Stylidiaceae comprises five genera and over 230 taxa which are clearly distinguished from related families in the Asterales by the presence of a central floral column which bears both the anthers and stigma at its apex. In the largest genus, Stylidium, column movement is triggered by an external stimulus creating a distinctive pollination mechanism. Whilst the relationships between families within the Asterales have been widely investigated, the generic interrelationships within Stylidiaceae have only recently come under evaluation using cladistic techniques and the species-level relationships remain largely unexplored. The primary aim of this dissertation was to survey the anatomical and morphological variation within the family, with particular reference to Stylidium, and to construct a phylogeny of the group based on these data.

Forty-three habit and vegetative anatomy characters are described. Stylidium leaves are particularly diverse in structure, with variation in the appearance and arrangement of the epidermal cells, presence of sclerenchyma, arrangement of the mesophyll cells, structure of the leaf margin and pattern of stomatal distribution. Scape anatomy is relatively uniform, with the exception of section Junceae which is unique in possessing specialised palisade cells in the scape cortex. Pericyclic cambial activity was found to be widespread in the stems of the perennial stylidia and also present in the annual species from subgenus Tolypangium. Variation in trichome structure is also of considerable systematic value, with 10 different types recognised. Fifty-seven floral characters are identified; the variation in the pattern of venation in the throat appendages, calyx lobes and the labellum are features not previously utilised for taxonomic or systematic purposes.

To investigate generic relationships within the family, cladistic analyses using 51 Stylidiaceae taxa and 100 morphological and anatomical characters were performed. Phyllachne and Forstera, characterised by actinomorphic flowers and insensitive columns, root at the base of the phylogeny. Oreostylidium, also with actinomorphic flowers and insensitive column, is sister to Stylidium; however, a position nested within Stylidium is thought more plausible on the basis of vegetative morphological and anatomical features and independent molecular evidence. Levenhookia nests within Stylidium in the analysis which included Oreostylidium, but is sister to Stylidium in an analysis which excluded this enigmatic taxon. The latter result corresponds to intuitive ideas about floral development in the family.

A larger analysis, which included an additional 57 stylidia, was used to assess relationships within Stylidium. Whilst support for a number of traditional groupings has been demonstrated (e.g. sections Squamosae, Verticillatae and Despectae), the subgeneric boundaries are ill-defined. A revised classification is not proposed here; however, on the basis of the species examined in this study two new alliances are suggested, both of which are supported by morphological, anatomical and cytological data: section Repentes and Appressae and S. drummondianum and S. tenuicarpum. It is also suggested here that section Despectae warrants subgeneric status and that two sections within the subgenus, composed of the annual species and the cormaceous perennials, should be recognised.

Whilst there is a general pattern of strong support for the monophyly of some species groups, there is weak support for the relationships between them. The lack of hierarchy is explained here as a consequence of rapid speciation which appears to have been driven by both ecological and genetic factors.