Petrova O. The lichen genus Usnea in eastern Gennoscandia. III Shrubby species. Ann. Bot. Fennici. 1999, 36, p. 235-256.

238 Halonen • ANN. BOT. FENNICI 36 (1999) usually less than 90 ° , but in U. fulvoreagens , for instance, they are often ca. 90 ° , which gives a dis- tinctly divergent habit for a thallus. The branches are circular in cross section, or slightly angular, and may have ridges and/or foveoles (round or oval pits). In longitudinal section, branches are either tapering, or about the same thickness in the whole length, or irregularly shaped. Branches are more or less segmented by well demarcated an- nular cracks, whichmay be lined by white, everted medullary rings. The base in most of the East Fennoscandian Usnea species is even in thickness or slightly fusi- form, and the colour varies from pale to jet black. The frequency of segmentation of the base has only minor diagnostic value in the East Fenno- scandian species, but thin longitudinal cracks are common at the bases of U. wasmuthii while they are rare or sparse in other species. Soralia may be tuberculate, superficial or ± excavate. They vary from minute and punctiform to large and expanded. In some species soralia may even encircle the branches, and may also be- come confluent. Soralia develop on local breaks of the cortex or arise from scars. Scars are borne when fibrils become detached, for instance. Clerc and Herrera-Campos (1997) call the tuberculate scars of fibrils “fibercles”. Soredia are farinose to granulose. In most species soralia have isidia (isi- diomorphs, according to Clerc and Herrera-Cam- pos 1997) at least when young, in which case the isidia may partly or totally be replaced by sore- dia. Isidia occur single or in clusters, and develop from the cortex or frommedullary tissues, i.e. scars and soralia. Many species, for instance, Usnea gla- brescens , U. lapponica and U. wasmuthii , have modifications with tuberculate and more or less confluent soralia, which may produce spinules. The thickness of the cortex, medulla and cen- tral axis have a significant taxonomic importance. The structure of the medulla varies from lax (loosely arachnoid in Usnea glabrata ) to dense, or only exceptionally compact in the treated shrub- by species. The surface of the cortex is usually mat, but U. glabrata has a somewhat shiny sur- face, which is typical for the species in the U. fra- gilescens agg. (Clerc 1987a, 1998, Halonen et al. 1998). In the East Fennoscandian species, the colour varies from pale straw-coloured, to yel- lowish-green, greyish-green, or deep yellow. Us- nea hirta often shows a more intense yellow col- our than the other species since it mainly grows in open sites. Specimens change colour with pro- longed storage in the herbarium, when some spe- cies may turn darker in colour or even deep brown (e.g., U. glabrata and U. lapponica ). Wart-like papillae vary in shape from low and verrucous to tall and cylindric. Tubercles are simi- lar, but differ in their often larger size and in the presence of medulla. Tubercles often produce so- redia and/or isidia. The distinction between pa- pillae and tubercles is not always clear, especially in Usnea subfloridana and U. wasmuthii , in which structures similar to papillae regularly develop into soralia. Cilia-like fibrils may be irregularly dis- tributed on the thallus, for instance, in U. glabre- scens , where fibrils are normally absent or sparse in the apical parts while they are more abundant near the base. Short, spinulose fibrils are often called spinules. Papillae, tubercles and fibrils have been found in every East Fennoscandian Usnea species, but their abundance may vary consider- ably within a species. All the boreal, northern circumpolar Usneae are secondary species, which only rarely produce apothecia. Fertile primary species, for example, U. florida (L.) F. H. Wigg. and U. rigida (Ach.) Mot. s. lato , are found in temperate areas of Eu- rope and North America (Clerc 1984, Halonen et al. 1998). Secondary species become dispersed almost solely by vegetative diaspores and thallus fragments, and therefore they may have apomic- tic populations. In East Fennoscandia, apothecia are most frequently present in U. hirta and U. sub- floridana , but are also found very rarely in U. di- plotypus , U. fulvoreagens , U. glabrata , U. gla- brescens and U. lapponica . ECOLOGY AND DISTRIBUTION All the East Fennoscandian Usnea species are primarily epiphytes. The majority of them are also occasionally found growing on lignum (especially Usnea hirta ) and more rarely on rocks. Local spe- cies may display more or less phorophyte prefer- ence. This may be partly due to bark properties such as acidity, but climatic conditions seem to have much more significance. For example, mem- bers of the U. fragilescens agg. are commonly