Abstract: In order to study the taxonomy of Boraginaceae further, pollen morphology of six species in Heliotro-pium, two species in Messerschmidia, and one species in Tournefortia from the subfamily Heliotropioideae was examined by light and scanning electron microscopy. Pollen grains are prolate. They are elliptic from the equato-rial view, and 6-lobate circular, 8-lobate circular or rarely triangular from the polar view. There are three aperture types in the subfamily Heliotropioideae, namely: (1) 3-colporate alternated with 3-pseudocolpate type, (2) 4-colporate alternated with 4-pseudocolpate type, and (3) 3-colporate type. The exine surface is faintly perforated, rugous decorative, or rarely psilate. The results showed that the three genera in the subfamily Heliotropioideae of Boraginaceae are closely related. Based on pollen features, the genus Tournefortia might be basal in Helio-tropioideae of Boraginaceae. Heliotropium seems to represent a more derived lineage than both Tournefortia and Messerschmidia, and the genus Messerschmidia lies between Heliotropium and Tournefortia. Pollen morphologi-cal features suggested that the subfamily Heliotropioideae is a transitional type from Ehretioideae to Boragi-noideae. Pollen grains in the genus Tournefortia of the Heliotropioideae are of 3-colporate aperture, implying a possible close relationship between Heliotropioideae and Cordioideae.

Key words: Heliotropioideae, pollen morphology, systematics