New insight into fern vascular evolution
A recent study led by Assistant Professor Jacob S. Suissa from the University of Tennessee, Knoxville, challenges a long-standing view of how ferns evolve their internal vascular systems. Rather than stem vasculature adapting primarily to environmental conditions, Suissa’s team found that shifts in leaf arrangement along the stem covary with the number and layout of vascular bundles. This implies that leaf placement can steer the evolution of fern vascular patterns, a perspective that reshapes our understanding of plant development over deep time.
The study, published in Current Biology, shows a striking 1:1 relationship between the number of leaf ranks along a stem and the number of vascular bundles. In other words, as the number of leaf positions increases, the vascular bundle count tends to rise in parallel. Even more telling is how the spatial arrangement of leaves influences the organization of vascular bundles within the stem. When leaves are arranged spirally, the vascular bundles form a radial pattern; when leaves cluster toward the dorsal side, the stem’s vasculature resembles a smiley-face arrangement, with an elongated ventral bundle and scattered dorsal bundles near the top.
“As leaf number increases, we see a direct 1:1 increase in vascular bundle number, and as the placement of leaves along the stem changes, we also see a shift in the arrangement of vascular bundles in the stem,” Suissa explained. The observation initially came from close field study of the tobacco fern, Mickelia nicotianifolia, a creeping fern with a long stem that climbs trees. In Costa Rica, Suissa noted that the leaves consistently emerge on the dorsal or upper side of the stem, and the vascular pattern appeared “whimsical” at first glance—whimsical because it resembled a smiley face with a distinctive arrangement of vascular bundles tied to leaf placement.
To test whether leaf arrangement truly governs stem vasculature, Suissa and his team examined 27 fern genera, representing roughly 30% of fern species diversity. They then employed a microCT scanner at UT’s Advanced Microscopy and Imaging Center to study internal stem anatomy noninvasively, avoiding physical dissection. The results revealed a robust correlation: the number and placement of leaves along the stem predict both the number and the spatial configuration of vascular bundles within the stem. The “smiley face” vasculature, for instance, aligned with leaves placed on the stem’s upper side.
Implications for how we study plant evolution
The findings run counter to more than 150 years of research that treated vascular patterning as an independently evolving trait shaped mainly by natural selection and environmental pressures. Suissa’s work suggests a developmental covariation—an intrinsic linkage—between leaf arrangement and stem vasculature. This implies that the evolution of fern vascular architecture cannot be fully understood without considering how leaves and stems co-evolve as an integrated system.
“Often, we think of research as requiring high-tech instrumentation, and intensive manipulative experiments in the lab, but natural history observations can yield quite impactful insights,” Suissa said. “This work combines field observations with modern imaging and quantitative methods to trace trait evolution over deep time.”
Looking ahead, Suissa is expanding this inquiry to a comprehensive dataset spanning over 300 species across all fern families. The goal is to map how vascular architecture—ranging from cellular details to large-scale stem patterns—has evolved across the fern lineage over roughly 400 million years. The research aims to reveal how various scales of construction in the vascular system have emerged and diversified, guided in part by leaf placement along the stem.
Why this matters for botany and evolution
Understanding the covariation between leaf arrangement and stem vasculature reshapes our models of plant development and evolution. It highlights the importance of looking at trait suites—the interconnected traits that evolve together—rather than isolating single features. For educators and researchers, the study offers a fresh framework for interpreting fern diversity and the evolutionary history embedded in their vascular anatomy.
