On the basis of light microscopic (LM), scanning electron microscopic (SEM) and transmission electron microscopic (TEM) investigations the Pertusaria-type of ascus is described as a particular functional type. The functionally unitunicate Pertusaria-type is characterized by its structure, staining properties, and by its particular mode of dehiscence. Tripartite ascus walls were observed in LM and TEM. The non-amyloid ascus wall is surrounded by a thin, amyloid outer layer. Both become amorphous at maturity and partly disintegrate. An apically thickened, amyloid inner layer reaches the base of the ascus. In its fine structure this amyloid inner layer resembles the material of the amyloid dome of Lecanora-type asci. It plays an important role during dehiscence and spore discharge. An elongation process was observed prior to dehiscence, at the end of which the ascus tip is situated above the hymenial surface. Dehiscence occurs by bursting or splitting of the whole ascus tip. The Pertusaria-type might represent a side-branch of evolution from bitunicate to unitunicate forms within the Lecanorales.

Pertusaria-type asci are restricted to a small number of genera within the Pertusariaceae. A considerable heterogeneity in ascus structure and staining properties was observed within the Pertusariineae sensu Henssen & Jahns (1973) and Henssen (1976).

A synopsis of 26 species belonging to 10 genera of Caliciales (s.lat.) occurring in Costa Rica is given. Short comments on the ecology and distribution of the species, indications of occurrence of secondary substances, and observations on the ultrastructure of the spores in some species are included. The mainly tropical genera Pyrgidium, Schistophoron and Tylophoron are given particular attention. Seventeen species are new to Central America and the following five new to science: Caliciurn constrictum sp. nov., Chaenothecopsis rubina sp. nov., Coniocybe sanguinea sp. nov., Schistophoron variabile sp. nov., and Tylophoron crassiusculum sp. nov. Two new combinations, Mycocalicium albonigrum (Nyl.) comb. nov. and Pyrgidium montellicum (Beltr.) comb, nov., are made. Lectotypes have been designated for Tylophoron annulatum Krempelh., T. cupulare Vainio, T. moderatum var. consociata Vainio and T. rufocapillatum Nádv. Names of several taxa in Pyrgidium and Tylophoron have been put into synonomy. Arthothelin, thuringione and thiophanic acid (in Calicium hyperelloides Nyl.) and lecanoric acid, 2'-O-methylperlatoic acid and lichexanthone (in Tylophoron) are reported for the first time in the Caliciales.

Gaseous diffusion resistances for carbon dioxide and water vapour, thallus water content and thallus water potential were experimentally determined on species of the Stictaceae. The diffusion resistance to water loss was high only at low water contents and correlated closely with thallus water potential. Carbon dioxide diffusion resistances, however, were high at both low and high water contents and, even at medium water contents, were still an order of magnitude greater than the water resistance. These results indicate that carbon dioxide and water vapour exchange occur by different pathways in these lichens. Consequently it is suggested that the lichens have structural adaptations which separate the functions of water uptake, water storage and carbon dioxide exchange.

A cyclic exchange between communities of mosses and the lichen Baeomyces rufus is described. The cushions of different mosses are supplanted by the lichen. After a lifespan of about 3 years the lichen dies off and the location is repossessed by mosses. The vegetative reproduction of the lichen is adapted to this ecological cycle.

Nylon mesh litter bags containing Parmelia caperata, P. flaventior, P. subrudecta, Evernia nesomorpha, Hypogymnia physodes and Umbilicaria muehlenbergii were placed on the floor of a black spruce bog in northern Minnesota to study the rates of decomposition. P. caperata and E. mesomorpha had decomposed after 10 months. Most of the lichens had completely decayed after 13 months but a few thalli of some species placed on top of hummocks were still alive. The species with usnic acid decayed as rapidly as those without. Lichens decompose rapidly but not more rapidly than some vascular plants.