Tree fern growth strategy in the Late Devonian cladoxylopsid species Pietzschia levis from the study of its stem and root system.

Portions of stems from five new anatomically preserved specimens of Pietzschia levis from a new Late Devonian plant locality of eastern Tafilalt, Anti-Atlas (Morocco), were analyzed to complete the preliminary reconstruction previously done with a single specimen. The basal part of the longest new specimen consists of an obconical portion of stem surrounded by a thick mantle of adventitious roots. Roots are connected to the peripheral strands of primary xylem specific to the stele of Pietzschia stems. Roots grow outwardly; they cross the cortex and the broad central pith at a steep angle and emerge from the stem lower down. The number of roots produced at one level increases conspicuously from the base towards the distal end of the obconical portion of stem. By contrast, cross-sectional dimensions of roots at their origin level decrease distally. Individual roots increase in diameter, and their stele gets more lobed as they grow through stem tissues. The large number of roots at the specimen base and their wider dimensions at this level contribute to the conspicuous enlargement of the stem base. Patterns assessed from the reconstruction of the Pietzschia levis root system may be close to those of the older cladoxylopsids Pseudosporochnales comprising an upright trunk. Growth strategies in the small-statured species P. levis and in younger arborescent ferns of the Psaronius type are compared. They differ mainly in the relative lengths of epidogenetic vs. apoxogenetic growth phases of the stem.