Development of inflorescences and flowers in Buxaceae and the problem of perianth interpretation

Recent phylogenetic analyses of angiosperms based on molecular and combined molecular and morphological data recognize a grade of basal eudicots situated between the ranunculids and the core eudicots. Buxaceae together with Didymelaceae form a well-supported clade within this grade. Flowers of representatives of this grade are characterized by a considerable variability in organ number and organ differentiation. In particular, perianth evolution of basal eudicots has been difficult to interpret because of a high diversity in number and form of perianth organs. In this investigation, development and structure of inflorescences and flowers of representatives of all buxaceous genera were studied comparatively to gain a better understanding of flower structure in Buxaceae and their relationships to basal eudicots. Inflorescences of Buxus, Notobuxus, and Styloceras kunthianum are most often botryoids with several lateral male flowers and a terminal female flower. Inflorescences of Pachysandra and sometimes Sarcococca are open spikes with lateral male flowers in the upper part of the inflorescence and female flowers below them. In dioecious species of Styloceras, male flowers form long spikes that may be terminated by a peloric flower; female flowers occur singly or in thyrses. In Buxaceae, all phyllomes preceding the reproductive organs on an inflorescence axis or floral axis look superficially similar. They are bractlike, and no obvious differentiation between bracts and tepals can be observed. However, detailed comparative studies of plastochrons, phyllotaxis, size, and shape of organs on the whole reproductive shoot show slight differences between organs. Bractlike phyllomes, if present, preceding the reproductive structures of male flowers of all genera are arranged in a decussate pattern. They are initiated with short plastochrons within each pair, whereas the plastochrons between pairs are longer. The uppermost mainly four bractlike phyllomes show modifications in the direction of tepals. The two pairs of stamens in Buxus are initiated in the same pattern as the bractlike phyllomes, whereas in Sarcococca and Pachysandra the plastochron within each stamen pair is almost zero. In contrast, Notobuxus has six or eight and Styloceras has up to 45 stamens in a more complex arrangement. The remaining floral apex forms a more or less developed pistillode in Sarcococca, Pachysandra, and most Buxus species. The floral center of Notobuxus is less differentiated than in the other genera, whereas a pistillode is even lacking in a few Buxus species and in Styloceras. Carpels of Styloceras, Sarcococca, Pachysandra, Notobuxus, and often Buxus are preceded by an initial pair of transverse phyllomes (prophylls) and several spirally arranged bractlike phyllomes that are only weakly differentiated in the direction of tepals. The nature of these organs is discussed in a broader systematic context.