OSMUNDA-WEHRII, A NEW SPECIES BASED ON PETRIFIED RHIZOMES FROM THE MIOCENE OF WASHINGTON

Silicified rhizomes from Miocene strata near Yakima, Washington represent a new species of Osmunda [named O. wehrii ]. The stems are 8-13 mm in diameter and are surrounded by a thick sheath of adherent leaf bases, each of which has stipular expansions typical of the Osmundaceae. The new species has an ectophloic siphonostele in which the xylem cylinder is dissected by leaf gaps with 12-14 strands visible in a given stem cross section. Such sections have 12-16 leaf traces in the cortex. The xylem of each leaf trace diverges from the xylem cylinder of the stem as an adaxially concave strand with its protoxylem organized into a single medial adaxial cluster. Initial bifurcation of the leaf-trace protoxylem occurs as the leaf trace passes through the outer cortex of the stem. In the basal part of the stipular region of the petiole base, thick-walled fibers form an arch on the abaxial side of the sclerenchyma ring around the petiolar bundle. This arch persists throughout most of the length of the stipular region, with the thick-walled fibers reorganizing into 2 lateral masses in the distal part of the stipular region. Similar thick-walled fibers form an elongate strip of tissue in each wing of the stipule with several small clusters scattered near the sclerenchyma ring. The new species belongs to the subgenus Osmunda and shows that during the Neogene, the latter existed as a group of closely related species much as it does today. O. wehrii combines features of the modern O. regalis, O. japonica and O. lancea with those of O. claytoniana and supports the inclusion of the latter species in the subgenus Osmunda.