Diagnostic characters of ferns

DISTRIBUTIONS
Local/widespread, rare/common: with even limited experience, it soon becomes obvious that some species are commonly seen over wide areas in a range of habitats, while others are absent from large areas of Britain or are restricted to very specific habitats in a few sites and rarely seen. Some rare species are locally abundant in the few localities where they occur.
Information on distributions can help to eliminate some possibilities when identifying a species.

HABITATS
It can be helpful to note the main features of the habitat because some ferns are more common in, or even restricted to, habitats of a certain type.
Types:
Wet/dry: this is usually obvious from the soil conditions. Wetter habitats occur on poorly-drained soil in high-rainfall areas and at higher altitude, as well as near open water.
Basic/acidic: although often indicated by the flowering plant community and geology, for inexperienced beginners the soil type is not always obvious. Acid soils are widespread, particularly in areas of high rainfall and hard rock such as granite. The plant communities of acid soils are often species-poor. Heather and related plants indicate some acid soils. Basic, usually lime-rich, soils are more local and are derived from a variety of rocks including limestone and basalt. Basic soils often support a rich diversity of plant species.
Exposed/shaded: it is not difficult to assess whether a fern is protected from direct sunlight when it is growing on a site with a northerly aspect or shaded by overhanging rocks or tree canopy. However, it must be remembered that wintergreen ferns in deciduous woodland may be in deep shade in the summer but receive direct sunlight during the winter. Even some deciduous woodland ferns may receive sunlight on expanding fronds in Spring before the tree canopy develops fully. Some fern species are intolerant of exposure to sun, others are inhibited by shade.
Montane/lowland: some fern species are only found at high altitudes in Britain, some others only at low altitude.

FROND SIZE Fronds are described as small, medium-sized, or large according to the length (base of stipe to apex of blade) of typical mature fronds of wild individuals (small ferns: typical fronds less than 25cm; medium-sized = 25-75cm; large = 75-120+cm); individuals growing in adverse conditions may be atypically small, while those in unusually sheltered or shaded conditions, and those grown in cultivation, may be atypically large.

FROND SHAPE. All fronds have a stipe, the length of which varies between species from less than one-sixth to more than two-thirds of the total length of the frond. Frond shape is mainly determined by the overall outline of the blade (lamina). For simplicity, only five terms are used in this key to describe the blade outline. These are defined precisely to ensure consistent usage in this key, although the definitions may not always coincide exactly with their usage by other authors.

For divided fronds, the blade outline may be :
triangular“=lowest pair of pinnae are the longest or equal longest with the adjacent pair;
lanceolate“= blade widest around the mid-point, tapering at the base (lowest pinna less than half the length of the longest pinna);
oblanceolate“= blade widest at or above the mid-point, truncated at base (lowest pinna more than half the length of the longest pinna);
ovate“= blade widest below the mid-point, tapering at the base (lowest pinna less than half the length of the longest pinna);
or
obovate“= blade widest above the mid-point, truncated at the base (lowest pinna more than half the length of the longest).
These can be qualified by “narrowly“: blade length > 6 times maximum pinna length, or “broadly“: blade length < 4 times maximum pinna length. If the terms ‘narrowly ‘ and ‘broadly’ are not used, either the blade width is consistently intermediate (blade length 4-6 times maximum pinna length) or it ranges from broad to narrow. The same terminology can be used to describe the shape of individual pinnae. For undivided fronds, the same terms can be used but in the definitions, read "blade base" in place of "lowest pinna", and "the width at the widest point" in place of "the length of the longest pinna". BLADE (LAMINA) DIVISION. The degree of subdivision of the blade is one of the most conspicuous characteristics of a fern. In only a few ferns, including Asplenium scolopendrium, are the blades undivided (also referred to as “simple”). In most ferns, the blades are to a greater or lesser extent sub-divided. Because the stipe is undivided in British species, the terms ‘divided blade’ and ‘divided frond’ are synonymous and the latter is the term most frequently used in published identification guides. In a few species (Asplenium septentrionale, and some plants of A. ruta-muraria) the fronds are irregularly sub-divided, but in most ferns there is regular and symmetrical division first into “pinnae” (singular “pinna”) and then perhaps further sub-divided into “pinnules” and even “pinnulets“. The terms “pinnate”, “pinnatisect”, “pinnatipartite” and “pinnatifid” used by other authors to describe the sub-division have not always been applied consistently. It is important that terms are clearly defined and precisely applied because frond division is an important character in distinguishing species. In order to avoid further confusion, this key refers to fronds only as “undivided”, “once-divided (=1-divided)”, “twice-divided (=2-divided)”, etc. A frond, pinna, pinnule or pinnulet is considered to be further divided when the division extends to at least three-quarters of the way from the margin to the midrib. Where the separation does not extend so far, the frond, pinna, pinnule or pinnulet margin is (deeply to slightly) “lobed“. If the margin resembles the teeth of a saw, it is “serrated“. If the margin is straight and smooth, without indentations, it is “entire“.

In describing a frond, it is the maximum extent of sub-division that is expressed. For 2- and 3-divided fronds, this is normally found on the largest pinna, which is sometimes (as in a triangular lamina), but not always, the lowest.

For many species, the degree of sub-division is consistent even when the shape of the frond and its sub-divisions varies. This is not true of some of the more variable species with finely divided fronds. For example, the fronds of some individuals of Asplenium adiantum-nigrum, Athyrium filix-femina and Cystopteris fragilis are 2-divided while fronds of other individuals of the same species are 3-divided. For this reason, these species appear twice in the Key. A few of the species that typically have 3-divided fronds (e.g. Pteridium aquilinum) are occasionally 4-divided, but they appear only once in the Key under “At least 3-divided fronds”. Asplenium ruta-muraria also appears in the key twice because, although typically 2-divided, on some plants the fronds are more irregularly and variably sub-divided.

SORUS DISTRIBUTION: their position on the frond. Sori are usually in the upper half of the blade, on the underside. They are often associated with the veins but in some species they are restricted to the margins of the pinna or pinnule. In a few species (Royal Fern, Osmunda regalis; Moonwort, Botrychium lunaria; and Adder’s Tongue, Ophioglossum vulgatum) conspicuously large sporangia are not in sori but aggregated like small grapes at the frond apex where the blade has almost no expanded green tissue.

The following features require examination under a x10 lens:

SURFACE FEATURES: for example, stipe scales and minute secretory hairs (the latter are not always conspicuous even under x10 lens and are liable to be lost as the frond ages)

SORUS SHAPE (when immature, before the shape is obscured after the sporangia split open to release the spores). With some diagnostic exceptions, most sori are approximately circular or linear.

INDUSIUM CHARACTERS. The indusium is a thin membrane of cells that covers each sorus while the sporangia develop, after which it shrivels. It is absent (sorus “naked”) in some species. In some species it is present but lost before the spores are ripe; in others, the indusium is persistent after spore dispersal (although in winter-green species it is likely to be lost eventually during the winter along with other sorus features). The shape of the indusium, and the appearance of the indusium margin, are diagnostic in some species.

SPORANGIUM APPEARANCE. The only conspicuous feature of the sporangium that is reliably visible under x10 lens is the annulus. The annulus is absent from the large sporangia of Royal Fern (Osmunda regalis), Moonwort (Botrychium lunaria) and the Adder’s Tongue Ferns (Ophioglossum species). In other species, the colour of the annulus cells can be diagnostic. (The number of annulus cells can be useful also, as in Polypodium species, but they can only be counted accurately under a microscope at a magnification of at least x80.)

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