Pollen grains of 16 species of Microula Benth. and six species of three

 related genera were examined under LM and SEM, and four of them also under  TEM. Pollen grains of Microula and three related genera are dumb-bell-shaped, 3-colporate apertures alternate with three pseudocolpi. Pollen grains are very small,ranging from 12.18 x 7.13 μm to 6.36 x 3.66μm. In general, colpi with os are

wider and shorter, rhomboid, but sometimes they are equal to pseudocolpi in length. Colpus margins are regularly or irregularly tooth-like. The surface of colpi is psilate or processed. Ora are circular or lalongated in outline, protruded or not;  surface of os membrane is smooth or scabrid.  The exine is usually indistinctly layered under LM. The exine surface is psilate, and more or less perforate. TEM examination shows that the pollen wall is differentiated into exine and intine: the exine includes ectexine and endexine, while the ectexine consists of

tectum, columellae and foot-layer. However, there are differences in constriction of equatorial area, apertural characters, ornamentation and exinous ultrastructure between these genera.  Pollen morphology indicates that the genus Microula Benth. is primitive, directly related to the genus Actinocarya Benth .;  the genus Asperugo L. Is more advanced. The genus Eritrichium Schrad. which has two ora or one os and is anisopolar, represents the most advanced group among them. Noteworthily, the diorate phenomenon is found for the first time not only