The centre for the usual autumn weekend meeting, held on 26 and 27 October, was the University of Bristol.
Papers were read on the Saturday morning and afternoon, about forty-five members and guests attending. The President, Dr E. F. Warburg, introduced the speakers; summaries of the six papers are given below.
Dr R. S. Clymo: ‘Some aspects of the physiology and ecology of Sphagna’.
The substantial ion exchange properties of Sphagnum were demonstrated experimentally. In both living and dead Sphagna, the capacity for cation exchange is probably dependent on mixed sugar-uronic acid molecules, unesterified polyuronic acid making up an appreciable proportion of the dry weight of the moss. An approximate description of the cation behaviour of Sphagna can be given in terms of Donnan theory, although there are some anomalies.
By the use of parameters deduced from Donnan theory, and from measurements of plant growth rates (obtained in several ways), rainfall rates and the cation concentration of rainfall, calculations may be made of the pH to be expected in a Sphagnum bog. Such calculations show good agreement with observed values of pH, and it is also possible to account for various other features of pH variation in carpets of Sphagnum.
Dr A. J. Willis: ‘Studies on the physiology of Tortula ruraliformis’.
The work which was described in this paper is published in full on pp.668-83 of this volume.
Mr P.J.Wanstall: ‘The growth and structure of carpets of Polytrichum formosum and P. juniperinum’.
The mode of growth of mature P. formosum by rhizome-like shoots was first described. It was then shown, from the results of contiguous cropped quadrats (1 cm. x 10 cm.) at right angles to the edge of the moss carpet, how, passing from the margin inwards, not only the average height of the shoots increased to a maximum but also the density. Towards the centre of the carpet the height and density of the shoots became irregular, so that the situation resembled that shown by Watt in an invading bracken association.
The shoots in carpets of the two species of Polytrichum from different localities and soils in Sussex and Essex were demonstrated to vary in density but generally the taller the shoots the lower the density, as might be expected. The proportions of shoots of different ages in cropped quadrats suggested that shoots of these two species might live for up to four years.
Mr D. T. Streeter: ‘Growth and yield in Acrocladium cuspidatum’.
The annual yield and period of maximum growth of A. cuspidatum are controlled by habitat conditions. Dune slack habitats gave the highest yield recorded, whereas dry chalk grassland produced the lowest annual yield.
The nutrient content of the moss carpet varies widely in different habitats and with the time of year. It is higher when the moss is growing under scrub than when in open habitats; there is a significant positive correlation between the monthly rainfall and the potassium content of A. cuspidatum in these conditions.
Removal of the moss carpet from the mown droves of Wicken Fen results in the loss of 1·13 kg./ha. sodium, 6·60 kg./ha. potassium, 4·28 kg./ha. calcium, 0·43 kg./ ha. phosphorus and 5·87 kg./ha. of nitrogen to the habitat. This is of importance when considering the implications of moss-gathering from the point of view of conservation.
Dr K. Benson-Evans: ‘Some aspects of the physiology of reproduction in bryophytes’.
Light duration effects on the initiation of the sexual state have been shown experimentally in gametophytes of four species of the Marchantiales, six of the Jungermanniales and one of the Anthocerotales and Sphagnales. In the first two orders, long days favour the formation of sexual organs and govern the number of sexual branches initiated; increased light intensity hastens the response. Application of auxins can induce structures similar to gametangiophores in Marchantia, Preissia and Conocephalum, but no sex organs develop and high auxin levels inhibit gametangial production on normal plants.
The importance of using cultures of known history was stressed in view of the apparent photoperiodic after-effects recorded for plants grown from gemmae. Anthoceros laevis and Sphagnum plumulosum become fertile in short days (8 hr.), remaining sterile in long days (18 hr.). Cryptothallus mirabilis and Lunularia cruciata have a low-temperature requirement that must be fulfilled prior to the stimulus of a temperature rise of 10-20° F. Regenerated growth from frost-damaged thalli of Lunularia, a native of the Mediterranean, will not give rise to gametangia when transferred to experimental chambers, whereas in Conocephalum conicum, a native of Britain, fertile gametangiophores can be induced on regenerative growth within two weeks in L.D. chambers at 70° F.
No evidence is yet available of light duration effects on gametangial initiation in mosses, but increased temperature may be stimulatory.
Mr P. D. Coker: ‘Ecology of epiphytic bryophytes in Hertfordshire’.
Studies on the epiphytes of the main oak-hornbeam woods of the county were illustrated by colour transparencies. Elder was found to support the highest number of species and next were rotten stumps of oak; fewest epiphytes were on oak and hornbeam. These differences may be attributed to the high water capacity of elder bark and of rotten wood as compared with that of the impermeable bark of oak and hornbeam.
The prevalence of propagules in ‘rain-track’ areas of all the trees was noted, and it was tentatively proposed that the reason for their almost complete failure to establish on oak and hornbeam was because of toxic substances in the bark. The structure of the bark was considered of great importance in this connexion.
The absence of Tetraphis and Zygodon was striking, the epiphyte flora corresponding to the Bryeto-Aulocomnietum association observed by Barkman in Holland. Species involved in the primary succession on elder were generally Aulacomnium androgynum, Bryum capillare and Dicranoweissia cirrata; these were eventually superseded by the more vigorous secondary succession of pleurocarpous mosses. The paucity of the epiphytes in the county was attributed to drainage and the effects of atmospheric pollution.
Dr E. F. Warburg thanked the speakers for their papers, which stimulated lively discussions.
During the evening a conversazione was held in the Department of Botany, by kind permission of Prof. E. W. Yemm.
The exhibits displayed were as follows:
|Miss R. J. MURPHY and Mrs J. A. PATON:||‘Lophocolea semiteres (Lehm.) Mitt. and Telaranea sp.; established on the Isles of Scilly’.|
|Mrs J. A. PATON:||‘Amblystegium saxatile Schimp.; new to the British Isles’.|
|Dr M. C. F. PROCTOR:||‘An Association-Analysis of the Census Catalogue of British Hepatics and colour transparencies of bryophytes’.|
|Mr E. C. WALLACE:||‘Photographs of bryologists from the album founded by the late Secretary, Mr A. Thompson’.|
|Dr E. F. WARBURG and A. R. PERRY:||‘Weissia levieri in Britain’ and ‘Mosses not recently seen in Britain’.|
Also displayed were papers of the late Miss E. Armitage. These papers, as well as her herbarium, are held by the Department of Botany, University of Bristol, and include personal correspondence, papers relating to bryophytes of the Azores, Madeira and Canary Islands, and her notebook containing records of Herefordshire hepatics.
On the Sunday, the field meeting in Leigh Woods was attended by about twenty-five members and guests. In fine weather, more than 110 bryophytes were listed in this old woodland which fringes the Somerset side of the Avon Gorge, an area of Carboniferous limestone well-known for its rare flowering plants.
The habitats visited included the shaded Nightingale valley with its rocky slopes, the towpath area along the Avon and the quarries which fringe the towpath. Fallen logs in Nightingale valley yielded Nowellia curvifolia, Ptilidium pulcherrimum and Dicranum montanum*. On the rocky areas were seen Cirriphyllum crassinervium, Fissidens cristatus, Isopterygium depressum, Isothecium striatulum and Rhynchostegiella pumila. Of interest was the find by Dr Warburg of Fissidens minutulus var. tenuifolius* in several sites.
[* = New v.c. record]
At least six species of Barbula were recorded along the towpath, and Encalypta vulgaris on the adjoining rocks. The quarries were examined fairly closely. Here were noted Scapania aspera, Solenostoma triste (with perianths), Bryum donianum, Campylium protensum, Distichium capillaceum*, Grimmia orbicularis and Gymnostomum calcareum. In one of the quarries a form of Tortella tortuosa with narrow leaves was collected.
Thanks are again due to Mr S. W. Greene, who arranged the programme of papers, and also to the Department of Botany for providing refreshments.
A. J. Willis