On the very stormy weekend of 26-27 October, 17 BBS members got together at Cardiff to examine sectioning techniques, and the taxonomic significance of sections, in bryophytes. Tom Blockeel produced and led the programme which covered topics at the cutting-edge of bryological research. Roy Perry was local organizer and had arranged facilities in the School of Pure and Applied Biology, University of Wales, Cardiff, by kind permission of Prof. John Fry. Sean Edwards had provided some of the presentational material on sectioning techniques. We were joined by two museum staff, Victoria Purewal, Conservator, and Kathryn Cliffe. In the Centenary year it was fitting for those unfamiliar with the BBSUK herbarium (and indeed the herbarium of NMW), to have the opportunity during the weekend to inspect it deep within the catacombs of the museum.
Saturday 26 September
We began the session by considering some leaf-section terminology, such as guide cells, hydroids and stereids, and hearing of the value of leaf sections in separating some recently described species from their near relatives, on the basis of leaf stratosity. The various methods that can be used to section bryophytes were then discussed. These ranged from the microtome (but who has one of these?) through wax embedding (too messy and time consuming) and elder pith or carrot (too much detritus gets on slide) to free-hand sectioning.
There is no doubt that free-hand sectioning of leaves gives quick and excellent results, but each individual may have their own preferred method that works for them. The method advocated at the workshop was to take some wetted leaves (no more than 4 or 5 normally suffice) and lay these side by side in a row on a microscope slide, and place over them another slide to hold them firmly in position, and at the same time provide a cutting-edge guide. By doing this it is easy to control where on the leaf one makes the section. A double edged razor blade (preferably broken in half) is then drawn at an angle across the leaves using the upper slide as a guide. The blade can be tilted incrementally as the leaf is sliced, giving several near-perfect sections. It is critical that sharp blades are used; a corner of a blade may be used for sectioning something up to four specimens, by which time it will have become blunt (but is still adequate for cutting stiffer tissue such as stems).
We then moved on to examine the question of Cinclidotus riparius in Britain, and had the opportunity to try out the method outlined above. This plant has always been of doubtful status in Britain, being very difficult to separate from C. fontinaloides in the absence of sporophytes (putative C. riparius has never been found with sporophytes in Britain). Recent work on the species, though, has suggested that there may be a good distinguishing character in the cross section of the thickened leaf margin. We were given continental C. riparius (with exserted capsules) and vouchers of British C. fontinaloides and putative British C. riparius to section and compare. To the surprise of everyone, specimens of putative C. riparius, all from the River Teme, matched continental C. riparius in terms of leaf margin anatomy. The margin in cross section of C. riparius seems to have more or less uniform isodiametric cells, whilst C. fontinaloides is quite different in having a distinct inner group of heavily incrassate, smaller stereid-like cells running through the centre of the thickened margin. By the end of the morning many people were converts to the sectioning method, but we also recognized that the Cinclidotus issue needs to be followed up. After lunch we had a session on Racomitrium and Grimmia, two genera that have always, and justifiably so, been regarded as difficult. There have also been many recent publications on these plants (Bednarek-Ochyra, 1995; Frisvoll, 1983, 1988; Greven, 1995; Maier & Geissler, 1995) and much emphasis has been placed on leaf sections.
Racomitrium. We had a quick run through some taxonomic characters that are relevant in distinguishing Racomitrium species:
- Papillosity: the British species fall into three groups, those with conical papillae (R. canescens agg.), those with smooth leaf cells (R. ellipticum and R. heterostichum agg.), and those with flat plate-like papillae which give a ‘cobblestone’ effect to the leaf surface when viewed in section. The plate-like papillae are particularly useful in identifying depauperate specimens of R. fasciculare and in separating R. aquaticum from muticous forms of R. heterostichum.
- Supra-alar cells: these are the marginal cells at the leaf base. In some species, especially the non-British R. microcarpum and to some extent also R. sudeticum, they form a pellucid band. Their shape is important in separating R. ericoides from R. elongatum.
- Leaf section profile: the profile of the leaf in section is determined by the width of the nerve, and is important in separating species of the R. heterostichum agg. Species with a wide nerve have a broadly channelled (canaliculate) leaf; those with a narrow nerve have a keeled leaf.
- Nerve section: the thickness of the nerve (in terms of the number of cell strata) is particularly important in the R. heterostichum aggregate.
- Outer perichaetial leaves: these are reflexed in R. himalayanum, erect-spreading in other species.
- Inner perichaetial leaves: these constitute a well-defined character for separating some similar species. In R. heterostichum and R. affine they are strongly differentiated, with hyaline esinuose cells. In R. sudeticum and R. macounii they are only moderately differentiated from the vegetative leaves, and the upper cells have sinuose walls.
Grimmia. With Grimmia it was noted that the distinction from Schistidium can be very subtle, but Schistidium generally has a deciduous columella that is lost with the capsule lid, it has a small calyptra not extending below the lid, the nerve cells in section are usually homogeneous and the plants often have a reddish tinge to them. A list of key characters of Grimmia was then presented:
- Insertion of lamina onto nerve: three general types were recognized. Some species (e.g. G. montana) have the lamina, as seen in section, arising vertically from the nerve. Consequently these species have a deep narrow slit running along the ventral surface of the nerve, and the leaf easily splits longitudinally under a coverslip. A second type is found in G. laevigata. Here the lamina is inserted sideways onto the nerve and the leaf is rounded in section. The third and most common type has the lamina attached obliquely and the leaf is therefore V-shaped or keeled in section. It was noted that G. ovalis and G. affinis, two much confused species, are readily separated by leaf sections (these being rounded and V-shaped respectively).
- Stratosity: though widely used in keys, the stratosity of the leaf lamina must be used with caution. Some normally unistratose species (e.g. G. trichophylla) are sometimes partly bistratose near the leaf apex. Other species (e.g G. donniana, G. sessitana) are not always uniformly bistratose.
- Hair points: in species with short hair-points (e.g. G. hartmanii) the cell lumen is usually readily discernible. However, examination of herbarium specimens suggests that this character may not always be clear-cut.
- Nerve section: species of Grimmia usually show some differentiation of the cells in the nerve section, hydroids or stereids often being present.
- Basal leaf cells: the shape of the basal cells (i.e. short or elongate) is a critical character in some species, and the marginal cells are often differentiated from the cells near the nerve. However in some species (e.g. G. donniana) this differentiation is weak. The marginal cells often have thickened cross-walls. G. laevigata is distinctive in having basal marginal cells that are incrassate and often wider than long.
- Cells above leaf base: these are often strongly incrassate. However, in G. incurva they are nodulose (irregularly thickened) rather than sinuose.
- Upper leaf cells: species with distinctive upper leaf cells include G. decipiens (cells elongate) and G. elatior (cells with rounded papillae).
- Gemmae: these are known in G. hartmanii (on the apices of the upper leaves) and in G. trichophylla and G. torquata (on the dorsal surface of the leaf at the base).
- Presence of innovations: these are distinctive in the interior of G. funalis tufts, having diminutive shell-like leaves and appearing cord-like.
- Leaf posture: some species have curled or twisted leaves when dry, but this is not always a reliable character. G. incurva fo. brevifolia and many forms of G. funalis have straight leaves.
It was noted that some dioecious species of Grimmia have dimorphic male and female plants.
Keys to Racomitrium and Grimmia were handed out, and BBS herbarium material made available to practise techniques upon and test the keys, or we had some of our own material that had always resisted naming.
As a postscript to Grimmia, we looked briefly at the recent revision of Scandinavian Schistidium species by Hans Blom (1996). Blom’s treatment has still to be applied to British material, but only twelve of the 31 Scandinavian species of the S. apocarpum complex are known to occur here at present. A key to the twelve British species, extracted from Blom’s book, was distributed.
On Saturday evening we were the guests of NMW, and honoured to be joined by Dr Eurwyn Wiliam, Assistant Director, and his wife. An excellent buffet with wine was laid on for us, and we thank Deb Spillards for her hard work. Then we were shown the Botany Department by research assistant George Hutchinson and Victoria Purewal. One of the museum galleries was opened specially for us but our only slight disappointment was that the giant bugs (‘Megabugs’) would not perform for us – apparently the air compressor was out of action.
Sunday 27 September
As something of a departure from the usual workshops of the past, it was decided not to have a field excursion but to continue the session on the second day in the lab. This had nothing whatsoever to do with the weather, but was more of an acknowledgement, made well in advance, that we would get much more out of the weekend by concentrating on methodologies and having recourse to expert tuition. We thus turned our attention to pleurocarpous mosses and the value of stem sections for separating a number of easily confused species.
The terms hyalodermis (large thin walled cells surrounding the stem) and central strand (small group of elongate cells running through the middle of a stem axis) were defined. Although the hyalodermis is readily observed in section, it is often collapsed or eroded and may therefore be overlooked on casual inspection. Its usefulness was illustrated in the case of the often difficult to separate Hygrohypnum ochraceum (hyalodermis present) and Hygrohypnum luridum (hyalodermis absent). Special attention however was given to Drepanocladus revolvens agg. and D. vernicosus. By making stem sections D. vernicosus is usually easily separated from D. revolvens agg.: it does not have a hyalodermis whilst the latter aggregate does. It is now recognized that we have two good taxa within D. revolvens aggregate: D. revolvens sensu stricto and D. cossonii. With some experience typical plants of these two plants can be distinguished by their jizz, but the best way to separate them is as below (see also Hedenäs, 1989):
D. revolvens s.s…… mid-leaf cells long (60-140m), tapered at the ends; autoecious
D. cossonii…………. mid-leaf cells short (20-95m), often with transverse blunt or oblique end walls; dioecious
Finally we briefly considered the small members of Hypnum. The presence or absence of a hyalodermis can be used to separate two Sections of the genus, Hamulosa (with hyalodermis) and Revolutohypnum (lacking hyalodermis). A plant that some people saw at Morrone in Scotland during the BBS summer meeting this year, pronounced in the field to be Hypnum hamulosum, turns out upon sectioning not to have a hyalodermis, so cannot be H. hamulosum. It was hoped that similar plants might be found in the BBS Herbarium, and one such plant was. The latter plant turned out to be not a Hypnum but a form of Ctenidium molluscum. Subsequent examination has shown that the Morrone plant is also Ctenidium, a confusing form with poorly differentiated stem leaves.
Herbarium material of Drepanocladus and Hypnum were made available for us to section. This transpired to be quite a revelation, since quite a lot of material purporting to be one thing (including new county record vouchers) was in fact something else. Mixed gatherings were also found to be a trap for the unwary!
Everyone was unanimous that this was an immensely interesting meeting, and our appreciation was duly given to both Tom and Roy. We are also most grateful to University of Wales, Cardiff and NMW for such a delightful setting in which to hold the weekend.
Blom HH. 1996. A revision of the Schistidium apocarpum complex in Norway and Sweden. Bryophytorum Bibliotheca 49: 1-333. Stuttgart: J. Cramer.
Bednarek-Ochyra H. 1995. The genus Racomitrium (Musci, Grimmiaceae) in Poland: taxonomy, ecology, and phytogeography. Fragmenta Floristica et Geobotanica Series Polonica 2: 3-307.
Frisvoll AA. 1983. A taxonomic revision of the Racomitrium canescens group (Bryophyta, Grimmiales). Gunneria 41: 1-181.
Frisvoll AA. 1988. A taxonomic revision of the Racomitrium heterostichum group (Bryophyta, Grimmiales) in N. and C. America, N. Africa, Europe and Asia. Gunneria 59: 1-289.
Greven HC. 1995. Grimmia Hedw. (Grimmiaceae, Musci) in Europe. 160 pp. Leiden: Backhuys Publishers.
Hedenäs L. 1989. The genera Scorpidium and Hamatocaulis, gen. nov., in northern Europe. Lindbergia 15: 8-36.
Maier E, Geissler P. 1995. Grimmia in Mitteleuropa: ein Bestimmungsschlüssel. Herzogia 11: 1-80.
R.D. Porley and T.L. Blockeel