Meeting report

Bryological symposium

A weekend meeting was held on 26-27 October in the Department of Botany, University College of South Wales and Monmouthshire, Cardiff, by kind permission of Professor G. F. Asprey. On the Saturday the President, Mr J. H. G. Peterken, F.L.S., took the chair, and the following papers were read, summaries of which are given below.

Dr O.L. Gilbert: ‘Bryophytes and atmospheric pollution.’

For a long time botanists have been aware that many common bryophytes are absent from a large area of the Lower Tyne valley. A brief literature and herbarium survey has indicated that their decline has taken place over the last 130 years. A thorough investigation into the distribution of these plants has revealed that not all species are equally affected and that various environmental factors such as shelter, pH, availability of nutrients, growth form of species, seral stage of community, water relations of the habitat, etc., can act to considerably increase or alleviate the harmful influence. (Constantly wet, high pH sites produce maximum alleviation and allow the survival of sensitive species in the centre of Newcastle.) The harmful influence was rapidly identified as air pollution by means of transplant experiments, chemical analysis and visits to other parts of the country. The effect of grit, sulphur dioxide, smoke, fluorine, vehicle exhaust and metal particles were investigated and it is now certain that sulphur dioxide is the most harmful pollutant in the air over Tyneside. The toxic limit of Grimmia pulvinata growing on sandstone walls has been calculated and it is found to be absent from this habitat in areas where the annual average sulphur dioxide concentration exceeds ca. 45 µg./m.³. Experimental work to explain how sulphur dioxide toxicity can be modified by the environment has revealed that the degree of ionization of sulphurous acid greatly affects its toxicity and that in thalli exposed to low concentrations (ca. 250 µg./m.³.) of sulphur dioxide, chlorophyll levels are affected long before respiration starts to fall. This must not, however, be taken as evidence that chlorophyll is the first system in the plant to be disrupted, as certain fungi are nearly as sensitive as green plants.

Mr G. Clarke: ‘Growth studies in bryophytes.’

The net annual productivity of Pohlia wahlenbergii var. glacialis on the sub-Antarctic island of South Georgia was measured using a coring technique for sampling. The dry weight of the current season’s growth gave a net annual productivity ranging from 479 to 849 g./m.²/year at different sites. The presence of running water at a site appeared to be correlated with the higher figures. Values for the Relative Growth Rate, obtained both in situ in the field and in pot culture experiments, were also greater at sites where running water was present. Measurements of the length of the current season’s growth showed that during the second half of the season the rate of increase of stem length was less than that of the dry weight per unit area. It was found that the weight per unit area could be correlated with the length if the closeness of packing of the stems was taken into account. Length measurements at various sites on Disko Island, West Greenland, indicated net annual productivity values of 665 to 1517 g./m.²/ year. The average productivity of Pohlia on Disko (latitude 69° N.) is evidently greater than that on South Georgia (latitude 54° S.). This is borne out by experiments with flowering plant seedlings and can be correlated with climatological measurements.

Dr N. G. Bayfield: ‘Polytrichum commune – a hardy perennial.’

Polytrichum commune is a perennial moss with potentially unlimited apical growth. The tussock habit can be maintained over long periods on the same site by a cycle of growth, collapse and regrowth. Individual shoots frequently continue growth through more than two complete cycles. Shoot density varies throughout the cycle with collapse usually taking place when the stems are both tall and relatively widely spaced. Wind-tunnel studies showed that risk of desiccation increased as density declined. Below a density of about 0·45 stems/cm.² desiccation risk increased very rapidly. This may explain field observations that stem elongation rates generally showed a decline with density. At low densities stems appeared to have little mutual shelter or support. Water conduction in this species was discussed and water-potential curves compared with those for species lacking a well-developed internal conduction system.

Dr J.G.Hughes: ‘Sexual and apogamous races of Phascum cuspidatum.’

Apogamous plants of P. cuspidatum may be obtained from seta-cuttings. They are green plants, at first protonemal then with erect radially symmetrical microphyllous shoots, becoming leafless above with a terminal cauline sporangium. The protonemata are of the caulonema type, resembling the grosschloronema of Tetraphis pellucida under some cultural conditions. The leafy shoot has short internodes and small leaves compared with the sexual plant. The leaves are squarrose. Rhizoids are rarely formed. Branching of the leafy shoot is sympodial. The terminal leafless stem may be much branched, with branches often in whorls. The leafless meristems have an apical cell with two cutting faces, in contrast with the tetrahedral apical cell of the typical sporogonium, and the two histogens (endothecium and amphithecium) are lacking in the apogamous plant. When sporangia are formed they lack the ring of five stomata found on the sporogonium. Both sexual and apogamous races are summer annuals. In the seasonal variation in leaf number there is evidence that the sexual plants are adapted in favour of late developers. In neither race is leaf number conditioned by light intensity. Only in the apogamous race was leaf number reduced in daylight by the use of coloured filters, especially red. The frequency of the vegetative development of sporangia in the apogamous plants was low in daylight. Reduction of leaf number under coloured filters was not accompanied by increased development of sporangia. In both races vegetative growth was prolonged in fluorescent light and this was due to its excessive blueness. Under yellow-filtered fluorescent light, the vegetative development of sporangia was much increased. Sexual reproduction was favoured by reduced light intensity and this was due to effects upon the mechanism of antheridial dehiscence and the frequency of fertilization. In a discussion of the low fertility of the apogamous plants of this and other species, it was proposed that the alternation of generations in mosses is based in part on a mechanism which ensures that neither sporophyte nor gametophyte alone can form sporangia.

Dr R.L.Jefferies: ‘Ionic relations of Cephalozia connivens and Leiocolea turbinata.’

The movement of ions into these two liverworts has been examined in relation to the ionic conditions which exist in the habitats where these plants grow. An analysis of measurements of electrical potential differences which exist between plant cells and the external solutions together with estimates of the concentration of ions in the plants suggests that at low external concentrations (0·1 mM) of potassium this ion is pumped into the tissues of these plants. The influx of potassium into plants of both species is sensitive to the inhibitor C.C.C.P. (1 x 10^-6 M) and is markedly influenced by the calcium concentration and the hydrogen ion concentration in the external solution. Maximum influx of potassium into plants of Cephalozia connivens occurs when the calcium concentration of the external solution is 0·1 mM and the pH is 4·0. These conditions are similar to those that exist in the habitat where this plant grows. Corresponding conditions for plants of Leiocolea turbinata are where the pH of the external solution is 4·0-8·0 and the calcium concentration is 3·0 m M. These results are in close agreement with field data from habitats in which this plant occurs.

Mr D. C. Sigee: ‘Some observations on the fine structure of Cryptothallus mirabilis.’

Electron-microscope observation of the fine structure of the apical cell and its surrounding cells revealed the presence of normal proplastids. In apical regions of the thallus well back from the apical cell, and in wall cells of the archegonia, the plastids showed no tendency towards chloroplast differentiation. It was considered that the fine structure of these plastids was consistent with the presence of carotenoids and the absence of chlorophyll in the organelles. Preliminary investigation of the egg cell indicated a stage in which the periphery of the cytoplasm was highly vacuolated, with large amounts of endoplasmic reticulum, and with discrete masses of ribosomes (resembling nucleoli). The egg cell has an additional (possibly fluid) inner wall layer, which contains particles and organelles derived from the cytoplasm, and which may serve to isolate the development of the egg cell from its surrounding (gametophyte) cells.

Dr J. H. Dickson: ‘Scanning reflexion electron microscopy of bryophyte spores with special reference to Polytrichum.’

The scanning reflexion electron microscope is an excellent tool for revealing the details of the sculpturing of bryophyte spores. Taxonomic potentialities were illustrated by reference to Polytrichum. In this genus every species examined thoroughly so far is determinable by spore characters.

Conversazione

After discussion and questions to speakers the President thanked all who had contributed and Dr Dickson for arranging the programme of papers.

Field excursion

On the Sunday a field excursion was held. In the morning a visit was made to Morlais Hill and Morlais Glen, an area of carboniferous limestone near Merthyr Tydfil, and bryologically one of the richest districts in the county. Unfortunately, heavy rain made field work difficult and the intended visit in the afternoon to the Pennant Sandstone scarp of Craig-y-Llyn had to be abandoned. About 100 species were found amongst which may be noted: Oligotrichum hercynicum, Diphyscium foliosum, Dichodontium pellucidum, Ditrichum heteromallum, Pottia heimii, Barbula hornschuchiana, Gymnostomum aeruginosum, Pohlia elongata, Bryum pallens, Thuidium philibertii, Campylium chrysophyllum, Cirriphyllum piliferum, C. crassinervium, Preissia quadrata, Riccia sorocarpa, Leiocolea badensis, Solenostoma triste, Cololejeunea calcarea and Frullania germana.

A. E. Wade
Kathryn Benson-Evans