Developing and mature inflorescences and flowers of several representatives of Platanus were studied to clarify various aspects of floral structure and organization. Special attention was given to perianth differentiation. Extant Platanaceae are monoecious with unisexual flowers aggregated into compact, spherical inflorescence heads. Development of hairs in a basipetal direction subdivides the undifferentiated inflorescence surface into floral zones. Development of both male and female flowers of Platanus ? hispanica begins with the initiation of a perianth whorl. Thereafter, the reproductive organs emerge on the floral apex: stamens in male flowers, staminodes and carpels in female flowers. The last organs to appear in both sexes are the small organs located between perianth and androecium. At anthesis, in both male and female flowers, organs of the first whorl are inconspicuous, scalelike, and only two to three cell layers thick. Alternating with these first thin organs is a whorl of short but fleshy organs. These second?whorl organs are basally united with the stamens, forming a short androecial tube. They also show some structural similarities with stamens. These features support the hypothesis that the second?whorl organs are of androecial (staminodial) origin. This hypothesis is further supported by the fossil record, where, in some taxa, second?whorl organs are particularly similar to stamens, as well as by morphological comparisons with flowers of Proteaceae.

Developing and mature inflorescences and flowers of several representatives of Platanus were studied to clarify various aspects of floral structure and organization. Special attention was given to perianth differentiation. Extant Platanaceae are monoecious with unisexual flowers aggregated into compact, spherical inflorescence heads. Development of hairs in a basipetal direction subdivides the undifferentiated inflorescence surface into floral zones. Development of both male and female flowers of Platanus × hispanica begins with the initiation of a perianth whorl. Thereafter, the reproductive organs emerge on the floral apex: stamens in male flowers, staminodes and carpels in female flowers. The last organs to appear in both sexes are the small organs located between perianth and androecium. At anthesis, in both male and female flowers, organs of the first whorl are inconspicuous, scalelike, and only two to three cell layers thick. Alternating with these first thin organs is a whorl of short but fleshy organs. These second‐whorl organs are basally united with the stamens, forming a short androecial tube. They also show some structural similarities with stamens. These features support the hypothesis that the second‐whorl organs are of androecial (staminodial) origin. This hypothesis is further supported by the fossil record, where, in some taxa, second‐whorl organs are particularly similar to stamens, as well as by morphological comparisons with flowers of Proteaceae.