Pollen morphology of the genus Skimmia (Rutaceae) and its taxonomic implications.

Pollen morphology of the genus Skimmia was studied. Of six species of the genus, five species were investigated by light microscopy and scanning electron microscopy. Pollen grains of Skimmia had striate to striate-reticulate sculpture and variable aperture number (3-7), and these characters differed from even the phylogenetically closest relatives such as Dictamnus and Casimiroa which have 3-colporate pollen grains with reticulate exine. In Skimmia laureola, pollen dimorphism is suggested in the aperture number in accordance with the floral system. Pollen grains of Skimmia japonica, a morphologically very variable species, were extensively surveyed covering its whole distribution area. The grains were also striate to striate-reticulate, and the exine sculpture was not different between varieties. However, the aperture number of S. japonica showed a geographical variation ranging from 4 to 6; in Sakhalin, Hokkaido, most part of Honshu and Taiwan, grains with 4-5 apertures were dominant, while in the southern part of Japan, including the Ryukyu Islands and the Izu Islands, those with 5-6 apertures were common.

Distyly and pollen dimorphism in Damnacanthus (Rubiaceae).

We document distyly in Damnacanthus (Rubiaceae) and investigate pollen morphology in the genus. Within nine taxa in Japan (four species, four varieties and one form), five show distyly. Non-distylous taxa of Damnacanthus always have a long style and short stamens. We examined the pollen morphology of all taxa of Damnacanthus in Japan. The pollen grains of Damnacanthus are 3- to 5-orthocolporate or sometimes loxocolporate. Polymorphic pollen is sometimes observed even within a single flower. Pollen dimorphism is associated with distyly; pollen grains from short-styled (thrum) flowers are significantly larger than those from long-styled (pin) flowers, and the muri are smooth in pin flowers whereas the pollen grains of thrum flowers have minute granules on the top and/or on the sides. In non-distylous taxa, in which the style is longer than stamens, the muri are always provided with minute granules. Interestingly, the pollen grains with granules in the non-distylous taxa contrast with the pollen of the distylous taxa, because granulate pollen grains are found only in thrum flowers. Although Damnacanthus is a stenopalynous genus, the number of colpi is useful for delimiting taxa. Based on palynological characters, we conclude that Damnacanthus should be revised.