Floral structure and development of Acoraceae and its systematic relationships with basal angiosperms.

Flower development and anatomy of Acorus calamus and flower anatomy of A. gramineus were studied. Findings were compared with published reports on paleoherbs. Important developmental features include an abaxially median tepal that is initiated first and is similar to a flower-subtending bract and unidirectional flower development with an inversion of organ initiation sequence in the second tepal whorl. The mature gynoecium is largely synascidiate, but early development of carpels is plicate, and the apocarpous portion persists up to anthesis. The carpels form dorsal bulges on the style, enclosing longitudinal intercarpellary slits. The dominance of the synascidiate portion and the apical position of the placenta result from a late and distinct basal elongation of the gynoecium. Stigma, pollen transmitting tract, and ovary are filled with secretion. Secretory papillae are present from the stigma to the placenta; papillae also occur on the rims of the integuments of the ovules. In the uppermost part of the inflorescence, the adaxial floral sectors are reduced in number and structure, and at the apex of the inflorescence, a peloria-like structure is formed. Developmental and morphological similarities seem to be closer between Acorus and Piperales than between Acorus and other magnoliids.