Floral development of the divergent endemic Hawaiian genus Schiedea (Caryophyllaceae), with special emphasis on the floral nectaries

Abstract

AbstractThe endemic Hawaiian genus Schiedea has among the most architecturally complex flowers of any genus of Caryophyllaceae. To understand the developmental basis of the unusual floral features that characterize Schiedea, we studied the floral organogenesis of 6 of the 34 species of the genus using SEM, supplementing these observations with a focused investigation of nectary histology. The early ontogeny of Schiedea flowers is like that of many other Caryophyllaceae in that: (1) the five sepals are initiated in a 2/5 spiral, with the first, antesepalous whorl of stamens initiated in an imperfectly contrarotating sequence relative to the sepals; (2) the alternisepalous stamen whorl initiates slightly external to and at a lower level on the floral apex than the antesepalous whorl; and (3) petal primordia, if initiated (rare), arise on a primordium common with an alternisepalous stamen. Hence, most floral features that distinguish the genus, such as the (often) reflexed sepals and elongate stamen filaments, arise late in floral ontogeny. The only unique trait to Schiedea flowers are the elongate, tubular nectary extensions. They are antesepalous and arise from the abaxial surface of a ring–shaped proliferation of tissue that surrounds the bases of the developing stamens. The shape of each nectary in early ontogeny is highly similar to the mature “swallow’s nest” nectaries present in many genera of a major crown clade of Caryophyllaceae (Pleurcaryophyllaceae, containing Schiedea), suggesting that the development of the needle–to scale–like nectary extensions of Schiedea might represent terminal additions onto a “swallow’s nest” nectary ontogenetic program.