Development of Pollen Grain Walls and Accumulation of Sporopollenin

By means of electron microscopy, we studied the development of pollen grain walls in Calendula officinalis L., Dimorphotheca aurantiaca DC., and Cichorium intybus L. (Asteraceae). As a reference, we studied the plants from the families Schisandraceae (Schisandra chinensis (Turcz.) Baill.), Lauraceae (Persea americana Mill.), Boraginaceae (Borago officinalis L.), and Cycadaceae (Encephalartos altensteinii Lehm.). In Asteraceae, we revealed two successively initiated layers of glycocalyx that form outer and inner layers of the ectexine. The formation of endexine is contributed by plasma membrane and small vesicles. Glycocalyx in the plants from the families Schisandraceae, Lauraceae, Boraginaceae, and Cycadaceae was found to consist of radially arranged helical cylindrical units, which are receptors of sporopollenin deposition. It is assumed that the receptor-independent accumulation of sporopollenin is also possible.