Ellerslie Hall at Leeds University was the scene of a well-attended meeting where the results of some notable contemporary study were presented. It was a particularly appropriate venue in view of the significant bryophyte research carried out in the Department of Genetics at Leeds itself.
Three papers from that source were concerned with the latest discoveries of developmental processes in Physcomitrella patens, considering physiological aspects of auxin transport as well as a monitoring of morphological changes using the novel method of time-lapse microscopic photography. Those papers were well complemented by others spanning a range of topics from field studies to electron microscopical work. Thus, one speaker put forward a strong and persuasive argument in favour of urgent attention to bryophyte conservation, whereas another unravelled the fascinating historical record, as seen in peat profiles, of the fluctuating fortunes of mosses in an area of the southern Pennines. There was also a significant ecological slant to a third paper, which called for a more critical stance in interpreting chemical analyses, paying particular attention to development factors. That patterns of development hold the key to other problems too, was shown by a speaker who, in dealing with the ultrastructure of liverwort oil bodies, opened up a new area of basic research as well as of Phylogenetic comparison. These papers combined to establish the present state and future promise of bryology which, in Britain, is built on the work of many bryologists including that of W.E. Nicholson, whose biographical details were presented by a further speaker. The papers are summarized below.
Dr N.W. Ashton (University of Regina, Canada): “The role of medium acidification and auxin transport in the morphogenesis of Physcomitrella patens.”
The following observations of the influence of certain environmental factors of gametophytic development in Physcomitrella patens have led to elaboration and testing of a model which can account for these observations.
- The morphology of P. patens grown in red light (RL) on agar medium, containing nitrate as sole nitrogen source and having a pH of approximately 6.8, resembles that of gametophytes cultured under white light (WL) on medium containing a low concentration of the synthetic auxin, naphth-lylacetic acid (NAA)
- In RL on medium with a pH of approximately 5.3, the appearance of the moss is similar to that of cultures grown in WL on medium supplemented with a higher concentration of NAA.
Key elements of my model which seeks to explain these findings are:
- P. patens gametophytes produce endogenous auxin(s) which is transported into the growth medium and which, subsequently, may be reabsorbed by the moss cells.
- In WL, the endogenous auxin is degraded while it is in the agar medium. This, in turn, leads to a reduction of the level of auxin within the moss cells.
- In RL, the endogenous auxin remains undegraded and accumulates in both culture medium and moss tissues.
- The accumulation of auxin by moss cells from their environment is driven by a pH gradient across the plasma membrane (PM), the pH inside being higher than the pH outside. This gradient may be generated by the cells extruding protons into their environment resulting in medium acidification or the gradient can be established by experimental manipulation of medium pH.
Some of the more important evidence supporting the model is as follows:
- P. patens gametophytic tissues grown in WL contain a low level of the endogenous auxin, indol-3ylacetic acid (IAA) (Ashton et al., 1985).
- UV spectroscopy studies have established that IAA is rapidy degraded in aqueous solution when exposed to WL. In RL, no degradation occurs even after prolonged incubation for more than three weeks (Ashton, unpublished data).
- The results of experiments in which P. patens has been cultured by the drip-feeding of liquid media suggest that endogenous auxin(s) can be released into the growth medium (Cove et al., 1980).
- Studies on the ability of P. patens to accumulate 14C- IAA and 3H-IAA have revealed that accumulation is highly pH- dependent, being greatest when the pH of the incubation medium is low. The data obtained also strongly support the existence of two kinds of auxin carrier in the PM, one, probably an IAA anion/H + symport, responsible for influx of auxin and the other, an IAA anion uniport, responsible for auxin efflux (Ashton & Bridge, unpublished data).
- P. patens gametophytic tissues are capable of both short and long term medium acidification (Ashton, unpublished data), suggesting that moss cells are capable of pumping out protons and, therefore, also of generating a pH gradient across their PMs.
Ashton, N.W., Schulze, A., Hall, P. & Bandurski, R.S. (1985). Estimation of indole- 3-acetic acid in gametophytes of the moss, Physcomitrella patens. Planta 164, 142-144.
Cove, D.J., Ashton, N.W., Featherstone, D.R. & WangT.L. (1980). In D.R. Davies & D.A. Hopwood (eds.), The Proceedings of the 4th John Innes Symposium, 1979, pp. 231-241.
Dr D.H. Brown (University of Bristol): “What can bryophyte mineral analyses really tell us?”
The ways in which chemical analyses of bryophytes have been successfully used as a method of biomonitoring to show patterns of heavy metal distribution in aquatic and terrestrial species at both local and regional scales were briefly described. Reasons why bryophyte analyses cannot, however, be used quantitatively to predict levels of elements in the environment or their biological consequences were discussed. These emphasised our lack of methods to distinguish accurately between metals present in the plant in theform of a) insoluble particles, b) chemically bound to exchange sites on the cell wall or c) biologically incorporated into the cell interior. The use of a sequential elution technique providing information on the latter two sites was discussed using examples taken from studies on Rhytidiadelphus squarrosus. Changes in the amounts and proportions of elements at extracellular and intracellular sites at different positions along the gametophore were considered. It was shown that retranslocation occurred from older regions to the growing apex and from extracellular sites into the cell. The need for careful selection of material of uniform age and a suitable weight basis on which to b ase concentration calculations was emphasised. Comparison between two control populations, showing differences in photosynthetic sensitivity to added cadmium, was related to the age structure and chemical composition of their apical segments and not to inherent differences in heavy metal tolerance.
Prof. J.G. Duckett (Queen Mary College, London) and Prof. K.S. Renzaglia East Tennessee State University, U.S.A.): “Transmission electron microscopy of oil bodies in hepatics: games with systematics and cytochemistry.”
Although widely used in taxonomy the origin and development of oil bodies in Hepaticae are poorly understood. Similarly their possible functions (repellance to herbivores, protection against cold, desiccation or high irradiance) remain in the realms of speculation untried by experiment save for the discovery made by virtually every bryologist that they rapidly disappear in the dried herbarium specimens. This dearth of knowledge about development and functions is reflected in definitions no more precise than “distinctive bodies always easily recognisable by their refractive index which differs from that of the surrounding cell content” (Schuster, 1966, p. 202).
Transmission electron microscopy (TEM) is an approach to oil bodies, which might reasonably be expected to provide new insights on their ontogeny and functions, both with obvious implications for Systematics. Inexplicably this is an area of Bryology marked by extreme neglect: developmental studies are limited to but one genus (Marchantia) and data on the ultrastructure of mature oil bodies to a mere handful of taxa (see Duckett, 1986, for review). However, these few published micrographs underline the unique nature of hepatic oil bodies. They are cytoplasmic bodies bounded by a single unit membrane and contain numerous lipid droplets in a proteinaceous matrix. This distinctive compound ultrastructure (so clearly different from the simple lipid droplets, lacking a limiting membrane, which are frequently encountered in the cytoplasm of all land plants) should be widely assimilated as a feature setting the Hepaticae apart from Musci and Anthocerotae, if not all othe r archegoniates. Whether or not structures akin to oil bodies also occur in the cells of particular groups of algae has yet to be explored.
Whereas oil body ultrastructure is a major diagnostic feature of Hepaticae as a whole, its potential at lower levels in the taxonomic hierarchy is at present impossible to assess. Satisfactory preservation of oil bodies for TEM is very difficult to achieve using standard glutaraldehyde-osmium fixation procedures. From our own observations on Riccardia, Aneura and Cryptothallus we confirm the findings of Galatis et al.(1978b) that redistribution of lipids from oil bodies onto the ER and plasma membrane and around the mitochondria and plastids during primary fixation in aldehydes is a recurrent problem. Further lipids may be lost during dehydration resulting in the apparently “empty” appearance of mature oil bodies. The dispersion of oil films on the boat surface during fine sectioning indicates mobility of the lipids even in resin- embedded specimens. Until the problem of lipid retention is solved we have no way of knowing if differences in oil body ultrastructure between taxa are real or merely a reflection of how much of their contents have been lost during preparation.
The meticulous developmental studies of Galatis et al.(1978a, b) reveal that, in Marchantia, oil body formation is a highly complex process directly involving Golgi bodies and coated vesicles, and intimately associated with ER, microbodies and a system of cytoplasmic tubules. Lipophilic materials accumulate exclusively within the oil body and lipid droplets in the cytoplasm never enter the same. Our discovery of ATPase activity associated with the oil body membrane in Cryptothallus is in line with the conclusion of Galatis et al. that oil bodies are an active cell compartment. Other cytochemical tests on Aneura, Riccardia, and Cryptothallus provide further insight into the nature of the endomembrane domains involved in oil body ontogeny.
Not surprisingly specific reagents for the plasma membrane (phosphotungstic-chromic acid and sodium silicotungstate) fail to stain the oil body membrane. In the Thiery procedure for non-cellulosic carbohydrates reaction products are limited to the cell wall and Golgi vesicles. Acid phosphatase activity is restricted to the Golgi bodies.
Pescreta & Lucas (1984, 1985), have recently demonstrated the existence of a “partially- coated-reticulum” in charalian algae and angiosperms. This endo-membrane system, possibly analogous to the endosome of animal cells, is intimately associated with both microbodies and lipid droplets and exhibits staining patterns different from both the Golgi and ER. Our cytochemical tests on the Riccardiaceae appear to preclude direct participation of the Golgi bodies in oil body development. Yet coated vesicles and microbodies are clearly involved in the same process in Marchantia. Taken together these results suggest that oil body ontogeny may involve a highly distinctive endomembrane system like the partially-coated-reticulum but hitherto unrecognised in the Hepaticae.
Duckett, J.G. (1986). Ultrastructure in bryophyte systematics and evolution: an evaluation. J. Bryol. 14, 25-42.
Galatis, B., Apostolakos, P. & Katsaros, C. (1978a). Ultrastructural studies on the oil bodies of Marchantia paleacea Bert. I. Early stages of oil-body cell differentiation: origination of the oil body. Can.J.Bot. 56, 2252-67.
Galatis, B., Katsaros, C. & Apostolakos, P. (1978b). Ultrastructural studies on the oil bodies of Marchantia paleacea Bert. II. Advanced stages of oil-body cell differentiation: synthesis of lipophilic material. Can. J.Bot. 56, 2268-85.
Pescreta, C. Th. & Lucas, W. J. (1984). The plasma membrane coat and a coated vesicle- associated reticulum of membranes: their structures and possible interrelationship in Chara corallina. J.Cell Biol. 98, 1537-45.
Pescreta, C. Th. & Lucas, W. J. (1985). Presence of a partially-coated reticulum in angiosperms. Protoplasma 125, 173-84.
Schuster, R. M. (1966). The Hepaticae and Antherocerotae of North America East of the 100th Meridian. Volume 1. Columbia University Press.
Miss C.D. Knight and Prof. D.J. Cove (University of Leeds): “Time-lapse microscopy of the gravitropic response of Physcomitrella patens.”
A moss protonemal filament grows by the elongation and division of its apical cell and the direction of growth is influenced by environmental stimuli. In the absence of light, certain filaments of the moss Physcomitrella patens respond to gravity by bending away from the gravitational vector. This represents a discrete developmental pathway.
We are using time-lapse microscopy, as part of a programme of study, to observe the timing and pattern of bending and to formulate a model for this developmental response. Observation of mutants altered in this response may reveal critical stages of the pathway.
A method of cell culture has been devised for the video recording of filaments under conditions necessary for a gravitropic response (Cove & Knight, in press). Important aspects of this method are:-
- The microscope lies horizontally so that the gravistimulated filaments grow in a plane perpendicular to the light source without affecting focus.
- The cultures are exposed to infra-red light; this does not appear to interfere with the gravitropic response which normally requires darkness. These wavelengths are detected by an infra- red sensitive camera.
- Cultures grow in a thin layer of agar, irrigated by medium pumped from the reservoir of a specially constructed culture chamber.
Using this method cultures have been maintained, without contamination, for up to 5 days.
Trends have emerged from recordings of the wild type response. The ability of filaments to bend away from gravity appears to be associated with stages of the cell cycle. More specifically, during nuclear division negatively gravitropic bending does not occur and those cells in the process of bending at the start of mitosis may undergo a temporary reverse bend. One explanation for this may be that bending requires an intact cytoskeleton and experiments are underway to test the effect of cytoskeletal drugs on this response.
The analysis of mutants in fine detail has revealed that one mutant, thought to be agravitropic, in fact responds by bending 10-15° away from gravity when reorientated 90° from gravity. The reason for this is unclear but since results from other mutants suggested that members of this class were altered in their perception of gravity, it is clear from this data that this may not be the case. Special attention is being given to the apparent sedimentation of cell organelles in this mutant on gravistimulation. Sedimentation of statoliths is proposed as a mechanism for graviperception in other organisms but has not been observed in the wild type response of Physcomitrella patens. It is possible that this strain may contain multiple mutations. One of these mutations might lead to the observed sedimentation of cell organelles and it is possible that this may be connected to the bending observed.
The advantages of this technique are manifold. It is hoped that, in addition to the detailed analyses of wild type and mutant responses, data obtained from the application of growth inhibitory or enhancing substances to cultures, will add to knowledge of the gravitropic response.
Cove, D.J. & Knight, C.D. (in press). In Thomas & Grierson (eds.), Developmental Mutants in Higher Plants. Cambridge University Press.
Mr D.J. McClelland (University of Leeds): “Protonemal branching patterns in Physcomitrella patens“.
Regular patterns of branching arising from caulonemata of the moss Physcomitrella patens are observed in cultures grown on agar based nutrient medium, at 25°C under bright white light. Radiating caulonemal filaments arise from primary chloronemal tissue inoculated onto the centre of a petri-dish of medium. Each cell of a caulonemal filament sequentially gives rise to a lateral side branch. The majority of side branches develop into secondary chloronemata, but some branches give rise to caulonemal filaments, and a few become buds, which grow out as gametophores. Caulonemal filaments elongate by division and extension of the apical cells, a process which occurs every 5½-6 hours, and branches arise from cells in the 2nd or 3rd sub- apical position. Therefore a single caulonemal filament observed at any particular instant represents a “snap-shot” of a dynamic system: an elongating caulonemata giving rise to lateral branches which, moving back sequentially from the apex, represent stages in development of branch initials following a number of alternate fates. Analysis of the branching observed in a population of filaments may provide insight into the mechanisms by which development and cellular differentiation are controlled in mosses.
Taking the apical cell as a reference position, the type of side branch arising from cells at each sub-apical position up to 30 cells back from the tip was noted. Twenty to thirty filaments in each culture were recorded in this way. The results obtained provide a statistical representation of the occurrence of branches of each type arising from cells in the 30 most apical positions in a population of filaments. To standardise the analysis, cultures were observed 21 days after inoculation; however, the branching patterns appear to be established after 15 days growth, and very little change occurs after this time. Assuming a constant cell division rate for caulonemal growth, cell position can be equated with a measure of time, metered in units of 5½-6 hrs: the time in which apical cell division occurs.
No branches are produced on cells nearer the apex than the second sub-apical position; however, at the 4th sub-apical position 97% of the cells have given rise to a side-branch initial (SBI). Not all caulonemal cells produce branches: a small proportion of cells (1%) at positions more than 20 cells back from the apex do not give rise to SBI’s. 85-90% of SBI’s develop into chloronemata, and the maximum level is obtained by the 6th sub-apical position. 5-6% of SBI’s give rise to caulonemal branches similar to the filaments from which they are branching; however, the maximum level of caulonemal branch production occurs further from the apex than the position at which maximal chloronemal branching is observed, i.e. on older cells. Buds arise from SBI’s at a frequency of less than 1% at particular cell positions; however, no more than 1 bud forms on each filament. The positions at which buds form range from the 12th sub-apical position (no buds form nearer the apex than this position) up to the 30th cell position, the extreme of this analysis. The differentiation of buds appears to occur later than that of other branch types, i.e. arising from initials on older cells. By the 17th sub-apical position along caulonemal filaments the differentiation of branch types appears to be fixed. In considering a population of filaments this shows the branching pattern to be in “steady-state”, i.e. the branches have become irreversibly committed to developmental fates. Thus no change in this pattern of branch ing is observed at positions further back from the apex than the 17th cell.
The pattern of branching described is characteristic of wild-type cultures grown under standard conditions; however, significant changes in the proportions of branch types produced occur when conditions are altered, or growth substances (e.g. auxin or cytokinin) are added to the substrate. Phosphate-starvation specifically inhibits the development of chloronemal branches. Tissue inoculated onto phosphate-free medium produces radial caulonemal filaments, as on phosphate-containing medium; however, there is a reduction in the number of branches produced: throughout the length of the filament (not counting the 3 most apical cell positions) the number of unbranched cells increases from 1% in the presence of phosphate to 10% in its absence. No chloronemal branches are produced in the absence of phosphate: SBI’s which would have given rise to secondary chloronemata are halted in growth and development at an early stage and persist as single cells. Branches of caulonemal cells arise at the same position, relative to the apex, in both the presence and absence of phosphate, but whereas caulonemal branching reaches a maximum level around the 12th cell position in the presence of phosphate, in its absence the number of caulonemal branches continues to increase at positions more distant from the apex, throughout the whole region analysed. This increase is accompanied by a corresponding decrease in the number of SBI’s present at those positions. It therefore appears that initials which might otherwise have given rise to chloronemata in the presence of phosphate, but are unable to do so in its absence, alter their developmental commitment under conditions of phosphate-starvation, and give rise to caulonemata. The production of buds is also reduced in the absence of phosphate.
Exogenously supplied growth factors such as auxin and cytokinin can also affect the branching pattern. Developmental roles for auxins and cytokinins in mosses have previously been established. In these experiments auxin (NAA) and cytokinin (BAP) were added directly to cultures, giving a final medium concentration of 1µM. The branch types arising at each cell position were analysed 5 days after the application of growth factors. The effect of auxin was to increase the level of caulonemata branches from 6% to 20%, whilst reducing chloronemal branching by a similar amount. However the production of buds was unaffected by auxin application. Treatment with 1µM NAA also causes a “shift” of the pattern of branching away from the apex by 1 cell position, i.e. the production and out-growth of SBI’s, giving rise to branches of all types is delayed (with respect to the apex) by 1 apical cell division cycle. This delay is not due to a change in the caulonemal growth rate, which is unaffected by the treatment. Cultures grown on phosphate-free medium show greater sensitivity to 1µM NAA: when treated with auxin in the absence of phosphate new SBI’s appear to have an equal probability of remaining as initials or giving rise to a caulonemata.
The effect of application of 1µM BAP is even more marked. In treated culture, almost all new SBI’s give rise to buds rather than chloronemal branches, and more buds are induced to form as growth continues over the 5 day period analysed. 80% of SBI’s develop into buds when treated with 1µM BAP. The number of chloronemata branches is reduced, but the production of caulonemata branches is unaffected by the cytokinin treatment. This suggests the existence of two populations of SBI’s predetermined, to a certain degree, to produce either chloronemal or caulonemal branches, and only the former is susceptible to BAP action in inducing bud formation.
Branching patterns provide a method of quantifying the stages of development as SBI’s differentiate, giving rise to chloronema, caulonema or buds. On solid substrate, filaments are held in position in the medium, thus it is possible to observe a complete cell lineage; in principle it is possible to follow the lineage of any particular cell back to the initial inoculum. It is therefore possible to observe the “context” in which branches of each type arise, with respect to cell lineage or the branch types arising from adjacent cells. Using this form of analysis it is also possible to follow the development of specific branches over a period of time as they differentiate. The effects of exogenously applied growth factors on branches at different stages of development can also be assessed, and morphologically abnormal mutants can be compared to wild- type cultures, as can the responses of mutant strains to exogenous growth substances. Branching pattern analy sis thus provides a technique to investigate aspects of differentiation and morphogenesis in mosses.
Dr P.H. Pitkin (Nature Conservancy Council, Edinburgh): “Bryophytes, the ‘poor relations’ in nature conservation?”
The world’s bryophyte flora is much better represented in Britain than the world’s vascular plant flora. Very few bryophyte species are endemic in Britain, but our flora is remarkable for its high proportion (c. 10% of the mosses, 12% of hepatics) which are very restricted in their distribution in Europe and the rest of the world, and for the many species which occur in Britain at the edge of their range or far distant from their other known occurrences. Many of these species are strongly Atlantic in their British or European distributions.
Afforestation and changes in farming have probably affected bryophytes less than other kinds of wildlife, though the current planting in the far north of Scotland will destroy large areas of Sphagnum-dominated peatland. The felling and underplanting of native broadleaved woodland has all but ceased, but there are several new threats to be added to Ratcliffe’s list of 1968. Epiphytic bryophytes in particular are sensitive to atmospheric pollution, and probably to the much more widespread acidification of rainwater and substrates. There is a new interest in hydro- electricity in Scotland; several new small-scale installations have been proposed which would intercept the flow of streams through gorges. The popularity of downhill skiing has increased in Scotland; planning permission has recently been granted for three new developments and for facilities at two of the already developed sites. Most of these developments are close to areas of fragile Racomitrium</ i> heath on bryophyte-dominated snow-bed communities which are likely to be damaged by construction work, by skiing or trampling.
Collecting by bryologists has probably caused more harm to the rarer bryophytes than any other influence, particularly in well known localities. On Ben Lawers (Payne, 1984) seven very rare species have not been recorded for more than fifty years, and there are no recent records of eighteen other uncommon ones.
The most significant development in conservation since Ratcliffe’s paper has been the statutory protection given to SSSI’s in the 1981 Wildlife and Countryside Act, which also required all SSSI’s to be renotified. Perth and Kinross and Lochaber are two local authority Districts in Scotland with many sites of bryological value. Out of 95 biological SSSI’s in Perth and Kinross the new citations of fifteen specifically mention some bryological interest, but the citations of a further eleven which probably have comparable interest do not. In Lochaber SSSI renotification is not yet complete, but in mid-1987, out of 26 sites renotified, eleven citations mentioned a special bryological interest.
In both Districts the treatment of bryological sites has greatly improved since 1981; in Perth and Kinross there were formerly only two sites for which bryophytes were separately mentioned.
Biological SSSI’s are selected either as examples of particular habitats or for their uncommon or threatened species. The criteria for selecting ‘species sites’ for insects and vascular plants are based on 10km-square distribution maps. Similar criteria are not used for bryophytes, partly because there is no complete Atlas. The Bryophyte Site Register project commissioned from the BBS by the Nature Conservancy Council has begun to review the interest of bryological sites and might be used to determine whether they are adequately represented In SSSI’s. The Register uses a complicated scoring system which is not recognised by NCC for selecting SSSI’s. The progress of the Register has been slow (it is only complete for four English counties), hampered partly by the lack of an Atlas. Work on it has now been suspended and the NCC is funding a two-year project to complete the Atlas.
The cause of bryophyte conservation could be considerably furthered in the following ways
- by the appointment of a lower-plant ecologist in the NCC (expected 1987)
- by the establishment of precise criteria for selecting bryological SSSI’s
- by the mention, where appropriate, of any bryological interest in the citations of SSSI’s
- by a review of the representation of the bryophyte flora in SSSI’s
- by an appreciation among bryologists of the need for site-based records in addition to a 10km- square system
- by bryologists communicating site records and lists to the NCC.
Payne, A.G. (1984). The rarer bryophytes of the Ben Lawers and Meall nan Tarmachan ranges. Unpublished report, The Nature Conservancy Council, Perth, U.K.
Ratcliffe, D.A. (1968). An ecological account of Atlantic bryophytes in the British Isles. New Phytol. 67, 365-439.
Dr P.E. Stanley (Cambridge): ” The life of W.E. Nicholson.”
William Edward Nicholson (1866-1945), a solicitor by profession, worked and lived in Lewes, E. Sussex (VC 14). He contributed greatly to the knowledge of British hepatics and especially the bryophyte flora of Sussex and The Lizard peninsula in Cornwall (VC 1). He travelled widely in Britain and Europe often with his friends H.N. Dixon and H.H. Knight. Like many of his generation he carried on a copious correspondence with bryologists throughout the world.
On retiring he moved to Mullion on the Lizard and on his death was buried at the churchyard at Landewednack, near Lizard Town.
He was President of the BBS during 1929-1930.
His extensive bryophyte herbarium together with his ‘diaries’ (which are accounts of his botanical field trips) are kept at the Botany School, Cambridge University.
Dr J.H. Tallis (University of Manchester): “Missing mosses from Holme Moss.”
Holme Moss is an upland blanket mire in the southern Pennines, with extensive areas of heavily eroded peat. Easily recognisable leaf remains of several species of Sphagnum (including S. imbricatum, S. papillosum and S. cuspidatum) and of Racomitrium lanuginosum occur widely in the upper layers of peat at Holme Moss, though these mosses are totally absent at the present-day. Also common in the upper peat layers are fragments of burnt plant material.
The abundance of remains of Sphagnum and Racomitrium, and also of burnt material, was assessed at different levels in 0.5-m peat cores from eleven sites on Holme Moss. A timescale covering the last 1000 years was constructed for each site on the basis of detailed pollen analyses, so that synchronous levels in all cores could be recognized.
Clear differences in the frequency of Sphagnum and Racomitrium leaves over the last 1000 years were discernible in the cores, and the frequency-patterns were different in cores from sites with eroded peat and with uneroded peat at the present-day. High frequencies of Racomitrium may indicate a lowered water table in the peat. Racomitrium was abundant at all sites in the early Middle Ages (a period of known drier climate), and persisted subsequently at eroded sites until the eighteenth century. At uneroded sites Sphagnum became dominant over large areas by the fifteenth century, and this expansion is interpreted as a response to colder wetter climatic conditions. The absence of renewed Sphagnum growth at this time at eroded sites suggests that peat erosion may have set in them.
The final disappearance of both mosses at Holme Moss coincides with evidence of at least one major fire event in the eighteenth century, when large areas of the mire surface must have been devastated. Air pollution at the same time (as evidenced by obvious soot particles in the uppermost peat layers) must also have contributed to the disappearance of these mosses. Certain features of the morphology of the peat margin at Holme Moss suggest that in addition there may have been an episode of limited peat sliding down the steep slopes bounding the Moss; a newspaper report of a catastrophic cloudburst over Holme Moss in July 1777 supports this conclusion.
The peat erosion currently visible at Holme Moss may thus have originated in a number of different ways and at several different times.
The Annual General Meeting held afterwards (Minutes in Bulletin 50) was followed in the evening by a conversazione during which the demonstrations and posters listed below were on display. There was also a much-appreciated opportunity to visit laboratories in the Department of Genetics to see some of the bryological work in progress. All contributed towards making this a most enjoyable and worthwhile meeting, the success of which was ensured by the enthusiastic and generous arrangements made by Prof. D.J. Cove and his colleagues, to whom we are greatly indebted.
|Anon.:||Margaret Plues and bryology.|
|Anon.:||Stereoscopic photographs and some Scottish bryophytes.|
|T.S. Futers, D.J. McClelland, R. Potter, T.L. Wang and D.J. Cove:||The interaction of light and cytokinin in bud induction in the moss Physcomitrella patens.|
|D.G. Long:||Andreaea blyttii in Scotland
B.B.S. summer meeting 1986.
|M.E. Newton:||Pellia borealis, a liverwort newly discovered in Britain.|
|R. Stevenson:||Unidentified moss from Yugoslavia.v|
|H.L.K. Whitehouse:||Leptobarbula berica and Gyroweisia tenuis growing on calcareous stones.|
Field meeting, Otley, 1986
Rich bryological sites close to Leeds are not easy to find, but the reservoirs of the Washburn Valley near Otley provided an opportunity to look at a temporary kind of habitat which was not familiar to many of the participants. The communities of exposed mud are inevitably somewhat unpredictable in occurrence from year to year, but on this occasion we were not disappointed. The richest site, the first venue of the day, was at Lindley Wood Reservoir. Here Physcomitrium sphaericum was plentiful and attracted much attention, but it was later upstaged by a small purple Riccia, well camouflaged against the dark mud. This proved to be a form of R. huebeneriana, in which development had possibly been arrested by recent unseasonal frosts. Other species at this locality included Riccia sorocarpa, Archidium alternifolium, Pseudephemerum nitidum, Ephemerum serratum var. serratum, Physcomitrella patens, Pohlia camptotrachela, P. bulbifera, Bryum klinggraeffii and B. tenuisetum. The next stop was higher up th e valley at Swinsty and Fewston Reservoirs, but these were less productive. The water level in the former was too high, and the banks of the latter were dry and stoney, although Archidium alternifolium was abundant in places and Philonotis arnellii was also found. A lateral inflow stream, known to harbour Discelium nudum, was found to have been scoured by flood water a few weeks earlier. However Richard Fisk with commendable perseverance took some bits of soil home and was able to find protonema and male plants under the debris.
Late in the afternoon a much reduced party visited Birk Crag near Harrogate, where it was a pleasure to walk upright after the hands and knees work at the reservoirs. This crag is in an area of oak/birch woodland on Millstone Grit and is of interest for the occurrence of a small quantity of Lepidozia cupressina, an Oceanic species now very rare in the Central and South Pennines, but which was probably once more widespread in this type of Pennine habitat. Two small patches were located and a third is known to occur, but the species is only just hanging on in this locality, where pressure from visitors scrambling over the rocks is a real problem. Also seen on and about the crag and in the adjacent woodland were Barbilophozia atlantica, a sterile Kurzia (probably K. trichoclados), Scapania umbrosa, Lejeunea lamacerina, Sphagnum quinquefarium and Heterocladium heteropterum.