Comparative ultrastructure of the megaspores of the Triassic lycopsid Pleuromeia rossica Neuburg

The ultrastructural study of the megaspores in Pleuromeia rossica clearly d emonstrates that these spores have, besides the features common to the mega spores of all the fossil and recent lycopsids, the few structural character istics regarded as distinctive of the isoetalean megaspores. These isoetale an characteristics of the megaspores of P. rossica, in addition to the ultr astructural similarities of its microspores with those of the living Isoete s (shown recently), reinforce the assumption of a close relationship betwee n these lycopsids. (C) 2000 Academie des sciences / Editions scientifiques ct medicales Elsevier SAS.