New Insight Into Fern Evolution Challenges Centuries-Old Assumptions
Leaf arrangement appears to steer the internal wiring of ferns, reshaping our understanding of how these ancient plants evolved. In a study led by Assistant Professor Jacob S. Suissa of the University of Tennessee, Knoxville, researchers found that the pattern of vascular bundles inside fern stems shifts in direct association with how leaves are distributed along the stem. This discovery suggests that leaf placement, not just environmental pressures, has a powerful influence on the evolution of fern vasculature.
The findings, published in Current Biology, indicate that as the number of leaves on a stem increases, there is a nearly one-to-one increase in the number of vascular bundles. Moreover, the spatial arrangement of these bundles changes in tandem with leaf placement. The result challenges a long-standing view that vascular patterns evolve primarily as independent responses to environmental conditions.
From Field Observations to High-Tech Imaging
The work began in the field in January 2025, when Suissa returned to Costa Rica to teach a Tropical Fern Biodiversity course he first took as a graduate student. While examining Mickelia nicotianifolia, commonly known as tobacco fern, Suissa noticed the leaves consistently emerge on the dorsal, or upper, side of the stem and that the stem’s vascular layout resembled a whimsical “smiley face.” This pattern featured an elongated ventral bundle with several separate bundles toward the top, seemingly tied to leaf placement.
Back in Knoxville, Suissa expanded the study to 27 fern genera, encompassing roughly 30% of fern species diversity. To examine internal stem anatomy without destructive cutting, the team employed a microCT scanner—an advanced imaging technique housed at UT’s Advanced Microscopy and Imaging Center. The noninvasive approach allowed researchers to map the internal vascular architecture while keeping the specimens intact for future study.
A 1:1 Link Between Leaves and Vascular Bundles
The results revealed a striking and consistent correlation: the number of leaf ranks along the stem mirrored the number of vascular bundles inside the stem. In some cases, five leaf ranks corresponded almost exactly to five vascular bundles. Even more telling was how leaf arrangement dictated the spatial pattern of these bundles. Spiral leaf placement produced a radial bundle arrangement, while a dorsal shift in leaf placement generated the smiley-face vascular pattern observed in tobacco fern.
According to Suissa, the implication is clear: the vasculature in the stem does not evolve as a standalone trait; it is developmentally covaried with leaf placement. “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,” he explained. This reframes our understanding of fern anatomy and evolution, showing a deep, coordinated link between leaf development and internal stem structure.
Implications for How We Study Plant Evolution
Historically, researchers examined fern vascular patterns as independent traits subject to natural selection. Suissa’s work suggests a more integrated view, where leaf architecture constrains and guides vascular evolution. He notes that successful interpretation of vascular evolution requires considering both leaf placement and stem anatomy together rather than in isolation.
Suissa’s ongoing research is expanding to a global dataset spanning hundreds of species and all fern families. The goal is to trace how vascular architecture has evolved over roughly 400 million years, across scales from cellular arrangements to larger stem and leaf patterns. By integrating field observations, high-resolution imaging, and quantitative evolutionary methods, the project aims to illuminate how different levels of plant construction—cellular and architectural—co-evolve in ferns.
Future Directions and Educational Impact
Beyond advancing scientific understanding, this work holds value for teaching and biodiversity conservation. Suissa emphasizes that natural history observations can yield powerful insights when paired with modern technology. He continues to mentor students in field-based data collection and imaging analyses, underscoring the collaborative and interdisciplinary nature of contemporary plant science.
As the fern family’s evolutionary story unfolds, researchers hope to reveal how universal principles of plant development shape the diversity of life we see today. The tobacco fern serves as a compelling case study, illustrating how a simple pattern of leaf placement can drive a complex internal architecture over deep time.
