Anatomy of Stigmaria asiatica Jongmans et Gothan from the Asselian (lowermost Permian) of Wuda Coalfield, Inner Mongolia, North China

Abstract

Stigmaria asiatica Jongmans et Gothan is a common species from the Permo-Carboniferous of East Asia. It is characterized by a relatively slender rhizomorph and represents the underground rooting system of lowland arborescent lycopsids. This species from the Wuda Coalfield (Asselian, lower Permian) represents the recovery taxon dominating the subsequent peat-forming vegetation after a volcanic eruption destroyed the previous flora, termed the Wuda Tuff Flora (Chinese “vegetational Pompeii”). It is characterized by novel downward axes, probably for access to deeper groundwater. Here, the anatomy of S. asiatica, including that of the axes and rootlets, is reported in detail for the first time. The axis contains stelar tissues, including pith, primary and secondary xylem, and phloem, and both primary and secondary cortical tissues. The primary cortex is tripartite and is divided into inner, middle, and outer zones. Secondary cortex is produced in the interior of the outer cortex splitting the outer cortex into inner and outer portions. The homogeneous pith and mesarch primary xylem maturation of S. asiatica are different from previously recorded species of Stigmaria Brongniart. Rootlets are composed of a central monarch vascular bundle surrounded by inner cortex and a ring of outer cortex. A vacant region is present between the inner and outer cortices. In some cases, connectives between the inner and outer cortices are present. According to the presented statistical analysis, it is proposed that rootlets of S. asiatica were highly branched, with at least for 7 bifurcations. Rootlets, together with the downward penetrating rhizomorph, helped to both anchor the plant and search for and absorb deep groundwater.