Derris solorioides (Fabaceae), a new limestone species with true-paniculate inflorescences from North-Central Thailand

Derris solorioides is described as a new species and illustrated. This species is only the second calciphilous and true-paniculate species of Derris ever recorded. The species was found in isolated and protected limestone areas surrounded by agricultural areas in Nakhon Sawan province, North-Central Thailand. It is characterized by its rather smaller flowers but with more ovules than other species of Derris, and 1-winged pods showing a dark-coloured pericarp around the seeds without thickening of the pericarp. The characters of the pods are similar to those found in Solori, a genus once synonymized with Derris and, therefore, the epithet ‘solorioides’ was assigned. This species appeared to be a distinct taxon in the molecular phylogeny, separate from its morphologically highly similar species, D. marginata. It is also a member of a lineage of Derris consisting of species with a deviating type of inflorescence: intermediate forms and true panicles, which is quite uncommon in this genus. The relationship with its closely related species is discussed, and a key to the species of Derris in the ‘deviating type of inflorescence’ clade is presented.