Ascus structure of eight yellow, two white and two brown Rhizocarpon species has been investigated by light microscopy. Ultrastructure and function in R. atroflavescens subsp. pulverulentum and R. montagnei were studied in TEM.

The Rhizocarpon-type ascus clearly differs from all other ascus types observed in the Lecanorales. It is bitunicate, opening with a slight ‘Jack-in-the-box’-mechanism. Its structure and function are related to patellariacean ascus types, but unlike those the ascus wall cytochemistry shows a certain similarity with Lecanora- and Peltigera-type asci. Rhizocarpon-type asci are embedded in a strongly amyloid hymenial gelatine. The nonamyloid ascus wall is surrounded by the strongly amyloid outer layer. The slightly amyloid expansible inner layer (= endoascus) is apically thickened; it shows the banded and pleated ‘accordion-structure’ characteristic of bitunicate asci. Prior to dehiscence, the ascus wall and its outer layer burst. Thereafter the pleatings of the expansible inner layer are stretched, forming the rather short beak which reaches the hymenial surface. During expansion gliding occurs between the expansible inner layer and an outer part of the endoascus, here described as the ‘inner layer’. In a few sections of aldehyde- fixed material of R. atroflavescens a small laminated plug was observed in the apex of the endoascus.

Rhizocarpon-lype asci are considered to be the most archaic in the Lecanorales. This supports a hypothesis that Rhizocarpon is a phylogenetically basal group, linking the evolved Lecanorineae, and possibly also the Peltigerineae and Teloschistineae with not yet recognized bitunicate ancestral forms similar to those occurring in the Patellariaceae.

Zoosporogenesis was observed in the algal symbiont (Trebouxia gelatinosa) of Parmelia caperata. This is the first detailed report of this phenomenon in a Trebouxia species and, more importantly, in lichens, where zoosporogenesis in this alga is usually suppressed and the arrested zoospores form aplanospores. Observations of zoospore formation by algal cells within the natural thallus and in thallus fragments incubated on a mineral medium suggest that zoospores are released within thalli and may escape from the thallus to form free-living microcolonies. These colonies potentially could unite with hyphae derived from spores of either the same, or genetically different mycobionts (fungal symbionts) to establish new lichen associations, as has been shown in laboratory studies. Such natural resyntheses could be one cause for the observed heterogeneity characterizing fungi in many lichen populations. Other implications of free-living Trebouxia populations are discussed.

Sociological tables are given for five communities found on Alnus incana in Troms, North Norway: Arthopyrenietum punctiformis P. James et al., Lecanoretum chloropoliae ass. nov., Lecanoretum subfuscae Hil., Lobarion pulmonariae Ochsn. and Parmelietum olivaceae Barkm. Snow cover and shading from large ferns are important factors governing the distribution of the communities on the tree trunks.

Fifteen specimens of four Acarospora species of subgenus Xanthothallia were analysed. All species contained (+)-rhizocarpic acid. A. gobiensis and A. schleicheri had only this compound and A. oxytona this and (+)-lichesterinic acid. A. chlorophana seems to exist in two chemical races, one with a mixture of (-)-acaranoic and (-)-acarenoic acids and the other with (+)-roccellic acid. The stereochemistry and biogenesis of these compounds is briefly discussed.

Bryoria chalybeiformis, as commonly interpreted, is an almost exclusively saxicolous or terricolous species with a largely arctic-alpine distribution; it produces small amounts of fumarprotocetraric acid in the soralia. The terricolous form of B. fuscescens var. positiva, distributed from sea level to the high mountains, is distinguished by the production of abundant fumarprotocetraric acid accompanied by protocetraric acid. B. intricans appears to be closely related to B. chalybeiformis and may possibly represent an environmentally induced morphotype of it. Attention is drawn to nomenclatural problems connected with B. chalybeiformis.