Solid mechanics of the torus-margo in conifer intertracheid bordered pits.

Bordered pits of many conifers include a torus-margo structure acting as a valve that prevents air from spreading between tracheids, although the extent of torus deflection as a function of applied pressure is not well known. Models were developed from images of pits in roots and stems of Picea mariana (Mill.) BSP.

Computational models evaluating the impact of sieve plates and radial water exchange on phloem pressure gradients.

The sugar conducting phloem in angiosperms is a high resistance pathway made up of sieve elements bounded by sieve plates. The high resistance generated by sieve plates may be a trade-off for promoting quick sealing in the event of injury. However, previous modeling efforts have demonstrated a wide variation in the contribution of sieve plates towards total sieve tube resistance.

Pit membrane structure is highly variable and accounts for a major resistance to water flow through tracheid pits in stems and roots of two boreal conifer species.

The flow of xylem sap in conifers is strongly dependent on the presence of a low resistance path through bordered pits, particularly through the pores present in the margo of the pit membrane. A computational fluid dynamics approach was taken, solving the Navier-Stokes equation for models based on the geometry of pits observed in tracheids from stems and roots of Picea mariana (black spruce) and Picea glauca (white spruce). Model solutions demonstrate a close, inverse relationship between the total resistance of bordered pits and the total area of margo pores.

Computational fluid dynamics models of conifer bordered pits show how pit structure affects flow.

• The flow of xylem sap through conifer bordered pits, particularly through the pores in the pit membrane, is not well understood, but is critical for an understanding of water transport through trees. • Models solving the Navier-Stokes equation governing fluid flow were based on the geometry of bordered pits in black spruce (Picea mariana) and scanning electron microscopy images showing details of the pores in the margo of the pit membrane. • Solutions showed that the pit canals contributed a relatively small fraction of resistance to flow, whereas the torus and margo pores formed a large fraction, which depended on the structure of the individual pit.

Xylem anisotropy and water transport--a model for the double sawcut experiment.

Early experiments with overlapping cuts to the stems of trees demonstrated that lateral flow within the stem must be possible to allow such trees to maintain water flow to their leaves. We present a mathematical approach to considering lateral flow in stems by treating the xylem as an anisotropic medium for flow and develop an expression of its conductivity in the form of a tensor. In both 3D models of tracheid-bearing stems with cuts (incorporating this tensor analysis) and experimental stems with steadily deepening cuts, it is shown that flow can continue despite the presence of even strongly overlapping cuts through 90% of the stem.

Water flow through junctions in Douglas-fir roots.

Roots are important conduits for the redistribution of water within the rooting zone. Root systems are often highly branched, and water flow between regions undoubtedly involves passage through junctions between individual roots. This study considered junctions in the roots of Douglas-fir with regard to the resistances encountered by water flow through the xylem.

Incorporation of transfer resistance between tracheary elements into hydraulic resistance models for tapered conduits.

The model of West, Brown and Enquist (1999) shows that hydraulic resistance in trees can be independent of path length, provided that vascular conduits widen sufficiently from tree top to base. We demonstrate that this result does not depend theoretically on branching architecture or cross-sectional conductive area of the stem. Previous studies have shown that pit membrane resistance, encountered when water moves between either tracheids or vessels, accounts for up to 60% of the total resistance in stem segments.

Branch junctions and the flow of water through xylem in Douglas-fir and ponderosa pine stems.

Water flowing through the xylem from the roots to the leaves of most plants must pass through junctions where branches have developed from the main stem. These junctions have been studied as both flow constrictions and components of a hydraulic segmentation mechanism to protect the main axes of the plant. The hydraulic nature of the branch junction also affects the degree to which branches interact and can respond to changes in flow to other branches.