Multiple xylem strands in an unidentified Lower Permian plant from Döhlen basin
deutsche Version
Multiple xylem strands are known from the stems of fossil trees: Ribbon-like strands of primary xylem run along the center of the Psaronius tree ferns, their number increasing as the center becomes wider with height [1,2]. Wooden strands of various shapes are present in the stem of medullosean seed ferns and in Rhexoxylon, an Upper Triassic /Lower Jurassic tree of uncertain affiliation. Thin multiple strands have been found in several paleozoic seed fern frond stalks [3]. Double xylem strands are a common sight in the Lower Devonian Rhynie chert, where the prongs of the forking central strand of Nothia keep growing closely together for some distance while the shoot does not yet fork. Even four-fold strands can form in this way, or three-fold ones if the second forking is not simultaneous in the two prongs. A peculiar type of unsymmetric growth after forking of the strand is observed with Trichopherophyton.multiple xylem strands, Lower Permian

Fig.1: Multiple xylem strands on a Lower Permian plant cross section. Width of the picture 7mm.

Apparently the xylem strands found in one chert sample from Döhlen basin (Fig.1) differ from any one of the various types of multiple strands mentioned above.  As it is often observed with xylem strands, they are much better preserved than the surrounding tissue. The latter has mostly vanished in the present sample. What has remained in addition to the strands is a well preserved envelope, probably a kind of sclerotic sheath, and a faintly seen epidermis with cells 25 to 40 µm wide. There had been some tissue between the sheath and the epidermis, judging from a gap or squeezed cells seen there in some places. (Two thin-walled squeezed empty sheaths are seen in Fig.1 below the big sheath with the 8 strands.)
kinked xylem strand, Lower Permian
Fig.2: Kinked xylem strand with cells cut both across (far left) and lengthwise. Note the arrays of pits faintly seen on some of the cell walls. Width of the picture 1.6mm.

The overall structure of the cross-sections with separate strands of primary xylem enclosed with a sheath (Fig.1) resembles that of the Psaronius center but the pitted cell walls of the tracheids (Fig.2) and the peculiar cells oriented across the strand do not seem to be compatible with marattialean ferns. The structure is partially similar to but also different from that of seed fern frond stalks, as of Stenomyelon, for example [3].
Detached sporangia resembling those of leptosporangiate ferns, sterile fern-like pinnules, and plant debris of uncertain origin contribute to the confusing but intriguing impression made by this chert sample.

This unique sample had been collected and provided by W. Schwarz (1952-2012).

H.-J. Weiss

[1]  G.W. Rothwell, A.H. Blickle: Psaronius magnificus ..., J. of Paleontology 56(1982), 459-468.
[2]  H. Steur, H. de Kruyk: Psaronius, een boomvaren ... (in Dutch), Grondboor & Hamer nr. 3/4 (2004), 75-83.
[3]  T.N. Taylor, E.L. Taylor, M. Krings: Paleobotany, Acad. Press 2009
Scolecopteris pinnule cross-section, Sardinia Permian Chert News11
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