An emphasis is being placed on integrating development into systematic and evolutionary research programs. Plants provide an excellent opportunity for ontogenetic studies, as they are modular organisms which record a temporally-ordered array of leaves arrested in successive stages of development, the heteroblastic series, that arises from a single initial cell (as opposed to a meristematic region of cells in seed plants). In acrocarpous mosses, a new branch module is initiated sympodially following the production of archegonia, so the heteroblastic series is reiterated along each new branch, providing an investigator with numerous replicates. Earlier studies have indicated possible parallels between the heteroblastic series and phylogenetic transformations. Within the haplolepideous mosses, a very distinct leaf architecture known as the 'leucobyroid' morphology has arisen at least five times. Leaves of mosses with this unique architecture are dominated by at least two strata of large thin-walled dead cells (hyalocysts), with one or more layers of green cells (chlorocysts) intercalated between them. Traditional interpretation of the tissue homologies for these leaves, derived from observations of Leucobryum and related taxa, contends that the leucobryoid morphology arose via a gradual expansion of the costa and simultaneous reduction of the lamina. Here I examine the heteroblastic leaf series for two taxa which represent independent derivations of the leucobryoid morphology, Leucobryum sanctum and Arthrocormus shimperi , and those of their respective sister taxa,Campylopus and Syrrhopodon . When placed in a phylogenetic framework, the heteroblasty of these taxa suggests that different developmental modifications have been involved in the origins of the leucobryoid leaf. In a comparative context, heteroblasty provides evidence for transformational homologies which may not be apparent at the level of the mature plants.

Key words: , Calymperaceae, heteroblastic series, homology