other species may have a thick white waxy covering or farina and are called farinose (e.g. Pityrogramme, some species of Cheilanthes). Glaucescence and farina are immediately obvious on live plants but the was may have melted in specimens dried with too much heat, or dissolved in specimens field-preserved in ethanol before drying, and may not be visible in herbarium material.
The vascular traces of the stipe and axes eventually end up in the venation of the lamina and the manner and pattern in which this is achieved is particularly important in fern classification. Veins are the ultimate vascular strands (and their supporting structure) running through the leaf, and are for the most part clearly visible on either surface of the lamina, being raised or impressed with cells of different colour, shape and orientation the rest of the lamina.
Veins with no branches or unions are called simple. Veins that do not reunite with other veins are called free and free veins may be simple or forked one of more times oftn in a more or less dichotomous manner. These veins may terminate at the margin or short of it, often in lobes or teeth, sometimes in the sinus between the lobes, or the margin may be entire. In many cases, the veins (forked or simple) are pinnately arranged around a main vein or costule, usually with a single costule in each lobe of the ultimate segment; the costules are in turn pinnately arranged about the midrib or costa of the leaflet. This is the common pattern in many Athyriaceae, Cyatheaceae, Thelypteridaceae. It is also common for the veins of adjacent groups to anastomose or reunite, sometimes forming another excurrent vein (= outward running) between the costules; this pattern is common in Thelypteridaceae. The area of lamina enclosed by veins is called an areole and a venation pattern involving few free veins is termed areolate. Anastomosing patterns are not restricted to between adjacent costules and may occur between adjacent veins forming a single series of areoles along the cost or costule (e.g. Stenochlaena, Pleocnemia, some Pteris) or at vein extremities to form a continuous intramarginal vein (e.g. "bird's nest" Asplenium, Vittaria, some Hymenophyllaceae) or even a marginal series of areoles (Coniogramme, some Syngramma). In some species there are no obvious costules, the veins forming a more or less even network or reticulum (e.g. Acrostichum); this form of areolate venation is often described as reticulate. In some species of Ophioglossum and Antrophyum there is no costa and the entire frond is reticulate. Sometimes the areoles themselves contain free veins that may be simple or forked; the number, position and orientation of the included veins (or veinlets) is often diagnostically important.
Supplimentary veins. In fertile fronds additional vascular networks or commissures may be present to serve the sporangia and soral tissue and may overlay or replace the venation of sterile fronds.
Hydathodes. The ends of veins some that terminate short of the margin may be swollen and are often involved in water and salt secretion. In some species (some Grammitis, Selliguea, Pyrrosia, Coniogramme) the secreted water evaporates leaving a persistent deposit of white salts visible in rows on the upper surface of the lamina. The presence of these deposits is a taxonomically useful character.
False veins. Also known as pseudoveins, these are strands of structural tissue running through the frond, often with the superficial appearance of true veins. However they are not involved with the translocation of water or nutrients and they are not connected to or continuous with the vascular network. They are sometimes hyaline or translucent, running between and parallel to the true veins, or outside the veins and parallel to the leaf margin. The presence and disposition of false veins is constant within a species and forms a useful taxonomic character.
Frond dimorphism. In addition to differences that may exist between sterile and fertile leaves, juvenile leaves are sometimes different to the adult leaves. The first fronds produced by pinnately divided species are often simple and differences in the shape of this juvenile foliage is sometimes diagnostic (e.g. Acrostichum). A particular case involves the high climbing epiphytes of the Lomariopsidaceae; the lower juvenile fronds near the ground (bathyphylls) are often finely divided and of quite different appearance to the adult fronds high on the trunk (acrophylls). In some bipinnate species some juvenile at the pinnate stage may be fertile and bear sporangia; examples of this precocious fertility are known in Cyathea and Lindsaea.
Bulbils. Bulbils are vegetative buds usually towards the apex of the frond (some species of Asplenium, Bolbitis, Diplazium, Ceratopteris) but sometimes along the rachis (Ampelopteris) or at the base of pinnae (some Diplazium). These may remain dormant and die with the frond, but under suitable conditions they are capable of producing roots and juvenile fronds and eventually developing into a new plant. By repeated propagation in this manner clonal populations may be produced in much the same manner as stolons. The presence of bulbils, whether dormant or active, is important taxonomically.
Fertile Leaves. The leaves that bear the sporangia are termed fertile as opposed to sterile for those without sporangia. In the fern allies sporangia bearing leaves are called sporophylls. Fertile leaves or fronds may or may not be different to the sterile. The sporangia of the fern allies are generally borne on the acroscopic or upper side of the sporophylls, at the base near the axil of the leaf. In true ferns they are generally produced in various ways on the abaxial or lower side of the leaf, remote from the base.
Sori. In the Osmundaceae the sporangia are more or less scattered along veins on the lower surface of the lamina, but the situation in the remainder of the ferns is for the sporangia to be aggregated in various ways into a sorus attached to a common receptacle which supplies nutrients from the veins. All sori and sporangia are associated with veins. The structure and distribution of these sori is very important diagnostically. Developing sori are protected in various ways such as thin, dry outgrowths of the lamina called indusia or by a reflexed fold of the lamina margin (a false- or pseudo-indusium). Sori with an indusium are called indusiate, those without are exindusiate. Sori may also be protected by paraphyses: simple, branched or stellate, glandular or eglandular hairs or scales mixed with the sporangia, the possible origin of which may be aborted sporangia or included hairs. The structure of the sorus and indusium, if present, is extremely diverse and very important taxonomically.
Commonly, sori are small and more or less circular. Their receptacles may be at the end of a free vein, at the margin or remote from it, or at the junctions of veins, or on the back of a vein, or on a free vein within an areole. The indusium, if present, can be circular, reniform (kidney-shaped), peltate or attached at the base, pocket-shaped (attached at the base and sides), cyathiform (cup-shaped) or variations of these (e.g. hemitelioid or half-cup-shaped). Sometimes a submarginal sorus may be protected by both a cupped inner indusium and a cupped reflexed lobe of the lamina (e.g. Dicksoniaceae) or the inner and outer parts may be more or less united into a tube or cup (Hymenophyllaceae, Dennstaedtia).
Sori may be elongate along free (e.g. Asplenium, Diplazium or anastomosing veins (e.g. Taeintis), or intramarginal veins (e.g. Vittaria), or elongate submarginal sori may link several free veins that would otherwise terminate at the margin (see coenosori below). The indusia of such elongate sori usually open acroscopically if along veins divergent from the costa (downward facing linear indusia are known in Diplora and some Athyriaceae) and outwardly if submarginal or parallel to the costa. Marginal linear sori may be sunk in a groove of the lamina (e.g. Vittaria, Scleroglossum), protected by an inner indusium only (e.g. Lindsaea), protected by an inner indusium and outer pseudo-indusium (e.g. Pteridium), or protected only by an outer pseudoindusium (e.g. Pteris).
Coenosori and acrostichoid sporangia. Sori that link veins that are normally free in the sterile condition or result from the spread of one sorus into another distorting the normal venation pattern are known as coenosori (or fusion-sori). They are often supplied by a supplimentary vascular system overlaying the usual veins of the sterile fronds. Lindsaea, Pteridium and Pteris are examples already mentioned; in Blechnum and Stenochlaena the coenosorus runs adjacent to the costa, and the remaining lamina is often very much reduced. The extreme case of the coalescing of sori is the acrostichoid condition (from the genus Acrostichum). Sporangia spread along all of veins of the lower surface of the fertile frond (or fertile region) and even between the veins; this fertile area may be slightly or greatly contracted when compared with the sterile. Examples of acrostichoid sori are common in the Polypodiaceae, Blechnaceae and to a lesser extent, Hypoderriaceae; all Plagiogyriaceae and most of the Lomariopsidaceae are acrostichoid.
Sporangia. The sporangia are the bodies in which the spores are produced. They are generally small and superficial but may be large and immersed in the sporophyll (Isoetes). The mostly minute spores are shed when the sporangium dehisces or splits although sometimes the spores are released by decay of the sporangial wall (Isoetes). In most cases the sporangia burst under dry conditions and dispersal is by wind.
The ferns are divided into two main groups on the basis of their basic sporangia type. Those with thick walled (several cell layers thick) sporangia that develop from several cells are known as eusporangiate and are considered to be more primative. The bulk of the ferns have thin-walled sporangia developed from a single cell and are termed leptosporangiate; these are the advanced ferns.
In the Psilotaceae, perhaps the most primative group of pteridophytes, the sporangia are large and united into twos or threes (synangia) each splitting when mature.
In the Ophiglossaceae, a primative group of ferns, the sporangia are relatively large and borne on erect or pendulous fertile spikes or branched structures produced near the base of the fronds. This is the most aberrant method of sporangia production in the ferns. The sporangia or Marattia, Angiopteris and Christensenia (also primitive ferns) are large, sessile and thick-walled and more or less united in groups (synangia) on the lower surface of the lamina. The synangia or Marattia and Angiopteris are composed of two rows of sporangia along a vein; in Christensenia the sporangia are arranged in a circle at vein junctions. Like the Ophioglossaceae, these genera do not possess an annulus (see later).
The sporangia of the remainder of the true ferns are all remarkably similar. They are thin-walled and dehiscence is caused by a region of cells with thick inner walls and thin outer walls, usually in a ring, called the annulus. The dehiscence occurs in a specialized region called the stomium. In Osmundaceae the annulus is a region of cells to the side of the sporangium; in Schizaeaceae it is a tight ring of cells at one end of the sporangium; in Gleicheniaceae, Hymenophyllaceae, Cyatheaceae, Plagiogyriaceae it is oblique and a complete ring; in the remainder of families it is more or less vertical over the top of the sporangium and interrupted by the stalk or pedical.
The microscopic structure of the sporangia yields many useful taxonomic characters. There may be hairs, which may be glandular or eglandular, on the pedicel or on the body of the sporangium near the annulus. There may also be sessile glands or stiff setae. Such characters have been used extensively in the Thelypteridaceae and also in the Grammitidaceae and may be useful in other families as well.
There are two basic types of spores in the pteridophytes: monolete (or bilateral) and trilete (or tetrahedral). Spores are always produced in tetrads (groups of four) and the spore shape results from the two possible cleavage patterns of the initial cell. If the second cleavages are in the same plane the result is a bilateral spore with a monolete spore; each spore as it develops has two neighbours, the two contact faces meet in a single ridge of scar and the free spore is more or less bean-shaped. If the second cleavages are in planes perpendicular to each other, the result is a tetrahedral spore with a trilete scar; each spore as it develops has three neighbours, the three contract faces meet at a three-armed ridge or scar, the four spores are positioned at the four corners of a tetrahedron and the resulting spore is more or less globular or tetrahedral with a rounded base. Generally, all genera have one or the other type of spore, but a few (e.g. Loxogramme, Dicranopteris) have both types, sometimes in the same species.
During development, the spores are nourished by an inner layer of the sporangial wall (the tapetum). In some species, a residue of this tapetal material may be deposited around the spore and is known as a perispore. The perisore is usually folded into ridges, wings or spines and its presence and structure are very important characters. Species that lack a perispore often have intricate ornamentation of the spore wall in the form of tubercules, depressions, ridges, valleys, warts etc. The morphology of the spore wall and perispore has been widely used in many groups and indeed in some genera (e.g. Isoetes) it is often the only reliable taxonomic character. Much of the spore detail can be seen with appropriate light microscopy techniques, but the use of scanning electron microscopes (SEMs) reveals much more detail and shows the previously know structure more clearly.
Photosysthetic spores. The spores of Equisetum and Grammitidaceae are dark green when alive and are capable of photosynthesis. This presumably confers some survival advantage on the germinating prothallus. In addition to photosynthetic pigments, Equisetum spores are provided with special coiled structures called elaters that extend and contract with changes in humidity, aiding dispersal.
Heterosporous pteridophytes are those that produce spores of markendly different sizes (e.g. Selaginellaceae, Isoetaceae, Marsiliaceae, Azollaceae, Salviniaceae). They produce both largemegaspores and minute microspores in much the same manner as the homosporous pteridopytes, the main difference being the greatly increased size of the megaspores. (See under Gametopyte above.)
In Platyzoma, spores are produced in two distinct. but not vastly diferent, sizes and are described as anisosporus. This condition is known as anisospory.
Sterile/fertile dimorphism. The sporophylls of some Selaginellaceae and Lycopodiaceae are contracted and much smaller than the foliage leaves and may be aggregated into a tight or lax radial cone or strobilus. The sporophylls of Equisetum, usually called sporangiophores, are also aggregated into tight strobili; the sporangiophores are peltate structures bearing sporangia on the lower surface.
In the true ferns, fertile fronds may be of different size, shape and orientation to the sterile, the difference being slight or very marked. Commonly, in species with dimorphic fronds, the fertile frond is held more erect, has a longer stipe, is narrower and often elongate compared with the sterile; this is widespread in many families. In the extreme case the fertile pinnae can be so contracted that there is hardly any green lamina visible beneath the sporangia (e.g Lomariopsidaceae, Stenochlaena, some Blechnaceae, Stenosemia, Plagiogyria, Leptochilus). When the difference between fertile and sterile regions occur within the same frond (e.g. Aglaomorpha, Belvisia) the frond is sometimes called internally dimorphic; more often the whole frond is modified and thus externally dimorphic.
A special case of spore production is involved in the aquatic heterosporuos families Azollaceae, Salviniaceae and Marsiliaceae (the last family is quite unrelated to the former two which are considerd closely related to each other and are in fact combined by many authors). In these families, the megaspores and microspores are produced in separate megasporangia and microsporangia which are produced in enclosed bodies called sporocarps. The stems of Marsiliaceae root in the mud and the sporocarps are borne near the base of the long-stalked leaves; the capsular fruit-like sporocarps are often called conceptacles. Azolla and Salvinia are floating herbs with short-stalked leaves below which the sporocarps are produced.