Populoxylon sp.

Populoxylon sp. cf. Populus alba L.,

Fig. 6 View Fig , photos a-i.

Studied material. Two samples of petrified wood collected from Aetohory Evros, from late Oligocene volcano-sedimentary deposits, show a similar xylostructure, typical for the genus Populoxylon . They are registered and kept in the Collection of the Faculty of Geol. & Geoenviron., of NKUA, under the field numbers: Aet915, Aet986.

Microscopic description. Growth rings – are present, usually having a diffuse-porous structure to slightly semi-ring-porous, with quite indistinct ring boundaries, difficultly guessed between the late-wood with smaller vessels and the early-wood, which starts with larger vessels.

The vessels – appear solitary and grouped in radial multiples of 2-3(-6) pores, their arrangement defining the diffuse-porous wood structure in which with larger vessels of the early wood, that slightly diminish only in the final wood. The shape of the solitary vessels is rounded to radial-oval and, within the radial multiples, they are slightly deformed and larger and smaller vessels are present together. The large solitary vessels have the lumina size of 110-150 / 60-85 μm the radial / tangential diameter, to 60-80 / 40-50 μm in the final wood. Their walls are not too thick, having 3-4 μm double wall. In the longitudinal view, the vessels show simple perforated plates, inclined. The numerous intervessel pits of bordered type are polygonal, contiguous or slightly spaced, in alternate arrangement or slightly irregular. Their mean size is between 7-10 µm. The vessel-ray pits have much reduced borders, to apparently simple, corresponding to the cross-fields pits, described below. Helical thickenings in vessel elements are not present. The vessels’ density is 20-40 vessels per square millimeter. Mean vessel element length is up to 350 µm, or slightly more. Inside the vessels' lumina no tyloses can be seen, but often granular dark content or fungi appear.

Tracheids or vascular fibres or vasicentric tracheids - not present.

The fibers – in cross-section, constitute the major part of the ground-tissue, showing a large polygonal lumen, 12- 17 μm in diameter, and relatively thick walls (4-6 μm the double wall). Longitudinally the libriform fibers show poorly preserved pitting on the walls, and are not septate.

Axial parenchyma – appears very few or almost indiscernible.

The rays – are exclusively uniseriate. Ray height is usually high to very high, i.e. more than 1 mm. As a composition, the rays are homocellular, with cells all procumbent, the marginals slightly taller. In cross-fields with vessels, rounded or polygonal simple pits appear, alternate, in 2-3 rows, often difficult to observe because of dark content or since are poorly preserved. Sheath-cells or tile-cells are not present. Ray density: 4-12 rays / mm.tg, sometimes more.

Storied structures – of rays, parenchyma or vessel elements – absent. Secretory elements – as oil or mucilage cells not present. Intercellular canals – as normal or traumatic axial canals, radial canals not present. Cambial variants – as included phloem not present. Mineral inclusions – as any kind of prismatic crystals in the structure – not present.

Affinities and discussions. Two samples of petrified wood showed a similar dicotyledonous xylotomy, characterized by diffuse porous structure, with solitary and/or grouped as radial multiples of 2-3(-6) pores, with exclusively uniseriate and tall rays with some rows of alternate pits in cross-fields and, consulting the papers of Greguss (1954), Schweingrüber (1990), Schoch et al. (2004), Wheeler et al. (2011, for InsideWood) and Akkemik & Yaman (2012), we have observed affinities with current Salicaceous genera, especially with the genus Populus L.

The correspondent fossil genus - Populoxylon was created by Mädel-Angeliewa (1968), based on the study of Pliocene wood remains from coal deposits of Baccinello (Toscana, Italy) with designated type-species P. priscum Mädel-Angeliewa, 1968 . In the genus diagnosis it's noted: „diffuse porous wood, with usually grouped vessels that have simple perforations and intervascular pits numerous, alternate; exclusively uniseriate rays, homocellular, with cells all-procumbent, marginals higher; cross-fields with 2-3 horizontal rows of slightly oval pits and axial parenchyma few“.

Doutrelepont et al. (1997), described a Populoxylon sp. as having slightly different vessel distribution in cross-section, as similar to the extant Populus euphratica Olivier , which is slightly different of our specimens.Other fossil forms, characterized by the presence of axial parenchyma which appears few, marginal and scanty diffuse were described as Populoxylon sp. (cf. P. tremula L.), by Greguss (1969) from the Sarmatian from Mikfalva ( Hungary), and Populus sp. ( P. tremula L.?) described by Nastschokin (1968) from the Quaternary of Yenisei-river basin ( Russia). Also, some Miocene remains with similar structure have been described as Populoxylon tremuloides (in a Ph.D. Thesis: Iamandei, 2002, unpublished) and, it was sometimes, cited like this, even if is not a valid name (see Iamandei & Iamandei, 2005; Iamandei et al., 2008c). More recently, a Miocene Populoxylon sp. (cf. Populus tremula L.) was described from the Bulgarian Black Sea shore (see Iamandei et al., 2023a,b).

A new species of Populoxylon was described recently from Anatolia, by Akkemik (2021), as P. sebenense Akkemik , having higher rays and obvious marginal axial parenchyma, so it is slightly different of our specimens here described.

Another form of Populoxylon was described, from Carpathian area, by Iamandei et al., (2005 and 2011), as Populoxylon (cf. Populus alba L.), which is very similar to here studied specimens, having axial parenchyma few, almost indiscernible. The here studied specimens, collected from Evros, from Oligocene volcano-sedimentary from Aetohory, have a structure xylotomically almost identical to the extant species Populus alba L. Thus , based on this discussion, we assign the studied specimens to Populoxylon sp. cf. Populus alba L., as a fossil equivalent of this current species, which is native to Mediterranean zone, living in Atlas Mountains, South and Central Europe, Central Asia ( Populus alba - Wikipedia - accessed 05.02.2023).