Floral Development in Mangrove Rhizophoraceae

Abstract

The flowers of mangrove Rhizophoraceae (tribe Rhizophoreae) are adapted to three different pollination mechanisms. Floral development of representative species of all four genera suggests that the ancestral flower of the tribe was unspecialized, with successively initiated whorls of separate sepals, petals, antisepalous stamens, and antipetalous stamens; at its inception, the gynoecium had a united, half-inferior ovary and separate stigmatic lobes. This developmental pattern is found in Rhizophora mangle (wind-pollinated) and Ceriops decandra (insect-pollinated). In Kandelia, all floral organs distal to the sepals are initiated simultaneously, and there has apparently been an evolutionary amplification in the number of stamens to about six times the number of petals. Explosive pollen release evolved independently in C. tagal and in Bruguiera. In the former, all stamens belong to one whorl and arise simultaneously upon a very weakly differentiated androecial ring primordium. In Bruguiera, the androecial ring is pronounced, and two whorls of stamens arise upon it; the primordia of the antisepalous whorl arise first but are closer to the center of the apex than the antipetalous stamen primordia. The antisepalous stamens bend toward and are enclosed by the petals early in development. In all genera, the inferior ovary develops by zonal growth of receptacular tissue; additional intercalary growth above the placenta occurs in Bruguiera. In general, floral specialization is accompanied by an increase in the width of the floral apex compared to the size of the primordia, increasing fusion of the stylar primordia, and decreasing prominence of the superior portion of the ovary. Apparent specializations of petal appendages for water storage, including the presence of sub-terminal hydathodes (previously unreported in any angiosperm), were found in two species in which flowers remain open during the day but were absent from two species normally pollinated at night or at dawn. Distinctive tribal characteristics that may aid in phylogenetic analysis include the mode of development of the inferior ovary; the aristate, bifid, usually fringed petals that individually enclose one or more stamens; the intrastaminal floral disc; and the initially subepidermal laticiferous cell layer in the sepals and ovary.