A set of artificial keys is presented to all those crustose lichens (except most species of Lepraria and Leprolomd) which commonly occur without fruiting bodies on rock or soil in the British Isles. A number of predominantly corticolous species that occasionally also occur on rocks are included, as well as several undescribed but well-understood entities.

Strigula macaronesica Sèrus., a new foliicolous species found in Madeira and Gomera, is distinguished by its almost linear thallus growing along leaf margins or scars and by its unusually small obclavate asci.

The monotypic genus Botryolepraria Canals et al. is described. The type species, B. lesdainii (Hue) Canals et al. is a sterile, cottony, pale green lichen, made of hyphae supporting subterminal bunches of chlorophycean algae.

The lichen vegetation of forty-five acid watercourses in England is described and related to ecological variables. The zonation scheme adopted for chalk and limestone streams that involves four overlapping bands of lichen vegetation related to length of submergence is also appropriate for this habitat. The flora involves around 90 species, a number of which are rarely recorded specialists. Evidence is presented that substratum chemistry is a more powerful ecological factor than water chemistry. An attempt has been made to relate the lichen flora to homogeneous stream segments from the headwaters down to the alluvial lowlands. An investigation into the effects of water quality showed that both suspended solids (silting) and eutrophication are highly deleterious to the lichen flora; the submerged and fluvial mesic zones are the first to be affected. The zonation across streams in England is compared with those reported from Germany and found to be closely similar.

Cladina subtenuis establishes and expands by mechanisms of the thallus mycobiont. Vegetative reproduction occurs through thallus fragmentation, branch ‘ de-differentiation ’, and the extension of the underground prothallus. Establishment hyphae can be considered as any hyphae that provide the mechanism for these modes of clonal thallus extension. They may originate in various parts of the thallus, laterally on branch surfaces, at the growing tips of branches, or from the inner fungal layers of the podetium. The thallus of C. subtenuis is structurally and functionally plastic. Parts of the photosynthetic lichen thallus can transform into nonphotosynthetic fungal aggregations, and non-photosynthetic aggregations may re-establish as a photosynthetic lichen thallus. These activities expand the area occupied by the clone, and potentially increase its photosynthetic surface.

The pustules on the surface of the umbilicate lichen Lasallia pustulata are moved away from the centre and towards the thallus margins with an average speed of 1.8 mm per year, revealing a growth pattern organized radially and flowing from the centre of the thallus. Pustules close to the margin move substantially faster than those close to the centre; for each 10 mm added to the initial distance from the centre, the speed of outward movement is about doubled. This suggests that intercalary growth over the entire thallus is displaced outwards in an accumulating manner, accelerating the pustules. Moving away from the centre the pustules expand, often collapse in the middle, and mature by developing isidia. In the less active (senescent ?) margins the pustules are eroded away and the supplementary intercalary growth seems insufficient to keep the outflowing thallus intact. Thus the margins are disrupted into irregular lobes.