The major works on British lichens published between 1724 and 1780 are briefly reviewed. The new lichens published in Withering's Botanical Arrangement of 1776 have been mostly overlooked; they were based on species described by Dillenius. These lichens are typified here, along with Withering's new lichens published in later editions of the Arrangement. Altogether 41 species are reviewed. The following 16 new combinations based on Withering's species are presented, with synonyms in parenthesis: Anaptychia runcinata (A.fusca), Caloplaca crenularia (C. festiva), Qadonia humilis (C. conista, C. conistea, C. conoidea), Qadoniapeziziformis (C. capitata), Cladonia ramulosa (C. anomaea, C. pityrea), Collema auriforme (C. auriculatum), Collema dichotomum (C. fluviatile), Collema fuscovirens (C. tuniforme), Dermatocarpon luridum (D. aqualicum, D. weberi), Leptogium gelatinosum (L. sinuatum), Lobaria virens (L. laetevirens), Peltigera didactyla (P. spuria), Peltigera lactucifolia (P. hymenina), Physconia distorta (P. pulverulacea), Ramalina lacera (if. duriaei), and Tornabea scutellifera (T. atlantica).

Later names affected by Withering's taxa are also investigated, and 12 are shown to be nomenclaturally superfluous. Other names are later homonyms, with the result that seven species require change of author citation as follows: Dactylina madreporiformis (Ach.) Tuck., Lecanora crenulata Hook., Psora lurida (Ach.) DC, Ramalina polymorpha (Liljeblad) Ach., Solorina spongiosa (Ach.) Anzi, Stereocaulon ramulosum Rauschel, and Sticta fuliginosa (Hoffm.) Ach.

The nomenclatures and systematic position of Cladonia conista C. Robb. ex Allen nom. illeg., Cliostomum griffuhii (Sm.) Coppins, Leptogium palmatum Mont., Lichen squamatus Dickson, Pachyphiale cornea (Ach.) Poetsch, and Pycnothelia papillaria Dufour is discussed, the correct name for Leptogium palmatum being L. corniculatum (Hoffm.) Minks and for Pachyphiale cornea being P. carneola (Ach.) Arnold. A history and account of Withering's herbarium is included.

Notes and keys are provided for British members of the Trapeliaceae. The genus Placynthiella Gyelnik is resurrected to accommodate the Lecidea uliginosa group: P. hyporhoda (Th. Fr.) comb, nov., P. icmalea (Ach.) comb, nov., P. oligotropha (Laundon) comb. nov. and P. uliginosa (Schrader) comb. nov. Aphanopsis Nyl. ex P. Sydow is recognized for the single species A. coenosa (Ach.) comb, nov., and attention is drawn to the enigmatic Biatora humida Kullhem. The Lecidea granulosa group is referred to Trapeliopsis, necessitating four new combinations, T. aeneofusca (Flörke ex Flotow) comb, nov., T. flexuosa (Fr.) comb, nov., T. gelatinosa (Flörke) comb. nov. and T. viridescens (Schrader) comb, nov., and the description of one new species, T. pseudogranulosa sp. nov. Special attention is given to sorediate species of Trapelia, including T. corticola sp. nov. and T. placodioides sp. nov.

The new species Placynthium arachnoideum Henssen is described from Patagonia. The lichen is characterized by incurved lobes retaining in part the false branching of the Scytonema phycobiont, and by two-celled, thick-walled spores with a perforated septum. Some localities in the Southern Hemisphere are given for P. asperellum, P. nigrum and P. subradiatum.

The rate of lobe division during thallus enlargement in Xanthoria elegans and Diploicia canescens is considered in geometric terms. The frequency of lobe engulfment by neighbouring lobes was measured, was fitted graphically to a simple equation and was found to result from constant lobe division with thallus size. Values for a constant in the equation and derived from the observed frequency of lobe engulfment provided a measurement of a new morphological parameter: ratio lobe length to width. Greatest departure from circularity in small thalli of D. canescens occurred when thallus size did not permit lobe competition. Increasing lobe engulfment and thallus circularity in larger thalli appeared to be correlated. Circularity in crustose thalli without lobes is discussed with definitions of ‘primary’ and ‘secondary’ areoles.

Cyanobacteria symbiotic with 17 members of the Lichinaceae and Heppiaceae were brought into unialgal culture and their free-living characteristics examined. A diversity of cyanobacterial types was observed; this diversity was much greater than that reported in the literature. Both the in situ identification of cyanobionts and their role as an aid to the identification of these lichens are questioned.

The sensitivity of lichen species to air pollution was assessed by examining aspects of their rate of disappearance over a 5 year period in an area with rising SO2 concentration. These rates of disappearance were compared with the ecological index of species proposed by LeBlanc & DeSloover in 1970, i.e. the index of average number of neighbours for a given species. Linear correlation showed that this ecological index is a reasonably accurate indicator of species sensitivity. When the selection of species was performed according to criteria other than SO2 sensitivity, viz. according to the predilection of lichens for eutrophicated bark, the ecological index proved reliable only when trees whose conditions were non-optimal for this group were excluded. When applied to the group of species that avoids eutrophicated bark the ecological index proved most reliable, although the group of ‘ nitrophilous ’ species may also be used in air pollution indicator studies.

In the Athabasca Oil Sands area of Alberta, a study was carried out to test a lichen community transplant technique and to determine the influence of oil sands extraction and processing emissions on lichen cover of transplanted communities. Measurement errors of a photographic technique for the determination of lichen cover were dependent upon lichen species but were not correlated to lichen cover. When lichen covers were small therefore, relative errors were very large. Changes in cover of naturally occurring lichen communities on black spruce branches were not significant over a four-year period. Lichen communities transplanted under jack pine and white spruce trees had cover changes with time not different from naturally occurring communities even over a period of years.

Significant (P<0.001) cover reductions of some lichen groups occurred on transplanted branches under white spruce within 8–3 km of a pollution source. Evernia, Cetraria, and Bryoria groups were reduced whereas Hypogymnia showed no response over a 3 year measurement period. Reductions in lichen cover were greater than the demonstrated precision of the technique.