Abstract
A new species, Cleisostoma yersinii J. Ponert & Vuong, is described and illustrated based on the material collected in the Hon Ba Nature Reserve in southern Vietnam. In addition to conventional (macro)morphological examination we comparatively investigated root and leaf anatomy (using light and fluorescent microscopy), assessed nectar characteristics (using HPLC analysis), determined nuclear genome size (using DNA flow cytometry) and reconstructed phylogenetic relationships (using nrITS sequences). Cleisostoma yersinii differs from its putative closest relative C. birmanicum in wider and shorter leaves, larger flowers, distinct lip with S-shaped tip of the mid-lobe, and a shallow spur with two large nectar sacks separated by prominent calli and septum. Nectar is sucrose-dominant and very rich in sugars. Stomata are developed on both sides of the leaf and have prominent hyperstomatal chambers and substomatal cavities. Roots with well-developed exodermis and tracheoidal idioblasts are covered by a two-layer Vanda-type velamen. Chloroplasts occur not only in the cortex but are also abundant in the stele. Mean 1C-value was estimated to 2.57 pg DNA. An updated identification key is provided for SE Asian sections and all Vietnamese species of Cleisostoma.