Early Floral Development and Nature of the Calyptra in Eupomatiaceae (Magnoliales)

Abstract

Early floral development in Eupomatia bennettii and Eupomatia laurina was studied with scanning electron microscopy and serial microtomy to focus on initiation and development of the calyptra and initiation and phyllotaxis of the floral organs. Phyllomes preceding the calyptra are distichous or spirodistichous in arrangement. In both species, the calyptra is initiated as a smooth ring meristem encircling the floral apex. There are no traces of distinct organ primordia on this ring. After initiation, the calyptra rapidly elongates into a tubular structure, and its rim closes by irregular lobation and local formation of hairs. These lobes also behave differently than young organs in their growth pattern. The closed rim ceases to enlarge and forms a minute closed pore at anthesis. A long plastochron follows initiation of the calyptra, and the floral apex widens considerably during this time. Since a perianth is lacking, stamens and staminodes are the first floral organs initiated. They appear in a regular spiral phyllotaxis according to a Fibonacci pattern, with the contact parastichy sets of 13 and 21 for E. bennettii and eight and 13 for E. laurina (with fewer floral organs). The carpels are also spirally arranged, with the contact parastichy sets of eight and 13 for E. bennettii and five and eight for E. laurina. The calyptra in Eupomatiaceae is best interpreted as a single, amplexicaul bract. This is indicated by the floral development of Eupomatiaceae and also by comparison with the closest relatives (Annonaceae, Magnoliaceae, Himantandraceae), which have calyptra‐like structures that more clearly correspond to bracts. For angiosperms in general, I suggest that bracts and tepals are not terms to be used at the same level; bracts represent a less differentiated ground state of phyllomes.