Quantitative comparison of floral development in Veronica chamaedrys and Veronicastrum virginicum (Scrophulariaceae)

Development in Veronica and Veronicastrum was studied to elucidate the growth patterns responsible for differences between their mature flowers. Nineteen floral dimensions were measured on buds from initiation to anthesis, and representative stages were recorded with the scanning electron microscope. Bivariate plots indicate the heterochronic changes responsible for the derived flower form of Veronica. The growth patterns of the gynoecium and androecium were little changed. The calyx of Veronica showed slower size increase at the early stages, but continued to grow after that of Veronicastrum had stopped. The most striking change occurred in the corolla lobes of Veronica, where growth was retarded until the beginning of style formation, but afterward was accelerated relative to that of Veronicastrum. The corolla tube of Veronica remained short due to a later onset of growth and slower enlargement later in development. Multigroup principal components analysis (M-PCA) was used to summarize the measurements. The distribution of points along M-PC 1 describes size increase during floral development. Along M-PC 2 the trajectories of the two taxa diverge after the beginning of style growth, while along M-PC 3 they differ from the earliest stages on, corroborating differences observed with the scanning electron microscope. M-PCA can thus be used to portray differences in patterns of floral development, facilitating simultaneous quantitative comparisons of two or more taxa