Spreading Ferns
What are they?
Ferns are primitive plants that evolved before the flowering plants and reproduce sexually by means of spores rather than seeds. This page covers the few species of ferns that spread out by means of underground rhizomes to form extensive mats or colonies of single leaves that emerge direct from the ground and do not form tight clumps of vegetation.
Where are they found?
Colonial, spreading ferns are found in a variety of habitats and the habitat can help to indicate the species involved. Wetlands, woodlands, hedgebanks, dry heaths and walls may all hold spreading fern species and the favoured habitats are listed below under each species.
Identification
Bracken and Marsh Fern are easily told by their appearance and their habiiat choice, but the polypodies are extremely difficult to tell apart without resorting to the use of high-powered magnifying lenses. For ease of describing the various parts, a couple of technical terms are used; the side arms that form the first division of a leaf are called pinnae (singular, pinna), while the secondary divisions of the pinnae are called pinnules, which make up the smallest parts of each leaf.
Bracken Pteridium aquilinum
Native. An abundant plant of both shady and sunny places, being most common on peaty, acid heathlands, where it may carpet large areas of ground. Bracken spreads readily by underground rhizomes which push up fresh, bright green leaves in great quantity in the spring and which then die down in the autumn, the dead leaves turning cinnamon-brown and remaining through the winter. Easily told from all other ferns by its spreading habit, coupled with the two- to three-metre long leaves, which start as a single stalk, but then branch into three main sections. Ripe spores appear in autumn and are covered by inrolled leaf margins.
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Intermediate Polypody Polypodium interjectum
Native. Widespread and by far the commonest of the two polypody species in East Anglia, being found in large colonies on shady hedgebanks, ditch sides, shady churchyards and commonly on old walls, where it is more tolerant of limey mortar than is Common Polypody. Ripe spores present in late autumn or early winter. The leaf shape tends to be broadest about the middle and tapers toward each end and the pinnae are somewhat pointed at the tip. Under high magnification, the spore cases can be seen to consist of two halves, held together by a ribbed 'strap'. In Intermediate Polypody, this strap has on average 7-9 (extreme 4-13) ribs or bands, while Common Polypody has on average 10-14 (extreme 7-17) ribs.
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and ribbed straps |
Manton's Polypody Polypodium x mantoniae
Native. Probably under-recorded but could occur in shady places or on walls throughout much of the region. The leaf is rather variable in appearance and may often be rather uneven or distorted in overall shape, but they often resemble those of Intermediate Polypody in being widest about half way along the length of the leaf. Under high magnification, the spore cases can be seen to consist of two halves, held together by a ribbed 'strap'; this strap has on average 9-10 ribs or bands. Manton's Polypody is a hybrid between Common and Intermediate Polypody and is sterile, so no yellow or orange spores will be found and the sori will be seen to be empty when viewed under high magnification.
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and ribbed straps |
Common Polypody Polypodium vulgare
Native. Despite its name, this is by far the least common of the two species in East Anglia, being found most often on sandy, acidic soils and especially on coastal sand dunes. Ripe spores present in late autumn or early winter. The leaf shape tends to be more or less evenly wide throughout much of its length, tapering only toward each end and the pinnae are rather blunt at the tip. Under high magnification, the spore cases can be seen to consist of two halves, held together by a ribbed 'strap'. In Intermediate Polypody, this strap has on average 7-9 (extreme 4-13) ribs or bands, while Common Polypody has on average 10-14 (extreme 7-17) ribs.
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and ribbed straps |
Marsh Fern Thelypteris palustris
Native. Frequent to common in parts of Broadland but rare elsewhere in peaty wetlands. Spore-bearing fronds may be found from late May to August. This species differs from all our other wetland ferns in that it has creeping rhizomes that do not form discrete clumps or tussocks of leaves but form spreading mats. A relatively low species, with leaves to around 80cm in length. The spores differ from those of the buckler ferns (which may be found in the same habitat) in that they appear in lines around the outer edges of the pinnules, not in clusters at the centre.
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