New findings overturn a long-standing view of fern vascular evolution
In a breakthrough study published in Current Biology, Assistant Professor Jacob S. Suissa of the University of Tennessee, Knoxville, reveals that the way leaves are arranged along a fern stem can steer the evolution of its internal vascular pattern. This challenges a century-and-a-half of research that has typically linked vascular bundle changes to environmental adaptation, suggesting instead that leaf placement on the stem plays a decisive, developmentally covarying role.
Suissa, a member of UT’s Department of Ecology and Evolutionary Biology, has spent years exploring the intricate world of plant vasculature. He notes that the classic view often treats stem vascular patterns as responsive to external pressures. His latest work, however, shows a tight developmental link between leaf arrangement and vascular architecture across the stem.
From field observation to high-tech validation
The research began in January 2025 during a Tropical Fern Biodiversity course in Costa Rica, where Suissa returned to one of his study species, Mickelia nicotianifolia, commonly known as tobacco fern. The long creeping stem of this species climbs trees and presents a clear canvas to observe how leaf placement relates to internal stem structure. While students gathered field data on the tobacco fern, Suissa used a razor blade and a simple, hands-on approach to examine the plant’s morphology, complemented by modern imaging.
Back in Knoxville, Suissa expanded the study to 27 fern genera, representing roughly 30% of fern species diversity. He leveraged the university’s Advanced Microscopy and Imaging Center to apply microCT scanning—an advanced, non-destructive imaging technique akin to a large CAT scan—to visualize the internal arrangement of vascular bundles without cutting the stems open.
A striking one-to-one link: leaves and vascular bundles
The results were striking. Suissa found a near 1:1 correspondence between the number of leaf ranks along a stem and the number of vascular bundles inside the stem. In some cases, five leaf ranks aligned with five stem bundles. But it wasn’t just the quantity that mattered—the arrangement of leaves dictated how vascular bundles were spatially organized. When leaves followed a spiral pattern, the vascular bundles adopted a radial configuration. When leaves clustered toward the dorsal side, the stem displayed a distinctive “smiley face” vascular arrangement.
“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. This relationship indicates that leaf placement along the stem exerts a developmental constraint that shapes the evolution of vascular patterns, rather than leaves adapting to an already fixed internal stem structure.
Implications for fern evolution and plant biology
The discovery reinterprets 150 years of fern research, which largely treated stem vasculature as adapting under direct natural selection to external conditions. Instead, Suissa’s study suggests a more integrated view: the stem’s vascular architecture evolves in concert with leaf arrangement, reflecting a coordinated developmental system that constructs both leaf positions and internal transport networks.
Suissa is expanding this work into a broad, cross-species analysis. He is assembling a massive dataset that covers over 300 species across all fern families to chart how vascular architecture has evolved across the approximately 400-million-year history of ferns. The aim is to understand how construction at multiple scales—from cellular to architectural—has evolved and co-evolved, revealing the deep connections between leaf placement and stem physiology.
Looking ahead
“Natural history observations can yield impactful insights alongside high-tech methods,” Suissa remarked. He envisions a future where models of fern evolution integrate leaf arrangement, stem vasculature, and lineage histories to predict how plant vascular systems may respond to changing environments. His ongoing work promises to illuminate not only fern biology but broader principles of how plant form evolves through coordinated development.
As researchers continue to parse the fern family tree, the key takeaway remains clear: in ferns, leaf arrangement is more than a surface pattern. It is a central driver shaping the vascular blueprint of the stem, guiding the evolution of a plant lineage that has thrived for hundreds of millions of years.
