Pollen morphology of Alstonia (Apocynaceae)

The pollen of Alstonia has been studied by light and scanning electron microscopy. The pollen grains are always colporate. Four distinct pollen types could be recognized. Type 1 (2‐aperturate) and type 2 (3‐aperturate) have rounded endopores, while type 3 (?<36 μm) and type 4 (?>49 μm) possess H‐endoapertures. Probably, all four types have harmomegathically inactive apertures and folding mesocolpia. Type 1 is found in five species of section Alstonia. The other species of this section and the monotypic section Winchia have type 2. The sections Dissuraspermum, Monuraspermum and Tonduzia have pollen type 3. This type may be subdivided on the basis of some minor (overlapping) features, which more or less segregate Dissuraspermum from Monuraspermum. Palynologically, the monotypic American section Tonduzia appears to be most related to the Malesian/Australian/Pacific sections Monuraspermum and Dissuraspermum, and not to the African species of section Alstonia.