Floral development elucidates the formation of a tubular calyx and connate stamens in Cecropieae species (Urticaceae)

Abstract

Small flowers with tubular calyces and connate stamens, a small number of whorls and organs per whorl are found in species of the tribe Cecropieae (Urticaceae). This study elucidates the processes that lead to such floral conditions by comparing the morphology of the developing flowers of Cecropia pachystachya, Coussapoa microcarpa and Pourouma cecropiifolia. Buds and flowers were examined under scanning electron and light microscopy. The tubular calyx originates from the activity of a peripheral annular meristem that results in a tube with two or three lobes. In the staminate floral meristem, the androecium primordium arises as a central bulge that elongates and originates two stamens with the filaments basally or totally united; the anthers can also be united. In the pistillate floral meristem, the gynoecium primordium also arises as a central bulge that elongates and originates two carpel primordia: one expands, forming a cleft and an ovule, and the other does not differentiate and remains rudimentary. Pistillate and staminate flowers result from the absence of the stamen or carpel, respectively, from inception. Petals are also absent from inception. The formation of the tubular calyx and connate stamens occurs very early in development, characterizing a congenital union. The union of anthers by the connectives in C. microcarpa is postgenital and occurs between epidermal cell walls through a weak cohesion. The floral development of Cecropieae is quite similar and less labile than in the other Urticaceae species.