Pollen of eight species of Cryptantha was studied using scanning electron microscopy. In addition, transmission electron microscopy was used in Cryptantha intermedia to elucidate the ultrastructural basis for their heterocolpate pollen, in which three "true" colporate apertures alternate with three "pseudoapertures." Both apertures and pseudoapertures are regions of the wall in which ektexine is largely absent. However, apertures have an outer band of verrucate exinous deposits, a band of inner intine wall material, and a concentration of cytoplasmic vesicles. In addition, apertures are shorter in length and wider at the equator than are pseudoapertures. The exine wall structure in Cryptantha intermedia consists of a typical inner, homogeneous endexine (thickened at both apertures and pseudoapertures) and, in non-apertural regions, an outer, essentially imperforate, tectate-columellate ektexine. Curious triangular "polar pseudoapertures," devoid of ektexine, occur at each pole and resemble the pseudoapertures in sculpturing and wall structure. The described heterocolpate pollen in the Boraginaceae may represent a major apomorphy for the tribes Eritricheae, Cynoglosseae, and possibly other taxa. However, further comparative studies within these groups are needed to definitively assess the extent of this putative apomorphy and to identify and characterize palynological features that may be useful within these groups.

Key words: Boraginaceae, Cryptantha, pollen, ultrastructure