Morphology and ultrastructure of orbicules in the subfamily Ixoroideae (Rubiaceae)

Orbicules were studied in 43 species belonging to 32 genera of the five tri bes of the Ixoroideae (Rubiaceae). The orbicules were investigated with sca nning electron microscopy (SEM), light microscopy (LM), and transmission el ectron microscopy (TEM). Orbicules are present in all species investigated of the tribes Pavetteae, Octotropideae and Coffeeae. In the tribes Gardenie ae and Aulacocalyceae orbicules were found in some species: while they were absent in others. 15 species out of 11 genera lack orbicules. Three orbicu le types (III, V, and VI) could be distinguished, mainly on the basis of ge neral morphological and ultrastructural variations. Orbicules that belong t o type III (0.50-1.29 mu m) have perforations in their wall, a regular or i rregular shape and two or three electron transparent cores. This orbicule t ype, exclusively found in all Pavetteae species investigated, can be divide d into two subtypes. Orbicules of subtype IIIa are present in all genera re lated to Ixora, and orbicules of subtype IIIb in those genera related to Ta renna. Orbicules of type V (0.97-1.86 mu m) are present in Himalrandia tetr asperma (Aulacocalyceae), and in all Octotropideae genera investigated, exc ept Feretia. Complexes of more than three individual orbicules characterize this type. They are irregularly shaped and have many perforations as well as sporopollenin granules on the orbicule wall. In all species investigated of tribe Coffeeae, type VI orbicules (0.56-1.60 mu m) are present. These o rbicules are characterized by their embedded position towards the tapetal m embrane, their aggregated form and by the presence of perforations as well as sporopollenin granules on their orbicule wall. In the tribe Gardenieae d ifferent types of orbicule were found (V, VI and orbicules that cannot be c lassified in our typology). Our results suggest that orbicule characters in the Ixoroideae may be systematically useful on tribal level. (C) 2000 Else vier Science B.V. All rights reserved.