To obtain a better understanding of the floral traits of sect. Heterotropa (genus Asarum, Aristolochiaceae), we conducted a detailed microscopic examination of the surface of the calyx, the floral part best representing the diversity of the lineage. The surface structures were highly variable among Heterotropa species, with remarkable variations in trichome morphology on the adaxial surface of the calyx lobes. Four types of trichome were identified on the calyx lobes, each of which was the sole type in most samples, although combinations of multiple types were occasionally seen. Variations were also found in the distribution of stomata on the surface of the calyx. Although the biological functions of the trichomes and stomata on the calyx are unclear, they may be related to pollination systems, e.g., through emission of the floral scents that are highly variable among Heterotropa species.
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