Pollen characteristics were studied in 24 species of Onosma L. (Boraginaceae) in Iran by scanning electron microscopy (SEM), showing the occurrence of the Tricolporate, Stenopalynous, (syncolporate), Tricolporate, prolate subprolate, ellipsoidal, isopolar or heteropolar pollen grains in these species. The pollen surface was granular except in O. rostellatum having micro granulating pollen surface. In mentioned study 7 characters including : Pollen size in Polar and equtourial view, relative size of Polar/equtourial size, Pollen shape in polar and equtourial view, Wall thickness and granulating size were assessed. Our result show pollen characters are useful for species differentiation of Onosma and relatively differentiation in Sections.
