Sprecher
Beschreibung
Recent fossil discoveries and advances in plant phylogeny have renewed debate about the most recent common ancestor (MRCA) of land plants and the evolution of its fundamental organs and tissues. Here, we re-examine the conducting system of Horneophyton lignieri (≡ Hornea lignieri), one of seven land plants preserved in the Rhynie Chert. Kidston and Lang (1920) originally described its conducting tissues as a stele composed of a ring of phloem surrounding a solid xylem core of tracheids—a view largely accepted in subsequent literature, though not without challenge.
Using confocal laser scanning microscopy combined with 3D modelling, we obtained higher resolution reconstructions of cell morphology than previously possible with white-light microscopy. Our analyses show that H. lignieri lacks discrete xylem and phloem tissues, contradicting long-standing interpretations. Instead, the conducting system is dominated by tissues bearing transfer-cell-like wall structures, comparable to those recently documented in charcoalified early land plants from other localities. Both cell type and cell wall architecture vary systematically with position in the plant.
These findings suggest that the ancestral conducting system of land plants comprised a single, multifunctional cell type capable of both solute transport and water conduction. Consequently, H. lignieri should not be regarded as a tracheophyte. Our results support emerging models in which the MRCA of land plants possessed a morphologically and cellularly complex body plan.