Floral development of the endangered genus medusagyne (Medusagynaceae-malpighiales): Spatial constraints of stamen and carpel increase

摘要

Premise of research. The floral development of the monotypic genus Medusagyne is investigated to understand the mechanisms of stamen and carpel increases in light of its proposed affinity with Ochnaceae. Methodology. Flowers at different stages of development were investigated with SEM. Pivotal results. Flowering shrubs are andromonoecious. Flowers arise in cymes at the end of shoots. Initiation of staminate and bisexual flowers is similar for sepals and petals, which arise in a spiral sequence. Stamen initiation is centrifugal on a broad circular primordium but becomes more or less irregular by the insertion of additional stamens between the older stamens. In staminate flowers the globular floral apex is completely covered with stamens, with no trace of carpels. Bisexual flowers emerge on the lower branches and develop a broad stamen ring surrounding a whorl of 13-16 carpels. Carpels initiate in a ring around the massive floral apex and develop mainly in a vertical direction. Carpels are progressively swamped in the stamen mass by a simultaneous expansion of filaments, and styles become irregularly compressed in older buds. The adaxial expansion of carpel tissue eventually overtops the floral apex as a roof. Measurements of buds show that carpel and stamen numbers are linked with the proportional size of the floral apex relative to the whole size of the flower bud. Conclusions. The unusual floral development of Medusagyne supports the separation of the genus in its own family, although several characters link it with the Ochnaceae clade. Stamen initiation and expansion shows much similarity with Eucryphia in Cunoniaceae. It is suggested that the larger number of carpels is intimately linked with the higher stamen numbers. Constraints of space and a vertical growth pattern of the floral apex cause the peculiar morphology of the gynoecium and the variably delayed stamen initiation.