Ultrastructure of Heterocolpate Pollen in Cryptantha (Boraginaceae)

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 central region 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 C. intermedia consists of a typical inner, homogeneous endexine and, in regions other than apertures and pseudoapertures, 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 and Cynoglosseae and possibly other taxa. However, further comparative studies within these groups are needed to assess definitively the extent of this putative apomorphy and to identify and characterize palynological features that may be useful within these groups.