134
The Essex Naturalist
[Seven other taxa of the Plagiotheciaceae occur in Britain, P. piliferum, P. platyphyllum, P. cavifolium, P.
denticulatum var. obtusifolium, Herzogiella striatella, Isopterygium pulchellum and Isopterygiopsis
muelleranum. All however are montane species unlikely to occur in the south east]
TIPS FOR FIELD IDENTIFICATION
Identification of the common species of the Plagiotheciaceae in the field can be achieved by a combination of
familiarity with the 'giz' of the plants and careful use of a x20 handlens for looking at leaf cells and auricles.
Once met, the robust shoots of P. undulatum can usually be recognized from a standing position, but the other
species often require a lens for certain identification. The yellowish clusters of deciduous 'twiglet-like'
propagules can usually be found on Isopterygium elegans shoots - even if only one or two are present - and the
narrowness of the leaf cells can be appreciated if observed by transmitted light with a x20 lens, when a shoot
is held up against a bright sky. Similarly, the narrow 8-10um cells of the P. curvifolium / laetum aggregate can
be judged with a x20 handlens once one gets a feel for the size, resembling at this magnification a series of
closely ruled parallel lines.
Once known, P. curvifolium can usually be recognised from its 'giz', the down-curved leaves giving it a
distinctive appearance when moist, the shoots being rather pale green and scruffy when dry. P. laetum is a
smaller, more delicate plant, forming almost smooth, very glossy patches, with rather short shoots that snap
off easily. The seta of P. curvifolium is said to be reddish, becoming brownish, whereas that of P. laetum is
said to be reddish at the base, becoming yellowish. The immature capsule of P. curvifolium is usually curved
and inclined on the seta, and a darker green than the capsule of P. laetum, which is straight and erect to slightly
inclined on the seta (Nyholm). P. laetum is rare and sporadic in south east England.
If the cells are obviously appreciably wider, when observed under a lens, then look for the distinctive abruptly
rounded auricles of the P. denticulatum/ruthei aggregate, which can be clearly seen in the field under x20, if
a few leaves are removed from a shoot with deft finger nails, and a still attached isolated leaf is looked at
against the sky. The outer row of rounded cells looking at this magnification like a cluster of bubbles. The
closely related P. ruthei, usually found in wet habitats, is distinctive in having paper-thin very flattened shoots,
most leaves undulate, and with one straight side. P. denticulatum is more exacting in its requirements than P.
curvifolium and is absent from strongly acid soils.
If, instead of forming distinct auricles, the decurrent leaf bases are narrowly tapering with the outer row of cells
rectangular in shape, then the P. nemorale/succulentum aggregate is indicated. Looking at the mid-leaf cells
against the sky, obvious regular rows of elongated hexagonal cells running across the leaf, (giving an
interference pattern of parallel bands even at xlO lens magnification), will usually distinguish P. nemorale,
which also tends to have a shorter, broader leaf, with a more abruptly tapered apex when moist, and wider cells
which shrink markedly to give a narrower leaf on drying. The cells of P. succulentum tend to be narrower when
moist, and overlap irregularly without exhibiting the storied appearance of P. nemorale. The two taxa are very
close to each other however, and in the south east some specimens appear to be intermediate and are therefore
best recorded as the aggregate. The colour and texture of the plants, golden-green or dark-green, glossy or dull,
appear to be habitat related, rather than species specific, and are unreliable in the south east (c.f. Smith).
P. latebricola is a very local and sporadic species. Look for it on the shuttlecocks of ferns in damp valleys, on
the dead stools of tussock sedges and on the bases of alder trees in alder carr. It can also occur on stumps and
tree bases in quite dry situations. Its distinctive, slender, semi-upright shoots, finely-pointed tapering leaves,
with very narrow cells, short single nerve, and almost invariably-present, leaf-tip fusiform gemmae, should
enable it to be suspected in the field. Taxiphyllum wissgrillii is a plant of the chalk and chalky boulder clay,
occuring on tree bases and flints in well-drained shaded woodland. The short acute apex, and fine denticulations
of at least the upper half of the leaf, can be seen with a x20 handlens against the sky. Herzogiella seligeri is
probably an introduced species. Virtually confined to south east England, it is most commonly found on rotting
stumps and logs of sweet chestnut, persisting for some time on the completely rotted remains. Its abruptly
tapering leaf apex with denticulations almost to the base of the leaf should be distinctive with a x20 lens. If
found in south east England the last three species should always be checked with a microscope, and, ideally,
vouchers kept.
Most species of the Plagiotheciaceae are probably somewhat sensitive to sulphur dioxide. Isopterygium elegans
however, is highly resistant, and is abundant on damp sloping banks in such places as Epping Forest where