Floral Morphology and its Systematic Implications in Drypetes integerrima (Koidz.) Hosok. (Euphorbiaceae, tribe Drypeteae) from Bonin Islands, Japan

Abstract

Flowers of the species of Drypeteae in Bonin islands have been very poorly understood with respect to their morphology and structure. Both male and female flowers in flowering stage of the species from Bonin islands, as well as of the three African species of Drypetes and two East Asian species of Putranjiva (including P. matsumurae), were examined by stereoscopy and on the basis of microtome sections as well as of scanning electron micrographs. Contrary to all earlier taxonomic treatment placing the two Japanese species in the same genus, the species from Bonin islands greatly differs in floral morphology from Putranjiva matsumurae, another Japanese species in Ryukyu islands. The species from Bonin islands has the nectariferous "disk" which is lacking in Putranjiva matsumurae and has 5-8 stamens in staminate flowers in contrast to 2(-3) stamens in those of P. matsumurae. Following the latest system of classification of Euphorbiaceae, two Japanese species of Drypeteae should be placed separately in two distinct genera, i.e., the species from Bonin islands as Drypetes integerrima and, the other from Ryukyu islands as Putranjiva matsumurae. It is also shown anatomically that the disk of Drypetes is a nectary composed of small, densely stained cells with rich cytoplasm and, having stomata-like pores on surface of the adaxial side.