A survey of morphological characters for phylogenetic study of the transition to pleurocarpy

Although the number of family or generic cladistic studies in bryophytes is increasing, there have so far been few cladistic analyses of moss relation ships at the ordinal level. Cladistic methods and theory provide powerful a nalytical tools in the study of character variation, and are used here to p rovide the basis for studies of morphological characters for the phylogeny of the transition to pleurocarpy, a problem at the ordinal level in mosses. Morphological characters from the gametophyte and sporophyte are assessed, and discussed in the context of cladistic analysis based on morphological data and molecular data from rbcL sequences. In order to understand the evo lution of branching systems we have deconstructed the traditional concept o f pleurocarpy into its constituent elements. These we have included in the analysis as independent characters. Among features potentially important to the understanding of the transition to pleurocarpy are: modular architectu re, hierarchy of the archegonial module, length of fertile module, differen tiation of perichaetial leaves, formation of sub-perichaetial branches, and intrusion of the foot into the supporting module. Position of the branch p rimordium is found to be less informative than previously thought Further s tudy of the pleurocarpous members of the Bryales, especially Pyrrhobryum an d its relatives in the Rhizogoniaceae, will be crucial for extending our un der standing of the transition to pleurocarpy.