Title : Cambial rearrangement in cycads: First evidence from a basal seed plant lineage
Abstract:
Cycads, the sister clade to all other gymnosperms, retain numerous ancestral features, including flagellated sperm, large compound leaves, and dioecy. Despite long-standing interest in this lineage, significant gaps remain in our understanding of its secondary growth, particularly the structure and dynamics of the vascular cambium. In most woody plants, a single vascular cambium forms a continuous cylinder that produces secondary xylem inward and phloem outward, maintaining its structure through coordinated periclinal and anticlinal divisions, as well as rearrangements driven by intrusive growth. In cycads, however, multiple successive cambial cylinders arise, often originating from cortical parenchyma, yet their developmental behavior is not fully understood. This study investigates cambial organization and activity in cycas revoluta, using anatomical material obtained from a trunk collected in Florida in 2025. Transverse and radial sampling allowed analysis of multiple vascular cylinders.
Tissues were processed through standard histological methods, including embedding, microtome sectioning, and PAS staining. The vascular cambium in cycas revoluta is narrow, often consisting of a single layer of initials with differentiating derivatives arranged in regular radial files. Because secondary xylem elements change little during maturation, successive layers preserve snapshots of cambial structure, enabling reconstruction of cambial history. Frequent anticlinal divisions generate new radial files, many of which are subsequently eliminated. Evidence of intrusive growth was observed in both fusiform and ray initials, where cells intrude between neighbors to establish new positions within the cambium. These events are often coupled with the elimination of competing initials, supporting the hypothesis that intrusive growth involves spatial competition between two initials. Characteristic oblique (slanted) cell walls, indicative of cambial rearrangement, were also documented.
