The genus Polythelis Clem, has been almost entirely ignored since its description in 1909. It belongs to the Pyrenulaceae and is close to Pyrenula Massal. from which it is distinguished in having a true septum (euseptum) near the base of the ascospores as well as locules (distosepta) identical to those of Pyrenula. The genus includes a single doubtfully lichenized species, Polythelis sexlocularis (Müll. Arg.) Clem. (syn. Microthelia sexlocularis Müll. Arg.), known only from the type material collected on the bark of a species of Croton, probably C. linearis Jacq., in the West Indies or southern Florida prior to 1824.

Species of Physcia reported to have a paraplectenchymatous lower cortex appear to have a dual cortex consisting of a lower paraplectenchymatous layer and an upper periclinally arranged prosoplectenchymatous layer. These species may have a black or pale lower surface but always contain atranorin. They should be retained in the genus Physcia.

Six species of Pyxine have been reported from Europe (including the Azores), but only two, Pyxine sorediata (syn. P. endochrysoides) and P. subcinerea (syn. P. chrysanthoides), are now accepted, the remaining four being excluded for various reasons. Buellia convexa is found to be a synonym of B. pulverulenta.

The Scandinavian distribution of the species of Squamarina is mapped and discussed. Squamarina magnussonii is reported as new to Scandinavia and S. gypsacea and S. pachylepidea as new to Norway. A key to the species is provided and some notes on their morphology, chemistry and ecology are given.

Since seasonal variation in the nitrogenase activity of Stereocaulon paschale is known, the infrastructure of Stigonema in the cephalodia was examined in thalli of S. paschale collected from under snow in late winter, and from bare ground in late autumn. In contrast to the Stigonema cells of the autumn material, those of the winter material were relatively devoid of osmiophilic (lipid) droplets and polyglucoside and cyanophycin granules, reflecting that carbon and nitrogen storage products had been metabolized. In young cephalodia dividing cells were common, while in old cephalodia degenerating cells predominated. Haustoria appeared in material collected in both seasons, but may be limited to degenerating Stigonema cells or to Stigonema cells with a low metabolic activity.

The energy balance processes at a surface are reviewed in relation to the development of extreme temperature profiles that constitute the normal operating environment of a lichen. The specific nature of this operating environment, irrespective of altitude or latitude is emphasized and illustrated with a number of specific examples. These range from the ‘desert’ thermal environment following forest fire in low-arctic spruce-lichen woodland to boundary layer conditions at 2000 m, which is the environment of Rhizocarpon superficiale. The importance of winter temperatures and the ameliorating effects of snow cover and thallus colour are also related to the ecology and thermal environment of arctic and alpine lichens. Finally the thermal regime of corticolous and pendulous epiphytic lichens is discussed, again in terms of boundary layer and canopy microclimate. It is concluded that the thermal environment of a lichen largely interacts physiologically with the thermal tolerance limits of the dry thallus on one hand and the temperature optimum of net photosynthesis on the other.