Diversity and Evolution of Seedling Baupläne in Perlargonium (Geraniaceae)

The genus Pelargonium (Geraniaceae) exhibits tremendous variation in growth form. We apply a broadly defined concept of Bauplan in our study of growth form, architectural and anatomical features of early seedling development in the type subgenus. We analyze variation in these features within a phylogenetic framework based on sequence comparisons of internal transcribed spacer regions (ITS) of nuclear ribosomal DNA. Preliminary ITS sequence comparisons show strong support for two major clades. One major clade contains two subgroups, one consisting of three previously recognized sections of more or less woody shrubs and subshrubs (sections Pelargonium, Glaucophyllum and Campylia), and a second clade of five sections consisting of geophytes and branched stem-succulents (sections Hoarea, Ugularia, Polyactium, Cortusina and Otidia). The shrubby subgroup is characterized by relatively high levels of variation in architectural determinants such as internode elongation and branching. All members of this subgroup exhibit similar patterns of tissue production by the vascular cambium wherein secondary xylem consists predominantly of lignified tracheary elements and fibers. The stem-succulent clade shows less variation in some architectural determinants such as internode length and branching. The location of the initial onset of axis thickening is variable among species. In all sections except Ligularia, axis thickening is correlated with the onset of cambial activity wherein both secondary xylein and phloem are composed of few conducting cells scattered among abundant storage parenchyma. Section Ligularia is characterized by cambial activity similar to that in the shrubby clade; epicotylar thickening in this section arises through expansion and division of cortical cells. The second major clade within the type subgenus contains the monotypic section Isopetalum, and sections Peristera and Reniformia. Members of these sections range from shrubs that are woody at the base to herbaceous scramblers. Species examined exhibit variation in branching but limited internodal elongation. These seedlings were thin stemmed and showed relatively limited activity of a vascular cambium, which produced secondary xylem consisting of lignified tracheary elements and fibers. The occurrence of a vascular cambium producing largely lignified tracheary elements and fibers in all members of the subgenus except the stem-succulent clade suggests that this type of secondary xylem is basal within the subgenus, and that section Ligularia has either retained or reevolved the basal state.