Monograph of the lichen genus Placopsis (Nyl): a contribution to the phytogeography of the Southern Hemisphere

Abstract

This monograph is the outcome of several years' study of this predominantly subantarctic genus. The work was undertaken in the first place with a view to obtaining accurate determinations of the specimens in the British Museum Herbarium, and the recognition of the fact that this would necessarily involve the revision of all accessible material led to its expansion to a mono- -graphic study of all the known species. As in many other genera of this neglected group of plants, the taxonomy was confused and uncertain, and reference to the original type specimens was necessary. Existing collections being relatively scanty ( with the exception of 1. gelida ), I have in many cases been unable to test the uniformity and validity of the distinguishing characters of many species, and have therefore made a point of giving in every case a detailed description of the type specimen. The purpose of this is twofold: firstly, although many of the minuter characters described have little or no bearing on the classification here proposed, it is realised that subsequent generations of workers with more refined methods and a greater amount of material for comparison may find some of them of considerable taxonomic importance; and secondly, there are reasons for believing that several entities here classed as of subspecific rank may eventually be regarded as good pro - -per species, and a full description of the types in this work will obviate the necessity of re- examining and fur - -ther demolishing the already often sadly exiguous type material. The type specimens always provide the nucleus of reference around which the circle of taxonomic distinction, whether narrow or wide according to individual views, is drawn.