Transfer cells in Horneophyton lignieri illuminate the origin of vascular tissues in land plants.

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. We re-investigate the vascular system of Horneophyton lignieri, an exceptionally preserved Rhynie Chert fossil central to understanding early plant evolution. Using confocal laser scanning microscopy combined with 3D modelling, we achieved higher resolution and precision in reconstructing cell morphology than earlier studies that relied on white light microscopy. We show that the vascular system of H. lignieri lacks distinct xylem and phloem tissues, contrary to prior assumptions. Instead, tissues with transfer cell-like structures are prominent, and both cell type and cell wall development vary with position in the plant. These findings indicate that the ancestral vascular system of land plants likely consisted of a single type of conducting cell capable of both solute transport and water conduction. Our results show that H. lignieri is not a tracheophyte, supporting emerging models of a morphologically and cellularly complex MRCA for land plants.