PERIANTH DEVELOPMENT IN THE BASAL MONOCOT TRIGLOCHIN MARITIMA (JUNCAGINACEAE)

Basal monocots exhibit considerable variation in inflorescence and floral structure. In some cases, such as Triglochin maritima, it is not clear whether the lateral and terminal structures of the inflores- cence are flowers or pseudanthia, or where the limits between flowers and inflorescence lie. To address these questions, morphological studies were carried out, and the results show that in T. maritima both terminal and lateral structures are flowers, not pseudanthia. The terminal flower of T. maritima de- velops from the apical inflorescence meristem, suggesting that the apical meristem identity changes from ‘‘inflorescence’’ to ‘‘flower’’ during inflorescence development. In addition, distal flowers of T. maritima are reduced, and there is no distinct flower-subtending bract; instead, the perianth develops unidirectionally, resulting in an abaxial-median bract-like tepal and bilaterally symmetrical flowers, similar to those of other basal monocots, such as Aponogeton and Acorus. It is possible that the leaf primordium changes its positional homology from ‘‘flower-subtending bract’’ to ‘‘tepal.’’ Therefore, in some basal angiosperms with abbreviated development of lateral flowers the demarcation of the flower vs. the inflorescence is ontogenetically ambiguous. In situ hybridization experiments show that a putative ortholog of the B-class gene APETALA3/DEFICIENS is expressed in developing stamens and carpels, and may also be expressed in the shoot axis of the very young inflorescence. This expression pattern seems to be consistent with the gradual transition between inflorescence and flower that was observed morphologically.