Abstract

L'รฉtude de nombreux fragments d'axes silicifiรฉs avec structure conservรฉe collectรฉs au Kazakhstan (U.R.S.S.) dans un niveau d'รขge dรฉvonien moyen et rapportรฉs ร  l'espรจce Xenocladia medullosina a permis de dรฉcrire leur structure complรจte, de prรฉciser que leurs faisceaux vasculaires sont anastomosรฉs et doivent รชtre considรฉrรฉs comme des mรฉristรจles. La stรจle de Xenocladia medullosina est fondamentalement monostรฉlique et non polystรฉlique; elle est polymรฉristรฉlique. Par ailleurs, la mise en รฉvidence de la prรฉsence de cette espรจce dans de Dรฉvonien moyen au Kazakhstan constitue un nouvel รฉlรฉment commun avec la flore de mรชme รขge des U.S.A. ce qui va ร  l'encontre de l'idรฉe d'une province floristique particuliรจre au Kazakhstan durant le Dรฉvonien moyen.The study of numerous silicified pieces of axis withtheir structure preserved collected in Kazakhstan (U.S.S.R.) from a horizon dated as middle Devonian and referred to the species Xenocladia medullosina has made it possible to describe their structure, and demonstrates that their vascular strands anastomose and should be considered as meristeles. The stele of Xenocladia medullosina is fundamentally monostelic and not polystelic; it is polymeristelic. The evidence for the occurrence of the species Xenocladia medullosina in the middle Devonian of Kazakhstan constitues a new common element with floras of the same age in the U.S.A., which weakens the concept of a distinctive floristic province in Kazakhstan during the middle Devonian.

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