Atypical tracheid organization in proximal wood of late Palaeozoic Sigillaria approximata Fontaine et White (Lycopsida)

It is thought that arborescent lycopsid cambial expansion kept pace with radial wood growth via tangential broadening of the fusiform initials, resulting in progressive outward expansion in tracheid diameters and an absence of de novo tracheid files. This pattern appears in distal lycopsid axes but has not been investigated in proximal-most trunk vasculature where wood was thickest and primary xylem thinnest. Here, a ground-level trunk vasculature fossil of Sigillaria approximata in transverse section is described. This proximal vasculature diverges from the expectations of the current lycopsid wood production model in two main ways: first, inner and outer wood tracheids have approximately the same lumen diameters, such that there is no trend towards centrifugally increasing lumen diameters; and second, de novo cell files commonly appear within the wood. The outwards rate of cambial circumference expansion closely tracks the outwards rate of new wood tracheid file appearance, suggesting that the addition of new files is the primary way the cambium expanded proximally. Because this vasculature can only be studied in transverse section, the developmental mode producing new files cannot be determined. However, evidence from this specimen is sufficient to demonstrate that arborescent lycopsid wood production was more complex than previously understood.

Automated summary

It is suggested that the expansion of arborescent lycopsid cambium in radial wood growth was accompanied by the broadening of fusiform initials, which led to the progressive outward expansion of tracheid diameters without the formation of new tracheid files. However, a fossil of proximal trunk vasculature of Sigillaria approximata shows some deviations from the expected lycopsid wood production model, with inner and outer wood tracheids having similar lumen diameters and de novo cell files commonly appearing within the wood. The rate of cambial circumference expansion is closely correlated with the rate of new wood tracheid file appearance, suggesting that the addition of new files is the primary way the cambium expanded proximally.