Convergent tip-to-base widening of water-conducting conduits in the tallest bryophytes

Background: Tip-to-base conduit widening is considered a key mechanism
that enables vascular plants to grow tall by decreasing the hydraulic
resistance imposed by increasing height. Widening of hydraulic anatomy
(larger conducting elements towards the base of the vascular system)
minimizes gradients in leaf-specific hydraulic conductance with plant
height, allowing uniform photosynthesis across the crown of trees.
Tip-to-base conduit widening has also been associated with changes in
conduit number.  However, in bryophytes, despite having
representatives with internal water conducting tissue, conduit widening
has been scarcely investigated. Methods: Here, we examined the changes in
hydroid diameter and number with distance from plant tip in Dawsonia
superba Grev. and D. polytrichoides R.Br., two representatives of the
genus containing the tallest extant bryophytes. Key results: We found that
the position of these moss species on the global scale of conduit size and
plant size was consistent with a general scaling among plants with
internal water transport. Within plants we found similar patterns of
conduit widening and number with distance from plant tip in endohydric
mosses to those observed in vascular plants. Conclusions:  Thus,
this study demonstrates that land plants growing upwards in the atmosphere
show analogous conduit widening of hydraulic structures, suggesting
efficient internal water transport is a convergent adaptation for
photosynthesis on land.