The Identification of Ferns

booThe Identification of Ferns

Every gardener knows what a fern looks like; there is no group of plants which can be recognized more easily, in the garden or in the wild, but for that very reason, when one considers that there are at least ten thousand species inhabiting this earth, the majority having feathery plumes of foliage built on the same basic design, the problem of identification becomes an extremely difficult one for the untrained observer.

However, by confining ourselves to the hardy ferns and those which can be grown in unheated structures, we eliminate at once a great many species. Fortunately the majority of our British species are fairly easy to spot, after a little practice and the will to learn the finer details. For accurate identification may depend on tiny details not at all obvious to the beginner, even when pointed out. It is so very true that so many people see but do not observe, even to the extent of not realizing that all ferns are not bracken.

Dryopteris filix-mas For the botanical names of ferns, which are international, consist of as it were a surname, the generic or family name which in plants comes first, as in the telephone directory, and the specific name, which is as it were the plant’s ‘Christian’ name, comes second. So the common Male Fern is a member of the Dryopteris genus, and is differentiated from its close relations by its specific name  —  filix-mas. Dryopteris filix-mas.

The trouble with accurate identification for the non-technical reader comes from the use of botanical terms, which are a kind of shorthand for eliminating long sentences of description in everyday language, but it is nearly impossible to describe the differences between ferns without the use of some technical terms.

In any case those who wish simply to grow ferns and find out something about them without diving very far into these deep waters can skip the tough parts as they are not necessary for successful cultivation. At the same time if one can take the trouble to familiarize oneself with the ‘jargon’, it will be found that increased pleasure and interest in these fascinating plants will be won.

Taking the most obvious characters first, the size of the fern plant is a good starting-point. There are mature ferns from one inch in stature to the majestic Tree Ferns which may attain thirty feet or more in height. The majority of garden ferns will range from six inches to four feet in height when mature. Any fern below or above these heights will belong to a limited number of species, and this will give a pointer to certain genera and eliminate a lot of others. For instance, below six inches, this will include certain Spleenworts, and above four feet, Osmunda and Tree Ferns.

Starting at the bottom and working upwards, next we may consider the root. This may be a stout rootstock, or caudex as it may be termed, covered with old frond bases, more or less obvious on the surface. For example our old friend the Male Fern again. In the case of the stout rootstock, the fronds above usually are in the form of a shuttlecock, or crown. Or it may take the form of a slender branching rhizome running about underground, throwing up fronds at intervals without the formation of a crown. This would eliminate all the Male and Buckler Ferns, Dryopteris species, and the Shield Ferns, Polystichum species, and many others, but would point to such species as the Oak and Beech Ferns and their allies.

The creeping rhizome might be of medium thickness, wandering on the surface of the ground or on the trunks of trees. At once the common Polypody springs to mind, or the New Zealand Microsorium diversifolium, and Davallia. The only fern which forms crowns and also spreads by underground runners is Matteuccia or Struthiopteris germanica.

Then there is the deep stout creeping rhizome of the Bracken, Pteridium aquilinum, which hardly needs mention as it is known to everyone, I imagine.

Next to consider is the frond. It is not correct to describe the foliage of ferns as leaves as they are not strictly comparable with the leaves of flowering plants, although they perform many of the same functions. The flowering-plant leaf is an assimilating organ, absorbing atmospheric gases and sunlight, manufacturing food therefrom with the aid of salts in solution drawn up from the roots, and also eliminating unwanted gases and water by transpiration.

The Identification of Ferns The fern frond does all of these things, but it is equally important as a spore-bearing organ, and for this reason, in botanical circles, spore-bearing fronds are termed sporophylls.

But there are certain groups of ferns which have separate fronds devoted to spore production, and others which are barren. This fact at once enables us to place a fern within a limited range of genera, for instance, Blechnum, Lomaria, Matteuccia, Onoclea, and Osmunda, also Botrichium and Ophioglossum.

Having established the type of rootstock, or rhizome, having noted the colour and shape of any scales thereon, the frond is examined. First the general shape of the frond is considered, and the relative length of stem to rachis — or stem to blade, as the rachis is the midrib of the blade. In some ferns the stem is quite short, in others it may be three times the length of the blade, depending somewhat on environment.

Then we should note the presence or absence of scales on stem and rachis; their shape and colour can also be an important means of separating two closely allied species, such as Dryopteris dilatata and D. Carthusian.

The next point to consider is the shape of the blade, and this means the outline, irrespective of the degree of elaboration. The blade may be simple or compound: simple when it is not divided into segments. Very few ferns have simple fronds : the Hartstongue, Phyllitis scolopendrium, and the New Zealand ferns Pyrrhosia and Grammitis are the only ones I mention in this site.

The varieties of Hartstongue depart from the simple frond when they become crested in various ways, but even so they are easily separated from the ferns which normally have compound fronds.

The shape of a frond may be linear, when it is very slender, several times longer than it is broad, tapering evenly to base and apex, being broadest about mid-frond. A slight modification of the linear frond is ‘oblong-linear’, when the greater length of the blade is parallel sided, as in the Hartstongue. The blade may be lanceolate or lance-shaped, in which the broadest part of the blade is about a third from the base, tapering evenly to the apex, and to the basal pinnae. A form intermediate between the linear and lanceolate shape is described as `linearlanceolate.

Then the blade may be more or less egg-shaped, in which case it is described as ovate when the broader part is nearer the base, or obovate when the narrower part is nearer the base. When we come to a blade which is widest at the base, tapering evenly to the apex, it is termed triangular, narrowly or broadly triangular as the case may be.

The deltoid frond, or blade, is a special case of the triangular shape, one in which the base and sides are about the same length, forming an equilateral triangle. Remember it is the overall shape which is being noted, however much the blade is divided within the triangle.

When we consider the compound blade, then is the time to get a cold compress to work, for the descriptions of the degree of segmentation of the blade involve the use of several terms with a strong family likeness, and it requires some effort to remember which is which.

a. The first degree of segmentation of a compound blade is termed `pinnatifid’. This means that the blade is deeply cut like a feather but the cleavage does not reach the midrib, the lobes between the clefts being termed segments. For example the common Polypody is pinnatifid, as the segments are continuous with one another, not stalked The Rustyback, Ceterach officinarum, is another example of the pinnatifid blade.

Having established this the outline of the segments should be considered, whether linear, lanceolate, triangular or oblong.

The apex of the blade and that of the segments may be blunt, acute, truncate or cut off square, or acuminate — gradually diminishing to a somewhat extended point.

b. The next degree of segmentation is termed ‘pinnate’ and in this case the blade is cut right down to the midrib or rachis. The segments now are termed pinnae (sing. Pinna) and they may be stalked or not stalked — sessile.

The pinnae themselves are not divided in any way. A good example of the pinnate blade is provided by the Hard Fern, Blechnum spicant, which can be found in most parts of the country where the soil is not calcareous.

c. The Common Male Fern provides us with a good example of the next degree of segmentation, because the pinnae themselves are deeply cleft, but not pinnate, ie. they are not divided down to their midribs — they are pinnatifid.

The whole frond is then described as `bipinnatifid’, or pinnate in the primary divisions, pinnatifid in the secondary divisions. Or the description might read: blade pinnate, pinnatifid

Again the shape of the pinnae and that of the segments into which they are cleft should be noted, and the type of apex.

d. When we come to the next degree, the pinnae themselves are pinnate, cleft to their midribs, and now the divisions are termed pinnules.

The pinnules may be stalked or sessile; their shape and tips should be noted.

Such a blade is termed `bipinnate’, pinnate in the primary divisions and pinnate in the secondary divisions. The soft Shield Fern in the original wild type is a good example of a fern having bipinnate fronds, and in this case the shape of the pinnules with their basal lobes is important, being characteristic of the genus Polystichum in general.

e. When the pinnules themselves are divided to their midribs, they then are pinnate, the pinna as a whole is bipinnate, and the whole frond is `tripinnate’, a characteristic of the divisilobum section of Polystichum setiferum. Dryopteris dilatata, the Broad Buckler Fern, is tripinnate in the lower part of the frond, becoming bipinnate towards the apex; the degree of segmentation usually diminishes towards the apex of a frond.

Such fronds are described as `tripinnate’, generally, perhaps with qualifications.

f. The greatest degree of segmentation is found in the plumose divisilobes of Polystichum setiferum. In these wonderful ferns which appear so delicate and yet are absolutely hardy the pinnules are bipinnate; therefore the whole pinna is tripinnate and the whole frond is `quadripinnate’.

Again a frond may be quadripinnate below, becoming tripinnate higher up, bipinnate at the apex.

 

Having established the shape and segmentation of the frond, there are still further details to make out.

First let us consider the edge or margin of the blade and of the pinna. This may be smooth and unbroken in any way and then is said to be `entire’. The margin may be lobed or rather deeply indented without sharp projections, or it may be waved or sinuate.

If the margin is fringed with fine hairs it is said to be ‘ciliate’, if the hairs are coarse or bristly it is said to be `setigerous’.

Then again the margin may be toothed in some way, and the degrees are as follows: rounded, mucronate or scalloped, serrate or finely toothed, dentate or deeply toothed. Sometimes the teeth have smaller teeth upon them; then the margin is said to be `biserrate’.

The surfaces of the blade, upper and lower, may be of different shades of green; they may be smooth or glabrous, hairy or hirsute, and they may be peppered with glands as in Dryopteris aemula. These glands are tiny objects, requiring a lens to be made visible.

Next the fertile fronds are examined to ascertain the shape of the indusium, if any. Indusia may be linear as in Asplenium, kidney-shaped or reniform as in Dryopteris, circular and peltate-shield-like, attached by their centres as in Polystichum, domed as in Cystopteris, saucer-shaped with fringed edges as in Woodsia, two-valved as in Hymenophyllum, and so on.

Or the indusium may be rudimentary, fleeting, disappearing before the sporangia are ripe as in Thelypteris, or missing altogether as in Gymocarpium and Polypodium. The edge of the indusium may be entire, fringed, or glandular.

Next the sorus demands our attention. This may be linear as in the Hartstongue and Asplenium, roughly circular, comparatively large or small. The colour may be black as in Asplenium, brown as in Athyrium, or yellow as in Polypodium vulgare.

The spores themselves may be green as in Osmunda, black in Asplenium, light brown in Athyrium, very dark brown in Polystichum, yellow in Polypodium. The spores often have characteristic sculpturing of their outer skin, but as this observation cannot be made without a microscope it need not concern us here, especially as the easily visible factors suffice for determination.

The shape and method of dehiscence of the sporangia also offer a minor point in identification. However, the position of the sorus relative to veins and pinna margin should be noted.

The venation, or veining of the pinnae, is always described in a full botanical description, but again is not essential for our purpose. Mention might be made of the Woodwardias which show examples of reticulate or netted venation highly characteristic of that genus. Islands of tissue surrounded by veins are termed ‘areoles’ and these can be seen alongside the pinna midribs in all species of Woodwardia. The majority of ferns have other types of venation.

Not to be confused with ‘venation’ is `vernation’, which describes the way in which the fronds are folded in the young state, and the way in which they unfurl. This character is to be observed only in the spring. All British ferns except the Moonwort and Adderstongue — Botrichium and Ophioglossum — have the same vernation, ‘circinate’, in which the fronds are enfolded like a crozier, the rachis coiled up like a watch-spring, the unfurling of which, like so many violin scrolls, is so attractive in early spring.

 

Schedule of Observations for Identification of Ferns

  1. Size of mature plant
  2. Type of rootstock or rhizome
  3. Colour and shape of scales if any on z
  4. Relative length of stem to rachis
  5. Presence or absence of scales on stem and rachis
  6. Shape and colour of scales
  7. Frond. All potentially fertile
  8. Frond. Fertile fronds different to barren ones
  9. Shape of blade
  10. Blade simple or compound
  11. Type of segmentation
  12. Shape of segments
  13. Type of margin
  14. Type of apex of blade
  15. Type of apex to pinna
  16. Average number of pinnae to frond
  17. Shape of indusium and kind of margin
  18. Indusium absent, or rudimentary
  19. Shape of sorus
  20. Position of sorus relative to veins and margin
  21. Colour of sorus and spores
  22. Frond surface, smooth or hairy, colour, texture
  23. Venation
  24. Vernation

 

16. May 2011 by Dave Pinkney
Categories: Ferns | Tags: , , | Comments Off on The Identification of Ferns

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