Floral development of Lavatera trimestris and Malva hispanica reveals the nature of the epicalyx in the Malva generic alliance

Abstract

The epicalyx is a structure below the calyx that is often integrated in floral display. In Malvales, the epicalyx is interpreted to be formed by bracts derived from inflorescence reduction. In this study, we compare the epicalyx and flower development of Lavatera trimestris and Malva hispanica, which are close relatives but show contrasting morphologies. Both species exhibit cymose branching, stipulate subtending leaves, a short plastochron between the appearance of the alternating epicalyx and calyx, a centrifugally developing androecium and a multicarpellar gynoecium. The predominantly trimerous structure and leafy morphology of the epicalyx suggest its origin from a former subtending leaf with leaf-like stipules. The bilobed epicalyx in M. hispanica represents a loss of the adaxial epicalyx lobe rather than modified bracts. In Malvoideae, the bracts and bracteoles in the flowering branches can be completely absent and are variable in position and number when present. Individual bracts and bracteoles could correspond to further reductions of former subtending leaves instead of precursors of the epicalyx. Although the centrifugal androecium behaves as a branched-like structure, it is a dynamic complex floral whorl with extended growth capacity. The umbrella in L. trimestris is a swollen part of the style without a well-understood role in floral or fruit morphology.