Floral phyllotaxis and floral architecture in Calycanthaceae (Laurales)

Calycanthaceae are sister to the remainder of Laurales (magnoliid clade), an order seemingly showing high variability in floral phyllotaxis. Previous studies of floral phyllotaxis in this family focused on one species, Calycanthus floridus. Therefore, a comparative study of floral phyllotaxis, comprising species of all four genera of the family, was undertaken. Floral phyllotaxis is spiral according to the Fibonacci pattern for all species studied. The more or less extensive transition zone from the vegetative decussate to the floral spiral phyllotaxis comprises three to 19 organs. There is usually a gradual transition between floral organ types, especially between different functional parts of the perianth and between stamens, staminodes, and carpels. The transition between staminodes and carpels sometimes shows irregularities in organ sequence along the ontogenetic spiral. There is a tendency to form series of organs of the same kind in Fibonacci numbers. Thecae dehisce via laterally hinged valves in Sinocalycanthus but via simple longitudinal slits in all other genera. Food bodies are present on the stamens and inner tepals of Calycanthus and Sinocalycanthus but differ in extent. All previously described calycanthaceous fossil flowers (Araripia, Virginianthus, Jerseyanthus) appear to show spiral floral phyllotaxis. In the three-dimensionally preserved Virginianthus and Jerseyanthus, arrangement of androecium organs in series of Fibonacci numbers appears to be present.