Tip-to-base conduit widening maintains hydraulic efficiency in aerial fern organs

Tip-to-base conduit widening is critical for hydraulic efficiency; yet few studies have investigated the full developmental potential of xylem. We utilized the diverse growth habits of ferns to test whether a wide developmental potential of xylem is constrained by growth habit. We documented xylem hydraulically weighted conduit diameter (Dh) across the vertical stem of the tall Equisetum giganteum L.; in the soil surface rhizome that roots at each node of Phlebodium pseudoaureum (Cav.) Lellinger and its vertical leaf. In addition, we determined Dh across aquatic and aerially suspended rhizomes of Marsilea hirsuta L. Considerable tip-to-base conduit widening was observed in the stems of E. giganteum, aerially-suspended rhizomes of M. hirsuta, and the vertical leaves of P. pseudoaureum, organs in which selection would favor the maintenance of hydraulic efficiency toward the apex. No change in Dh was observed along the soil-surface rhizomes of P. pseudoaureum or the aquatic rhizomes of M. hirsuta. Our results indicate that there is considerable developmental potential for xylem conduit diameter in ferns apparent under conditions in which hydraulic efficiency is not required for survival; but that tip-to-base conduit widening is consistently observed in fern organs that require an efficient long-distance transport of water to the apex.