Define 'Trichostomum'

The moss in the above photo Icopyright Hermann Schachner) generally goes by the name of Trichostomum crispulum. Trichostomum is a cosmopolitan genus in the Pottiaceae, the largest recognised family of mosses with about 1500 species overall. But with great diversity comes great difficulty of identification. Pottiaceae tend to be small mosses that are common in harsh habitats. Features of pottiaceous mosses are often hard to distinguish and may be quite variable, making it difficult to confidently define taxa. As a result, Pottiaceae is a prime example of what I like to call 'taxonomic blancmange': something that tends to just get prodded nervously then backed away from when it wobbles ominously.

Characteristic features of Trichostomum as it is commonly recognised tend to include symmetric leaves with more or less plane margins, and with the basal cells of the leaf differentiated straight across the blade or in a U-shape. The peristome of the capsule also tends to be short and straight, and the sexual system is usually dioicous (with separate male and female plants) (Flora of North America). However, none of these features are entirely reliable, and some species have been the subject of extensive disagreement about whether they should be placed in Trichostomum, or in a related genus such as Weissia or Tortella.

To date, only a selection of Pottiaceae species have been subject to molecular analysis, but these analyses have confirmed the unsatisfactory nature of the current system. A molecular phylogenetic analysis of the pottiaceous subfamily Trichostomoideae by Werner et al. (2005) did not identify Trichostomum species as a monophyletic clade; instead, various representatives of the 'genus' were scattered throughout the subfamily. The type species of Trichostomum, T. brachydontium, was associated with a few close relatives such as T. crispulum in a broader clade containing numerous species of the genus Weissia. As a result, it has been suggested that the two genera should perhaps be synonymised, in which case the name Trichostomum would be absorbed by the older Weissia. But first, someone would need to work out just how such a genus could be recognised...

REFERENCE

Werner, O., R. M. Ros & M. Grundmann. 2005. Molecular phylogeny of Trichostomoideae (Pottiaceae, Bryophyta) based on nrITS sequence data. Taxon 54 (2): 361–368.