Abstract
A new species of Derris Lour. (Fabaceae), Derrislongiracemosa Boonprajan & Sirich., sp. nov., is described as the species bearing the longest inflorescences recorded in the genus to date, reaching up to 155 cm in length. Endemic to limestone areas in southwestern Thailand, it is possibly the third known limestone-adapted Derris species. Compared with its closest morphological relative and a partially sympatric species, D.solorioides, D.longiracemosa exhibits a longer and differently structured inflorescence, lower numbers of ovules per ovary (fewer than five ovules), and flowers that turn pinker with maturity. Leaf anatomical study reveals several differences, such as the shape of epidermal cells, the presence of secretory-like cavities, an atypical stomatal type that occasionally occurs, the presence of schizogenous cavities in the midrib cortex, and the distinct absence of lysigenous cavities in the pulvini cortex, as well as a thicker mesophyll compared to D.solorioides. Molecular phylogenetic analysis using nuclear ITS and plastid trnL-F and trnK-matK sequences confirms all four sampled populations as a single, well-supported species, distinct from other Derris taxa but showing a close relationship with D.rubrocalyx only in Bayesian inference. This combination of morphological, anatomical, and molecular evidence supports the recognition of Derrislongiracemosa as a distinct species. A detailed description, distribution map, line drawing, photographs, and preliminary IUCN conservation status are provided.