Department of Biological Sciences, Humboldt State University, Arcata, California 95521, USA.
Am J Bot. 2013 Apr;100(4):754-63. doi: 10.3732/ajb.1300024. Epub 2013 Mar 26.
Secondary xylem (wood) produced by a vascular cambium supports increased plant size and underpins the most successful model of arborescence among tracheophytes. Woody plants established the extensive forest ecosystems that dramatically changed the Earth's biosphere. Secondary growth evolved in several lineages in the Devonian, but only two occurrences have been reported previously from the Early Devonian. The evolutionary history and phylogeny of wood production are poorly understood, and Early Devonian plants are key to illuminating them.
A fossil plant preserved anatomically by cellular permineralization in the Lower Devonian (Emsian, ca. 400-395 million years old) Battery Point Formation of Gaspé Bay (Quebec, Canada) is described using the cellulose acetate peel technique.
The plant, Franhueberia gerriennei Hoffman et Tomescu gen. et sp. nov., is a basal euphyllophyte with a centrarch protostele and metaxylem tracheids with circular and oval to scalariform bordered multiaperturate pits (P-type tracheids). The outer layers of xylem, consisting of larger-diameter P-type tracheids, exhibit the features diagnostic of secondary xylem: radial files of tracheids, multiplicative divisions, and a combination of axial and radial components.
Franhueberia is one of the three oldest euphyllophytes exhibiting secondary growth documented in the Early Devonian. Within the euphyllophyte clade, these plants represent basal lineages that predate the evolution of stem-leaf-root organography and indicate that underlying mechanisms for secondary growth became part of the euphyllophyte developmental toolkit very early in the clade's evolution.
维管形成层产生的次生木质部支撑着植物体型的增大，是维管植物中最成功的分枝模式的基础。木本植物建立了广泛的森林生态系统，极大地改变了地球的生物圈。次生生长在泥盆纪的几个谱系中进化，但此前仅从早泥盆世报道过两次发生。木质部的产生的进化历史和系统发育还知之甚少，早泥盆世的植物是阐明这些问题的关键。
通过在加拿大魁北克加斯佩湾下泥盆统（埃姆斯期，约 4.00-3.95 亿年前）电池点组中通过细胞质全矿化保存解剖结构的化石植物，使用醋酸纤维素剥离技术进行了描述。
该植物，Franhueberia gerriennei Hoffman et Tomescu gen. et sp. nov.，是一种基生真叶植物，具有中心原木质部和具圆形、椭圆形至梯状具缘多角形孔（P 型管胞）的后生木质部管胞。木质部的外层由较大直径的 P 型管胞组成，具有次生木质部的特征诊断：管胞的径向排列、倍增分裂以及轴向和径向成分的组合。
Franhueberia 是早泥盆世记录的三种最古老的具次生生长的真叶植物之一。在真叶植物分支中，这些植物代表了茎-叶-根器官发生进化之前的基础谱系，表明次生生长的潜在机制在分支的进化早期就成为了真叶植物发育工具包的一部分。
