The predominantly tropical family Rubiaceae is with ca. 13.000 species one of the largest families of angiosperms. Contrary to some other huge families, the pollen morphological variation within the family is impressive. Although the tricolporate pollen type dominates by far, a number of tribes (e.g., Psychotrieae, Spermacoceae, Sabiceeae) are highly eurypalynous. In many cases, the pollen morphological diversity observed has a high systematic value. Pollen grains are mostly dispersed as monads, but a small number of genera are characterized by permanent tetrads; polyads occur in a single genus, Massularia. Pollen size ranges from 6 to 120 Ám; shape varies from oblate to prolate, but is most often subspheroidal. Number of apertures ranges from 0 to 30. They are generally compound, i.e. made up by different apertures one above the other. The sexine pattern is also variable, including psilate, perforate, microreticulate to reticulate, and rugulate patterns. Supratectal elements are mostly absent, but if present they show a wide array of morphological variation. Orbicules are often present and can be spherical, doughnut-shaped, irregularly folded or spinulate. The results presented are a summary of the pollen morphological observations achieved in the last 7 years at the Laboratory of Plant Systematics and the National Botanic Garden of Belgium. We aim to explore the pollen and orbicule morphological diversity of the Rubiaceae in order to assess the systematic value of the respective characters. The database of all genera investigated with reference to our papers will be available soon at http://www.kuleuven.ac.be/bio/sys/pollen.

Key words: orbicules, pollen morphology, Rubiaceae, systematics