CHROMOSOMES AND SYSTEMATIC POSITION OF THE INOPERCULATE MOSSES, PLEURIDIUM AND BRUCHIA

Abstract

VIRTUALLY ALL modern systems of moss classification recognize the peristome as the principal criterion for establishment of the major groupings of the Bryales. Most of the families of acrocarpous mosses are arranged more or less strictlv according to peristome structure. However, among the acrocarpous mosses more than nine hundred species have capsules without a peristome. Many of these aperistomate mosses lack functional opercula as well. In the absence of peristome characters, systematists have been forced to assign these mosses to genera and families largely on the basis of gametophytic characters. Thus, the cleistocarpous and gymnostomous species have been distributed rather widely among some eighty genera and eighteen families of acrocarpous mosses. The systematic position of many of the cleistocarpous mosses has remained in doubt, however, for lack of strong morphological characters, and the problems of relationship are open to any clarifying evidence. Almost no cytological data are available for any of the functionally cleistocarpous mosses. Therefore, a series of studies was undertaken in ten genera of these mosses (Bryan, 1956; unpublished results). Results for two genera, Pleuridiuim Brid. and Bruchia Schwaegr., are presented here. The relationship of Pleuridium and Bruchia has not been well understood. Bruch et al. (1850), Lindberg (1879), Limpricht (1890), and Grout (1936) considered the genera closely related; all these workers placed both genera in one family. In other treatments (Schimper, 1876; Fleischer, 1915; Brotherus, 1924) the two genera are put in separate families: Pleuridium in the Ditrichaceae (Fleischer and Brotherus); Bruchia in the Trematodontaceae (Fleischer) and in the Dicranaceae (Brotherus). Ten species of Pleuridium and Brucha were studied, as well as four other possibly related species. Meiotic chromosome number, the behavior and gross morphology of the bivalents, and the reactions of the sporocytes to standard methods of fixation and staining were observed. The cytological mounts were prepared using the methods described by Steere et al. (1954). The studies were made with the aid of an N.A._ 1.30 Ach. SI. Th. 1.25 condenser, a 1.4 mm. oilimmersion objective with N.A. 1.30, X15 compensating oculars, and a Kodak Wratten filter no. 74. The magnification of the drawings, made with a camera lucida and X30 ocular, is X4350. The magnifncation of all published figures is X2170. Each population studied is represented by a