Thaigardenia (Rubiaceae: Gardenieae), a new genus distributed from Thailand to South China

Abstract

Identified as Gardenia over a century ago, three known species from Thailand to South China differ considerably from typical members of that genus, from which growth habits, aspects of branch architecture and corolla shape set them apart. They form a new genus, here named Thaigardenia, the species of which are scrambling to thicket-forming shrubs to sometimes treelets or small trees. They have typically unequal (asymmetric) development of each internode that offsets what began as opposite pairs of axillary buds (and potential axillary branches) from subtending leaf axils at the same level, and small infundibular corollas with insignificant tubular bases. In contrast, typical Gardenia are non-scrambling shrubs or trees, often have extra-axillary buds or branches that consistently continue to develop at the same level (i.e., remaining opposite); and showy hypocrateriform (salverform) corollas with elongate tubular bases. The unequal development of different sides of an internode that brings an initially opposite pair of axillary buds (branches) to different levels, so that they do not appear paired subsequently, is, as far as is known, unique and unknown in other Rubiaceae or opposite-leaved plants; this shared feature is a key synapomorphic character for species of the newly recognised genus.