NEWTON, ANGELA E. Botany Department, Natural History Museum, Cromwell Road, London SW7 5BD, UK. - Phylogenetic constraints on branching architecture in mosses.
The range of possible morphologies for any given lineage of plant is
constrained by it’s phylogenetic history. Mosses are relatively simple
plants, with correspondingly few structural elements available to
provide the units of different branching architectures. Consequently,
phylogenetically diverse moss lineages may have superficially similar
growth forms, such as dendroid, pendulous, or weft forming. For
example, dendroid growth forms, with an upright "stipe" and
spreading branches, appear to have developed independantly in at least
the Climaciaceae, Hypnodendraceae, Pilotrichaceae, Pterobryaceae and
Thamnobryaceae. In this study, representatives of different lineages
were dissected out, and the branching architecture analysed.
Relatively few structural elements were found to interact in different
combinations to result in the observed growth forms. In the case of
dendroid mosses the growth form results from the interaction of
characters relating to termination and origin of the primary module,
"stipe" orientation (where a stipe may be a primary or
secondary module), origin, density and plane of secondary modules,
formation of a stolon, and reiteration. The distribution of the states
of these characters was examined on a phylogenetic tree derived from
molecular and morphological data. Although the states of some of these
characters were found in several different lineages that are not
closely related (based on this tree), others were restricted to
monophyletic lineages. In the pleurocarpous mosses, release from
morphological constraints through innovations in orientation,
branching and modularity have permitted the development of a wide
range of different growth forms.
Key words: branching architecture, mosses, phylogeny, pleurocarps