Martin Godfrey

The Ferns – an Identification Guide: Part 4

Polystichum setiferum (Martin Godfrey)

Polystichum species

We have again had a rather odd year although it has been simpler to get out to more places to see more plants. I have decided to keep this article fairly straightforward and introduce the Shield-ferns, genus Polystichum which, for two of the species, is common but which can be confusing for the novice.

Introduction

There are three species of Polystichum found in Britain – the common and abundant P. setiferum Soft Shield-fern and P. aculeatum Hard Shield-fern, and the rather rare P. lonchitis Holly-fern. The first two are large and grow in the classic “shuttlecock” form of many ferns, while P. lonchitis is smaller and tends to have a rather untidy growth form amongst the rocks where it usually grows. Because of their growth form and obviously “ferny” appearance, the first two species may be mistaken for Male-ferns, Dryopteris, by the beginner. A good way of telling them apart is to look at the indusia on the underside of the pinnae – in Polystichum they are circular (Fig. 1), while in Dryopteris they are kidney shaped (Fig. 2). All three Polystichum species have fronds with a rather stiff leathery texture and long points on the edges of their pinnae. However, P. setiferum (Soft Shield-fern) is, as its common name suggests, rather less stiff and the points have a less “prickly” feeling. P. setiferum and P. aculeatum both have bipinnate fronds while those of P. lonchitis are once-pinnate.

Polystichum setiferum

This species is common in base-rich woodlands in Shropshire where is can form quite abundant stands. Superficially it looks rather like P. aculeatum, but as mentioned above, the frond has a rather less leathery and stiff feel to it. There are also two good reliable distinguishing features: The first is to look at the base of the frond; the basal pinnae are more or less the same length as those above, giving the frond a truncated look, and below this the stipe is quite long and narrow and not heavily scaly (Fig. 3). The second is to examine the structure of the

Martin Godfrey

Fig. 1 Polystichum sori (Martin Godfrey)

Fig. 2 Dryopteris sori (Martin Godfrey)

Fig. 3 P. setiferum (Martin Godfrey)

Fig. 4 P. setiferum (Martin Godfrey)

pinnules – these look a bit like little mittens with a thumb-like lobe held close to the stem (Fig. 4).

Polystichum aculeatum

Like P. setiferum this species grows in base-rich woodlands but also occurs on somewhat damper ground and around rock crevices. Again, it is abundant in Shropshire and can be found together with P. setiferum. As well as being more prickly and leathery to touch, it has the following two features to distinguish it: Look at the base of the frond – the lower pinnae are much shorter than those further up the frond giving it a tapering appearance. The stipe is shorter, thicker and more scaly than in P. setiferum (Fig. 5). The pinnules are slimmer, lack the “mitten and thumb” impression, and are held at a distinct acute angle to the stem (Fig. 6).

Polystichum lonchitis

P. lonchitis is a rare upland species of the north and west. It isn’t found in Shropshire but it is quite abundant where it does grow in Snowdonia, Cumbria and Northern Scotland. Its habitat is amongst calcareous rocks, in cracks and under ledges, and it is common in grykes (fissures in limestone pavement) where it is protected from sheep grazing. There is occasionally a bit of habitat overlap with P. aculeatum amongst rocks but its once-pinnate form and very prickly lamina make it unmistakable (Fig. 7 and Fig. 8)

Fig. 6 P. aculeatum (Martin Godfrey)

Fig. 7 P. lonchitis habitat (Martin Godfrey)

Fig. 8 P. lonchitis frond showing once-pinnate structure and prickly edge (Martin Godfrey)