Transitional Pitting in Tracheids of Psilotum

Abstract

Most of the anatomical studies upon Psilotum have been in connection with the interpretation of the spore-bearing structures and with the general organization of the stele. Accounts of the anatomical organization of the axis have been largely topographical, discussing the distribution of the cortical tissues and the differentiation of the stelar tissues, principally as seen in cross-section (Bertrand, '85, Boodle, '04, Ford, '04, Stiles, '10). The position and order of development of the wood elements have been very definitely emphasized, whereas the nature of the elements themselves has scarcely been more than mentioned. It has been stated generally that the protoxylem elements are spiral tracheids, and that the metaxylem elements are scalariform or somewhat reticulate tracheids. Previous anatomical studies have furnished two illustrations of pitting types in the tracheids of Psilotum. Bertrand ('85) illustrated diagrammatically several types of tracheids which he had seen in longitudinal section in the stele of the subterranean axis. In the accompanying text he described spiral protoxylem elements and scalariform metaxylem elements with.