Despite the limitations of a vascular system composed solely of primary xylem ferns have persisted and are adapted to a wide range of habitats, including exposure to low water availability. Two key components of drought tolerance in plants are cavitation resistance and the isolation of subsequent embolisms. Resistance to embolism spread is necessarily dependent on the size, shape, strength, and distribution of pit membranes that act as a physical barrier to embolism spread between adjacent conduits. Previous research has shown a broad range of cavitation resistance in ferns, but the role that pit membranes play has yet to be determined. We used the micro-capillary technique to determine the air-seeding threshold of pit membranes within individual tracheids in 10 fern species and three fern allies. These data were then paired with TEM analysis of pit membrane thickness and light microscopy to determine pit characteristics and tracheid dimensions.
Across all species in this study increasing tracheid diameter was associated with lower resistance to embolism spread. The air seeding threshold of the three fern allies were significantly lower than the true ferns, likely due to the smaller diameter of the tracheids and pits. We also found that species with thick pit membranes were more resistant to air seeding than those with thin pit membranes, similar to findings in woody plants. The one outlier in this study, Pteridium aquilinum, is a fern known to have vessels in addition to trachieds within its vascular bundles. In P. aquilinum conduit diameters and pit membrane thickness were significantly higher than the other fern species, suggesting that to maintain high transport rates pit membrane thickness has increased to compensate for the large, vulnerable conduits. Overall, these data give support to the idea that the rare pit hypothesis operates not only in woody plants, but in ferns as well. In addition, it appears that selection has acted on fern pit membranes with regard to cavitation resistance.