Pollen development of Rondeletia odorata (Rubiaceae)

Pollen wall ontogeny of Rondeletia odorata was studied with transmission el ectron microscopy (TEM) and scanning electron microscopy (SEM) from tetrad stage until maturity. The ontogenetic sequence of wall development in Ronde letia follows, to some extent, the basic scheme in the angiosperms, i.e., d evelopment starts centripetally with the pro-columellae in a plasmalemma su rface coating (primexine) at the early tetrad stage when the microspores ar e still enveloped by callose, until intine formation in young pollen grains . The main ontogenetical features of Rondeletia odorata pollen are (1) the very thin irregular foot layer, (2) development of a continuous layer of ra dially oriented membranous granular material under the thick endexine, (3) initiation of intine before first mitosis with characteristic radial plasma lemma invaginations, and (4) a strong stretching force upon engorgement jus t prior to dehiscence, which leads to reduction in thickness of all wall la yers. The possible function of Golgi vesicles in the considerable increase in surface area of the plasmalemma at intine initiation is discussed. The e ndocingulum observed on acetolyzed and sectioned mature grains is explained ultrastructurally.