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.

中文翻译：

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晚古生代 Sigillaria approximata Fontaine et White (Lycopsida) 近端木材中的非典型管胞组织

据认为，树状石松形成层扩张通过纺锤状初始的切向加宽与径向木材生长保持同步，导致管胞直径逐渐向外扩张，并且不存在从头管胞文件。这种模式出现在远端石松轴中，但尚未在最近端的树干脉管系统中进行研究，其中木材最厚，初生木质部最薄。这里描述了近似 Sigillaria approximata 的地面躯干脉管系统化石的横截面。这种近端脉管系统在两个主要方面与当前石松木生产模型的预期不同：首先，内部和外部木管胞具有大致相同的管腔直径，因此不存在离心增加管腔直径的趋势；第二，从头细胞文件通常出现在木材中。形成层周长向外扩张的速率与新木管胞锉出现的向外速率密切相关，表明新锉的添加是形成层近端扩张的主要方式。由于这种脉管系统只能在横切面上进行研究，因此无法确定产生新文件的发育模式。然而，该标本的证据足以证明树状石松木的生产比以前理解的更为复杂。无法确定生成新文件的开发模式。然而，该标本的证据足以证明树状石松木的生产比以前理解的更为复杂。无法确定生成新文件的开发模式。然而，该标本的证据足以证明树状石松木的生产比以前理解的更为复杂。