Comparative pollen development in Dioscoreales

Pollen and orbicule ontogeny in representatives of three genera of Dioscoreales - Narthecium ossifragum Huds. (Nartheciaceae), Tacca artocarpifolia Seem. and Tacca chantrieri Andre ( Dioscoreaceae), and Dioscorea communis ( L.) Caddick & Wilkin ( Dioscoreaceae) - is described and illustrated using LM, SEM, and TEM. The main difference is in microsporogenesis, which is successive in Narthecium Huds. and simultaneous in Tacca J.R. & G. Forst. and Dioscorea L. This is reflected in the tetrad configuration but not in the apertures: Narthecium and Tacca are monosulcate, while Dioscorea has two equatorial apertures. Other features of pollen development are similar in all three genera. Exine development starts at the proximal pole, and during the tetrad stage, the exine remains thicker at this pole. Intine development begins before mitosis. A conspicuously channeled intine forms beneath the aperture(s) in all three genera, and in Tacca it also occurs in nonapertural regions. After mitosis, the generative cell is situated at the proximal pole in all genera. Orbicules are present in Narthecium and Dioscorea but not in Tacca, although it has a secretory tapetum. In the tetrad stage, pro-orbicules appear on the tapetum in Narthecium and Dioscorea, and pro-sporopollenin is consequently deposited to form the orbicule wall.