Ontogeny and morphology of the fertile flowers of Hydrangea and allied genera of tribe Hydrangeeae (Hydrangeaceae)

The monophyletic Hydrangeeae (Hydrangeaceae) consists of the clade Cardiandra + Deinanthe and its sister, the Hydrangea clade, which includes the paraphyletic Hydrangea as well as Broussaisia, Decumaria, Dichroa, Pileostegia, Platycrater and Schizophragma. The plesiomorphic imbricate corolla aestivation and polystemony of Cardiandra and Deinanthe distinguish these two genera from most members of the Hydrangea clade. Deinanthe has postgenitally fused styles and cushion-like, dorsally positioned stigmas. The Hydrangea clade is notable because most species of Hydrangea share a floral morphology characterized by small size; tetramerous-pentamerous perianths; inconspicuous sepals; reflexed petals; diplostemony; stamens that are longer than the styles; completely inferior ovaries; separate styles; terminal, papillate stigmas; and dimerous-tetramerous gynoecia. This suite of states is termed the ‘Hydrangea floral syndrome’ (HFS). Various members of the Hydrangea clade lack the HFS, including (1) Platycrater; (2) Hydrangea anomala; (3) H. paniculata + H. heteromalla; (4) the Schizophragma clade (Schizophragma, Pileostegia and Decumaria); and (5) the macrophylla clade (H. macrophylla, H. scandens, H. hirta, Dichroa and Broussaisia). The meristic uniqueness of Decumaria reflects mutations observed in Arabidopsis (clavata) and Lycopersicon (fasciated) that cause organ number increases because of changes in meristem capital. The modification of early perianth development to form a prominent corolline torus at a point when sepals are diminutive is present in H. anomala and Hydrangea section Cornidia and may be synapomorphic for them. Various transformations in the perianth, androecium, and gynoecium lie behind the floral diversity of Hydrangeeae. Some morphological transformations have been homoplastic, including shifts to polystemony, calyptrate corollas, and synstyly.