ULTRASTRUCTURAL CHARACTERS OF THE POLLEN OF SOME PODOSTEMALES - AFFINITIES WITH ADVANCED ROSIDAE

The order Podostemales are two pantropical families of aquatic planes living in running water: Tristichaceae (five genera, ten species) and Podostemaceae (35 genera, 200 species). Pollen of the 26 genera and 62 species studied is characterized by: a granular infratectum in which the granules are sometimes organized as columellae like units, and a l amellar and/or granular endexine in all pollen types, single or in dya ds. Most of the apertural characters and the ornamentation of the exin e allow us to distinguish the previously established taxonomic groups: Weddellinoideae have tricolporate rugulo-areolate, pollen with a smoo th apertural membrane; in Tristichoideae, pollen is periporare and the microspines of the tectum and of the apertural membrane are massive; in Podostemaceae, the rectal spines with their broad bulbous base are formed from numerous masses of sporopollenin, the endexine is microfib rillar at the base, the apertural membrane is constituted of structure d ectexinous masses, and the endexine is granular at the aperture. Pre sence of the tricolporate pollen type associates the Podostemales with the higher eudicotyledons. Most of the pollen characters of Podostema les and their variations are found among advanced Rosidae (Hamamelidal es, Polygalales, Santalales, Violales, Euphorbiaceae). ((C) Academie d es sciences/Elsevier, Paris.)