ИСТИНА

Anatomical structure of the permineralised stems from the Upper Devonian deposits of North Russia (North, and Middle Timan) has been studied. Anatomically-preserved trunk of Callixylon was firstly found in the Upper Devonian of the Middle Timan. Pith cells are badly preserved in the stem. The metaxylem is of a mesarch type. In secondary branches metaxylem elements is represented by narrow polygonal tracheids with scalariform bordered pits both on radial and tangential walls of the tracheids. Secondary xylem is of a pycnoxylic type of the wood. In the radial section the tracheids of secondary xylem are narrow (19-45 μm in diameter) and very long (about 1 mm long). The tracheids on radial walls have distinct groups of bordered pits horizontally aligned in various tracheids. Such type of radial pitting is designated as cohortoid pitting (Snigirevskaya, 2000), which is typical for genus Callixylon. The pits are hexagonal in outline; 8 - 11 μm in diameter. Xylem rays are low, mostly uniseriate, and rarely biseriate. Ray cells are rectangular in shape. Cross-fields show 6-14 pits with inclined narrow apertures. Earlier stem fragments of Callixylon were found in the Late Devonian locality of North Timan (Snigirevskaya, Snigirevsky, 2001). Other permineralised stem fragments originate from the Upper Devonian (Upper Frasnian) of the Cape of East Ludovaty (North Timan). Anatomical features of Pseudobornia have been firstly studied with a scanning electron microscopy. The branches of different orders were examined. Exceptionally the tracheids of primary xylem were found in these branches. Primary xylem is characterized by narrow long tracheids with spiral and scalariform thickenings both on radial and tangential walls. The tracheid pitting is uniseriate. The anatomical features of the branches of different orders are similar. Most likely the stems of Pseudobornia did not have secondary growth.