Stigma of the African Genera Rutidea and Nichallea (Rubiaceae‐Ixoroideae‐Pavetteae): Highly Modified Receptive Surfaces

Abstract

The tropical African genera Rutidea and Nichallea possess secondary pollen presentation, as do many other Rubiaceae, but the style‐stigma complex shows some special features. Style and stigma form one slender structure that terminates in a fusiform to clavate or broadly elliptic to globose head. The stigma consists of two permanently fused stigmatic lobes, a common enough situation within the Rubiaceae, but it is further differentiated into three zones: a narrow stigmatic base, a stigmatic head, and the free tips of the stigmatic lobes. The pollen‐receptive surfaces are highly modified. They are restricted to the adaxial sides of the free stigmatic tips and to the lines of fusion between the stigmatic lobes, visible as two longitudinal furrows on the style‐stigma complex. Compared to the structure of the style‐stigma complex in other genera of the Rubiaceae with permanently fused stigmatic lobes, a major morphological trend can be observed in Rutidea and Nichallea, notably the displacement of the pollen‐r...